Why perform strain analysis?

Why perform strain analysis?

Why do we perform strain analysis?. It can be important to retrieve information about strain from deformed rocks. First of all, strain analysis gives us an opportunity to explore the state of strain in a rock and to map out strain variations in a sample, an outcrop or a region. Strain data are important in the mapping and understanding of shear zones in orogenic belts. Strain measurements can also be used to estimate the amount of offset across a shear zone. It is possible to extract important information from shear zones if strain is known. 

In many cases it is useful to know if the strain is planar or three dimensional. If planar, an important criterion for section balancing is fulfilled, be it across orogenic zones or extensional basins. The shape of the strain ellipsoid may also contain information about how the deformation occurred. Oblate (pancake-shaped) strain in an orogenic setting may, for example, indicate flattening strain related to gravity-driven collapse rather than classic push-from-behind thrusting. 

The orientation of the strain ellipsoid is also important, particularly in relation to rock structures. In a shear zone setting, it may tell us if the deformation was simple shear or not. Strain in folded layers helps us to understand fold-forming mechanism(s). Studies of deformed reduction spots in slates give good estimates on how much shortening has occurred across the foliation in such rocks, and strain markers in sedimentary rocks can sometimes allow for reconstruction of original sedimentary thickness. 

Strain in one dimension

Two elongated belemnites in Jurassic limestone in the Swiss Alps. The different ways that the two belemnites have been stretched give us some two-dimensional information about the strain field: the upper belemnite has experienced sinistral shear strain while the lower one has not and must be close to the maximum stretching direction.

One-dimensional strain analyses are concerned with changes in length and therefore the simplest form of strain analysis we have. If we can reconstruct the original length of an object or linear structure we can also calculate the amount of stretching or shortening in that direction. Objects revealing the state of strain in a deformed rock are known as strain markers. Examples of strain markers indicating change in length are boudinaged dikes or layers, and minerals or linear fossils such as belemnites or graptolites that have been elongated, such as the stretched Swiss belemnites shown in Figure above. Or it could be a layer shortened by folding. It could even be a faulted reference horizon on a geologic or seismic profile. The horizon may be stretched by normal faults or shortened by reverse faults, and the overall strain is referred to as brittle strain. One-dimensional strain is revealed when the horizon, fossil, mineral or dike is restored to its pre-deformational state.

Strain in two dimensions

Reduction spots in Welsh slate. The light spots formed as spherical volumes of bleached (chemically reduced) rock. Their new shapes are elliptical in cross-section and oblate (pancake-shaped) in three dimensions, reflecting the tectonic strain in these slates.

In two-dimensional strain analyses we look for sections that have objects of known initial shape or contain linear markers with a variety of orientations (Figure first). Strained reduction spots of the type shown in Figure above are perfect, because they tend to have spherical shapes where they are undeformed. There are also many other types of objects that can be used, such as sections through conglomerates, breccias, corals, reduction spots, oolites, vesicles, pillow lavas (Figure below), columnar basalt, plutons and so on. Two-dimensional strain can also be calculated from one-dimensional data that represent different directions in the same section. A typical example would be dikes with different orientations that show different amounts of extension.

Section through a deformed Ordovician pahoe-hoe lava. The elliptical shapes were originally more circular, and Hans Reusch, who made the sketch in the 1880s, understood that they had been flattened during deformation. The Rf/f, center-to-center, and Fry methods would all be worth trying in this case.

Strain extracted from sections is the most common type of strain data, and sectional data can be combine to estimate the three-dimensional strain ellipsoid.

Changes in angles 

Strain can be found if we know the original angle between sets of lines. The original angular relations between structures such as dikes, foliations and bedding are sometimes found in both undeformed and deformed states, i.e. outside and inside a deformation zone. We can then see how the strain has affected the angular relationships and use this information to estimate strain. In other cases orthogonal lines of symmetry found in undeformed fossils such as trilobites, brachiopods and worm burrows (angle with layering) can be used to determine the angular shear in some deformed sedimentary rocks. In general, all we need to know is the change in angle between sets of lines and that there is no strain partitioning due to contrasting mechanical properties of the objects with respect to the enclosing rock.

If the angle was 90 degree in the undeformed state, the change in angle is the local angular shear. The two originally orthogonal lines remain orthogonal after the deformation, then they must represent the principal strains and thus the orientation of the strain ellipsoid. Observations of variously oriented line sets thus give information about the strain ellipse or ellipsoid. All we need is a useful method. Two of the most common methods used to find strain from initially orthogonal lines are known as the Wellman and Breddin methods, and are presented in the following sections.

The Wellman method 

Wellman’s method involves construction of the strain ellipse by drawing parallelograms based on the orientation of originally orthogonal pairs of lines. The deformation was produced on a computer and is a homogeneous simple shear. However, the strain ellipse itself tells us nothing about the degree of coaxiality: the same result could have been attained by pure shear.

This method dates back to 1962 and is a geometric construction for finding strain in two dimensions (in a section). It is typically demonstrated on fossils with orthogonal lines of symmetry in the undeformed state. In Figure above a we use the hinge and symmetry lines of brachiopods. A line of reference must be drawn (with arbitrary orientation) and pairs of lines that were orthogonal in the unstrained state are identified. The reference line must have two defined endpoints, named A and B in Figure above b. A pair of lines is then drawn parallel to both the hinge line and symmetry line for each fossil, so that they intersect at the endpoints of the reference line. The other points of intersection are marked (numbered 1–6 in Figure above b, c). If the rock is unstrained, the lines will define rectangles. If there is a strain involved, they will define parallelograms. To find the strain ellipse, simply fit an ellipse to the numbered corners of the parallelograms (Figure above c). If no ellipse can be fitted to the corner points of the rectangles the strain is heterogeneous or, alternatively, the measurement or assumption of initial orthogonality is false. The challenge with this method is, of course, to find enough fossils or other features with initially orthogonal lines typically 6–10 are needed.

The Breddin graph 

The data from the previous figure plotted in a Breddin graph. The data points are close to the curve for R=2.5.

We have already stated that the angular shear depends on the orientation of the principal strains: the closer the deformed orthogonal lines are to the principal strains, the lower the angular shear. This fact is utilized in a method first published by Hans Breddin in 1956 in German (with some errors). It is based on the graph shown in Figure above, where the angular shear changes with orientation and strain magnitude R. Input are the angular shears and the orientations of the sheared line pairs with respect to the principal strains. These data are plotted in the so-called Breddin graph and the R-value (ellipticity of the strain ellipse) is found by inspection (Figure above). This method may work even for only one or two observations. 

In many cases the orientation of the principal axes is unknown. In such cases the data are plotted with respect to an arbitrarily drawn reference line. The data are then moved horizontally on the graph until they fit one of the curves, and the orientations of the strain axes are then found at the intersections with the horizontal axis (Figure above). In this case a larger number of data are needed for good results.

Elliptical objects and the Rf/f-method 

Objects with initial circular (in sections) or spherical (in three dimensions) geometry are relatively uncommon, but do occur. Reduction spots and ooliths perhaps form the most perfect spherical shapes in sedimentary rocks. When deformed homogeneously, they are transformed into ellipses and ellipsoids that reflect the local finite strain. Conglomerates are perhaps more common and contain clasts that reflect the finite strain. In contrast to oolites and reduction spots, few pebbles or cobbles in a conglomerate are spherical in the undeformed state. This will of course influence their shape in the deformed state and causes a challenge in strain analyses. However, the clasts tend to have their long axes in a spectrum of orientations in the undeformed state, in which case methods such as the Rf/f-method may be able to take the initial shape factor into account.

The Rf/f method illustrated. The ellipses have the same ellipticity (Ri) before the deformation starts. The Rf–f diagram to the right indicates that Ri=2. A pure shear is then added with Rs=1.5 followed by a pure shear strain of Rs=3. The deformation matrices for these two deformations are shown. Note the change in the distribution of points in the diagrams to the right. Rs in the diagrams is the actual strain that is added. Modified from Ramsay and Huber (1983).

The Rf/f-method was first introduced by John Ramsay in his well-known 1967 textbook and was later improved. The method is illustrated in Figure above. The markers are assumed to have approximately elliptical shapes in the deformed (and undeformed) state, and they must show a significant variation in orientations for the method to work.

The Rf/f-method handles initially non-spherical markers, but the method requires a significant variation in the orientations of their long axes.

The ellipticity (X/Y) in the undeformed (initial) state is called Ri. In our example (Figure above) Ri=2. After a strain Rs the markers exhibit new shapes. The new shapes are different and depend on the initial orientation of the elliptical markers. The new (final) ellipticity for each deformation marker is called Rf and the spectrum of Rf-values is plotted against their orientations, or more specifically against the angle f' between the long axis of the ellipse and a reference line (horizontal in Figure above). In our example we have applied two increments of pure shear to a series of ellipses with different orientations. All the ellipses have the same initial shape Ri=2, and they plot along a vertical line in the upper right diagram in Figure above. Ellipse 1 is oriented with its long axis along the minimum principal strain axis, and it is converted into an ellipse that shows less strain (lower Rf-value) than the true strain ellipse (Rs). Ellipse 7, on the other hand, is oriented with its long axis parallel to the long axis of the strain ellipse, and the two ellipticities are added. This leads to an ellipticity that is higher than Rs. When Rs=3, the true strain Rs is located somewhere between the shape represented by ellipses 1 and 7, as seen in Figure above (lower right diagram). 

For Rs=1.5 we still have ellipses with the full spectrum of orientations ( 90 to 90 ; see middle diagram in Figure above), while for Rs=3 there is a much more limited spectrum of orientations (lower graph in Figure above). The scatter in orientation is called the fluctuation F. An important change happens when ellipse 1, which has its long axis along the Z-axis of the strain ellipsoid, passes the shape of a circle (Rs=Ri,) and starts to develop an ellipse whose long axis is parallel to X. This happens when Rs=2, and for larger strains the data points define a circular shape. Inside this shape is the strain Rs that we are interested in. But where exactly is Rs? A simple average of the maximum and minimum Rf-values would depend on the original distribution of orientations. Even if the initial distribution is random, the average R-value would be too high, as high values tend to be over represented (Figure above, lower graph). 

To find Rs we have to treat the cases where Rs >Ri and Rs <Ri separately. In the latter case, which is represented by the middle graph in Figure above, we have the following expressions for the maximum and minimum value for Rf:

In both cases the orientation of the long (X) axis of the strain ellipse is given by the location of the maximum Rf-values. Strain could also be found by fitting the data to pre-calculated curves for various values for Ri and Rs. In practice, such operations are most efficiently done by means of computer programs.

The example shown in Figure above and discussed above is idealized in the sense that all the undeformed elliptical markers have identical ellipticity. What if this were not the case, i.e. some markers were more elliptical than others? Then the data would not have defined a nice curve but a cloud of points in the Rf/f-diagram. Maximum and minimum Rf-values could still be found and strain could be calculated using the equations above. The only change in the equation is that Ri now represents the maximum ellipticity present in the undeformed state. 

Another complication that may arise is that the initial markers may have had a restricted range of orientations. Ideally, the Rf/f-method requires the elliptical objects to be more or less randomly oriented prior to deformation. Conglomerates, to which this method commonly is applied, tend to have clasts with a preferred orientation. This may result in an Rf–f plot in which only a part of the curve or cloud is represented. In this case the maximum and minimum Rf-values may not be representative, and the formulas above may not give the correct answer and must be replaced by a computer based iterative retrodeformation method where X is input. However, many conglomerates have a few clasts with initially anomalous orientations that allow the use of Rf/f analysis.

Center-to-center method 

The center-to-center method. Straight lines are drawn between neighbouring object centers. The length of each line (d') and the angle (a') that they make with a reference line are plotted in the diagram. The data define a curve that has a maximum at X and a minimum at the Y-value of the strain ellipse, and where Rs= X/Y.

This method, here demonstrated in Figure above, is based on the assumption that circular objects have a more or less statistically uniform distribution in our section(s). This means that the distances between neighboring particle centers were fairly constant before deformation. The particles could represent sand grains in well-sorted sandstone, pebbles, ooids, mud crack centers, pillow-lava or pahoe-hoe lava centers, pluton centers or other objects that are of similar size and where the centers are easily definable. If you are uncertain about how closely your section complies with this criterion, try anyway. If the method yields a reasonably well-defined ellipse, then the method works.

The method itself is simple and is illustrated in Figure above: Measure the distance and direction from the center of an ellipse to those of its neighbours. Repeat this for all ellipses and graph the distance d' between the centers and the angles a' between the center tie lines and a reference line. A straight line occurs if the section is unstrained, while a deformed section yields a curve with maximum (d' max) and minimum values (d' min). The ellipticity of the strain ellipse is then given by the ratio: Rs =( d' max)/(d' min).

The Fry method

The Fry method performed manually. (a) The centerpoints for the deformed objects are transferred to a transparent overlay. A central point (1 on the figure) is defined. (b) The transparent paper is then moved to another of the points (point 2) and the centerpoints are again transferred onto the paper (the overlay must not be rotated). The procedure is repeated for all of the points, and the result (c) is an image of the strain ellipsoid (shape and orientation). Based on Ramsay and Huber (1983).

A quicker and visually more attractive method for finding two-dimensional strain was developed by Norman Fry at the end of the 1970s. This method, illustrated in Figure above, is based on the center-to-center method and is most easily dealt with using one of several available computer programs. It can be done manually by placing a tracing overlay with a coordinate origin and pair of reference axes on top of a sketch or picture of the section. The origin is placed on a particle center and the centers of all other particles (not just the neighbours) are marked on the tracing paper. The tracing paper is then moved, without rotating the paper with respect to the section, so that the origin covers a second particle center, and the centers of all other particles are again marked on the tracing paper. This procedure is repeated until the area of interest has been covered. For objects with a more or less uniform distribution the result will be a visual representation of the strain ellipse.The ellipse is the void area in the middle, defined by the point cloud around it (Figure above c). 

The Fry method, as well as the other methods presented in this section, outputs two-dimensional strain. Three-dimensional strain is found by combining strain estimates from two or more sections through the deformed rock volume. If sections can be found that each contain two of the principal strain axes, then two sections are sufficient. In other cases three or more sections are needed, and the three-dimensional strain must be calculated by use of a computer.

Strain in three dimensions

Three-dimensional strain expressed as ellipses on different sections through a conglomerate. The foliation (XY-plane) and the lineation (X-axis) are annotated. This illustration was published in 1888, but what are now routine strain methods were not developed until the 1960s.

A complete strain analysis is three-dimensional. Three dimensional strain data are presented in the Flinn diagram or similar diagrams that describe the shape of the strain ellipsoid, also known as the strain geometry. In addition, the orientation of the principal strains can be presented by means of stereographic nets. Direct field observations of three-dimensional strain are rare. In almost all cases, analysis is based on two-dimensional strain observations from two or more sections at the same locality (Figure above). A well-known example of three-dimensional strain analysis from deformed conglomerates is presented in below heading. 

In order to quantify ductile strain, be it in two or three dimensions, the following conditions need to be met:

The strain must be homogeneous at the scale of observation, the mechanical properties of the objects must have been similar to those of their host rock during the deformation, and we must have a reasonably good knowledge about the original shape of strain markers.

The strain must be homogeneous at the scale of observation, the mechanical properties of the objects must have been similar to those of their host rock during the deformation, and we must have a reasonably good knowledge about the original shape of strain markers.

The second point is an important one. For ductile rocks it means that the object and its surroundings must have had the same competence or viscosity. Otherwise the strain recorded by the object would be different from that of its surroundings. This effect is one of several types of strain partitioning, where the overall strain is distributed unevenly in terms of intensity and/or geometry in a rock volume. As an example, we mark a perfect circle on a piece of clay before flattening it between two walls. The circle transforms passively into an ellipse that reveals the two-dimensional strain if the deformation is homogeneous. If we embed a coloured sphere of the same clay, then it would again deform along with the rest of the clay, revealing the three-dimensional strain involved. However, if we put a stiff marble in the clay the result is quite different. The marble remains unstrained while the clay around it becomes more intensely and heterogeneously strained than in the previous case. In fact, it causes a more heterogeneous strain pattern to appear. Strain markers with the same mechanical properties as the surroundings are called passive strain markers because they deform passively along with their surroundings. Those that have anomalous mechanical properties respond differently than the surrounding medium to the overall deformation, and such markers are called active strain markers. 

Strain obtained from deformed conglomerates, plotted in the Flinn diagram. Different pebble types show different shapes and finite strains. Polymict conglomerate of the Utslettefjell Formation, Stord, southwest Norway. 

An example of data from active strain markers is shown in Figure above. These data were collected from a deformed polymictic conglomerate where three-dimensional strain has been estimated from different clast types in the same rock and at the same locality. Clearly, the different clast types have recorded different amounts of strain. Competent (stiff) granitic clasts are less strained than less competent greenstone clasts. This is seen using the fact that strain intensity generally increases with increasing distance from the origin in Flinn space. But there is another interesting thing to note from this figure: It seems that competent clasts plot higher in the Flinn diagram (Figure. above) than incompetent(“soft”) clasts, meaning that competent clasts take on a more prolate shape. Hence, not only strain intensity but also strain geometry may vary according to the mechanical properties of strain markers. 

The way that the different markers behave depends on factors such as their mineralogy, preexisting fabric, grain size, water content and temperature-pressure conditions at the time of deformation. In the case of Figure above, the temperature-pressure regime is that of lower to middle greenschist facies. At higher temperatures, quartz-rich rocks are more likely to behave as “soft” objects, and the relative positions of clast types in Flinn space are expected to change. 

The last point above also requires attention: the initial shape of a deformed object clearly influences its postdeformational shape. If we consider two-dimensional objects such as sections through oolitic rocks, sandstones or conglomerates, the Rf/f method discussed above can handle this type of uncertainty. It is better to measure up two or more sections through a deformed rock using this method than dig out an object and measure its three-dimensional shape. The single object could have an unexpected initial shape (conglomerate clasts are seldom perfectly spherical or elliptical), but by combining numerous measurements in several sections we get a statistical variation that can solve or reduce this problem.

Three-dimensional strain is usually found by combining two-dimensional data from several differently oriented sections.

There are now computer programs that can be used to extract three-dimensional strain from sectional data. If the sections each contain two of the principal strain axes everything becomes easy, and only two are strictly needed (although three would still be good). Otherwise, strain data from at least three sections are required.

Deformed quartzite conglomerates

Quartz or quartzite conglomerates with a quartzite matrix are commonly used for strain analyses. The more similar the mineralogy and grain size of the matrix and the pebbles, the less deformation partitioning and the better the strain estimates. A classic study of deformed quartzite conglomerates is Jake Hossack’s study of the Norwegian Bygdin conglomerate, published in 1968. Hossack was fortunate he found natural sections along the principal planes of the strain ellipsoid at each locality. Putting the sectional data together gave the three dimensional state of strain (strain ellipsoid) for each locality. Hossack found that strain geometry and intensity varies within his field area. He related the strain pattern to static flattening under the weight of the overlying Caledonian Jotun Nappe. Although details of his interpretation may be challenged, his work demonstrates how conglomerates can reveal a complicated strain pattern that otherwise would have been impossible to map. 

Hossack noted the following sources of error:

• Inaccuracy connected with data collection (sections not being perfectly parallel to the principal planes of strain and measuring errors).  
• Variations in pebble composition.  
• The pre-deformational shape and orientation of the pebbles.  
• Viscosity contrasts between clasts and matrix.  
• Volume changes related to the deformation (pressure solution).  
• The possibility of multiple deformation events.

Credits: Haakon Fossen (Structural Geology)

Mathematical description of deformation

Mathematical description of deformation

Deformation is conveniently and accurately described and modelled by means of elementary linear algebra. Let us use a local coordinate system, such as one attached to a shear zone, to look at some fundamental deformation types. We will think in terms of particle positions (or vectors) and see how particles change positions during deformation. If (x, y) is the original position of a particle, then the new position will be denoted (x',y'). For homogeneous deformation in two dimensions (i.e. in a section) we have that 

The reciprocal or inverse deformation takes the deformed rock back to its undeformed state.

The deformation matrix D is very useful if one wants to model deformation using a computer. Once the deformation matrix is defined, any aspect of the deformation itself can be found. Once again, it tells us nothing about the deformation history, nor does it reveal how a given deforming medium responds to such a deformation. For more information about matrix algebra, see below.

MATRIX ALGEBRA

Matrices contain coefficients of systems of equations that represent linear transformations. In two dimensions this means that the system of equations shown in Equation 2.1 can be expressed by the matrix of Equation 2.2. A linear transformation implies a homogeneous deformation. The matrix describes the shape and orientation of the strain ellipse or ellipsoid, and the transformation is a change from a unit circle, or a unit sphere in three dimensions. 

Matrices are simpler to handle than sets of equations, particularly when applied in computer programs. The most important matrix operations in structural geology are multiplications and finding eigenvectors and eigenvalues: 

Matrix multiplied by a vector:

The determinant describes the area or volume change: If det D=1 then there is no area or volume change involved for the transformation (deformation) represented by D. Eigenvectors (x) and eigenvalues (lambda) of a matrix A are the vectors and values that fulfil

 If A=DDTT, then the deformation matrix has two eigenvectors for two dimensions and three for three dimensions. The eigenvectors describe the orientation of the ellipsoid (ellipse), and the eigenvalues describe its shape (length of its principal axes). Eigenvalues and eigenvectors are easily found by means of a spreadsheet or computer program such as MatLabTM.

Pyrite (Marcasite)

What is Pyrite?

Pyrite, often called "Fools Gold", has a silvery-yellow to golden metallic colour. It is very common and may occur in large crystals. It has been used by ancient civilisations as jewellery, but is hardly used nowadays. Pyrite is sometimes incorrectly known as Marcasite in the gemstone trade. Marcasite is mineral that is a polymorph of Pyrite, and can be fragile and unstable, and is not fit for gemstone use.

The mineral pyrite, or iron pyrite, also known as fool's gold, is an iron sulphide with the chemical formula FeS2. This mineral's metallic luster and pale brass-yellow hue give it a superficial resemblance to gold, hence the well-known nickname of fool's gold. The colour has also led to the nicknames brass, brazzle, and Brazil, primarily used to refer to pyrite found in coal.

Pyrite is the most common of the sulphide minerals. The name pyrite is derived from the Greek (pyritēs), "of fire" or "in fire", in turn from (pyr), "fire". In ancient Roman times, this name was applied to several types of stone that would create sparks when struck against steel; Pliny the Elder described one of them as being brassy, almost certainly a reference to what we now call pyrite. By Georgius Agricola's time, c. 1550, the term had become a generic term for all of the sulphide minerals.

Pyrite is usually found associated with other sulphides or oxides in quartz veins, sedimentary rock, and metamorphic rock, as well as in coal beds and as a replacement mineral in fossils. Despite being nicknamed fool's gold, pyrite is sometimes found in association with small quantities of gold. Gold and arsenic occur as a coupled substitution in the pyrite structure. In the Carlin–type gold deposits, arsenian pyrite contains up to 0.37 wt% gold.

Occurrence of Marcasite

Marcasite can be formed as both a primary or a secondary mineral. It typically forms under low-temperature highly acidic conditions. It occurs in sedimentary rocks (shales, limestones and low grade coals) as well as in low temperature hydrothermal veins. Commonly associated minerals include pyrite, pyrrhotite, galena, sphalerite, fluorite, dolomite and calcite.

As a primary mineral it forms nodules, concretions and crystals in a variety of sedimentary rock, such as in the chalk layers found on both sides of the English Channel at Dover, Kent, England and at Cap Blanc Nez, Pas De Calais, France, where it forms as sharp individual crystals and crystal groups, and nodules.

As a secondary mineral it forms by chemical alteration of a primary mineral such as pyrrhotite or chalcopyrite.

Marcasite and Pyrite

A mineral is defined both by its chemical composition and its crystal structure. In some cases two different minerals have the same chemical composition, but different crystal structures. Known as polymorphs, these intriguing cases illustrate how the different crystal structures can result in quite different physical properties.

Perhaps the most famous case of a polymorph pair is diamond and graphite. Though both are composed entirely of pure carbon, diamond has a cubic structure with strong bonds in 3 dimensions. Graphite, by contrast, forms in layers with only weak bonds between layers. As a result of their structural differences, diamond has a hardness of 10 on the Mohs scale, while graphite rates only 1. Diamond is the ultimate abrasive, while graphite is a superb lubricant.

Another interesting polymorph pair is marcasite and pyrite. Both minerals are composed of iron sulphide. But where no one could ever confuse diamond and graphite, it can be difficult to tell pyrite and marcasite apart. In fact pyrite is often sold under the name marcasite in the gemstone trade. But despite their apparent similarities, they have some important differences, such that one can be used as a gem material while the other cannot.

Pyrite has a cubic structure, metallic luster and a yellow-gold colour that has earned it the nickname "fool's gold". With a hardness of 6 to 6.5 on the Mohs scale, pyrite is hard enough to be used in jewellery. Pyrite is also exceptionally dense, with a specific gravity of 5.0 to 5.2. Only hematite has a higher density.

Marcasite tends to be lighter in colour, and is sometimes referred to as "white iron pyrite". Sometime marcasite has a greenish tint, or a multi-coloured tarnish that is the result of oxidation. But marcasite has an unstable orthorhombic crystal structure and is liable to crumble and break apart. In some cases marcasite will react with moisture in the air to produce sulphuric acid. For these reasons marcasite is never used in jewellery. When a gemstone is sold as marcasite you can be quite sure that it is actually pyrite.

Varieties and blends

Blueite (S.H.Emmons): Nickel variety of marcasite, found in Denison Drury and Townships, Sudbury Dist., Ontario, Canada.

Lonchidite (August Breithaupt): Arsenic variety of marcasite, found at Churprinz Friedrich August Erbstolln Mine (Kurprinz Mine), Großschirma Freiberg, Erzgebirge, Saxony, Germany; ideal formula Fe(S, As)2.

Synonyms for this variety:

• kausimkies,
• kyrosite,
• lonchandite,
• metalonchidite (Sandberger) described at Bernhard Mine near Hausach (Baden), Germany.

Sperkise designates a marcasite having twin spearhead crystal. Sperkise derives from the German Speerkies (Speer meaning spear and Kies gravel or stone). This twin is very common in the marcasite of a chalky origin, particularly those from the Cap Blanc Nez.

Properties of Pyrite (Marcasite)

Colour	Metallic, Yellow, Gray
Hardness	6 - 6.5
Crystal System	Isometric
SG	4.9 - 5.2
Transparency	Opaque
Double Refraction	None
Luster	Metallic
Cleavage	None
Mineral Class	Pyrite

Prehnite

What is Prehnite?

Prehnite is an inosilicate of calcium and aluminium. Prehnite crystallises in the orthorhombic crystal system, and most often forms as stalactitic or botryoidal aggregates, with only just the crests of small crystals showing any faces, which are almost always curved or composite. Very rarely will it form distinct, well-individualised crystals showing a square-like cross-section, including those found at the Jeffrey Mine in Asbestos, Quebec, Canada. Prehnite is brittle with an uneven fracture and a vitreous to pearly luster. Its hardness is 6-6.5, its specific gravity is 2.80-2.90 and its colour varies from light green to yellow, but also colourless, blue, pink or white. In April 2000, rare orange prehnite was discovered in the Kalahari Manganese Fields, South Africa. Prehnite is mostly translucent, and rarely transparent.

Though not a zeolite, prehnite is found associated with minerals such as datolite, calcite, apophyllite, stilbite, laumontite, heulandite etc. in veins and cavities of basaltic rocks, sometimes in granites, syenites, or gneisses. It is an indicator mineral of the prehnite-pumpellyite metamorphic facies.

History and Introduction

Prehnite is a translucent to transparent gem-quality hydrated calcium aluminum silicate. It was the first mineral to be named after an individual, and it was also the first mineral to be described from South Africa, long before South Africa became one of the most important sources for precious and semi-precious gems. It was first described in 1788 after it was discovered in the Karoo dolerites of Cradock, South Africa. Prehnite was later named after its discoverer, Colonel Hendrik von Prehn (1733-1785), a Dutch mineralogist and an early governor of the Cape of Good Hope colony.

Until recently, prehnite was a rare collector's gemstone, but new deposits have now made it more readily available. In China, prehnite is sometimes referred to as 'grape jade' owing to its typical nodule formations which often resemble a bunch of grapes. Its colour is usually a soft apple-green, which is quite unique to prehnite, but it can also occur in rarer colours including yellow, orange and blue.

Identifying Prehnite

Prehnite is typically semi-transparent to translucent with a chemical formula of Ca2Al(AlSi3O10)(OH)2. Its color is usually yellow-green to apple-green. Prehnite is considerably hard with a rating of 6 to 6.5 on the Mohs scale. It has a specific gravity ranging from 2.82 to 2.94 and a refractive index of 1.611 to 1.669. Prehnite is in the orthorhombic crystal class, usually found in radiating botryoidal (grape-like) aggregate forms, and rarely as tabular and pyramidal crystals. When heated, prehnite crystals can sometimes give off water. It has a brittle tenacity and an uneven fracture. When polished, prehnite has a vitreous to pearly luster. Prehnite may be confused with apatite, jade or serpentine.

Prehnite: Origin and Sources

Prehnite occurs in the veins and cavities of mafic volcanic rock. It is a typical product of low-grade metamorphism. Primary deposits of prehnite are sourced from several locations around the world. Some of the most important deposits come from Africa (Namibia, South Africa), Australia (Western Australia, Northern Territory), Canada, China, Germany, Scotland, France and the United States (New Jersey, Pennsylvania and Virginia).

Rare, orange coloured prehnite has been discovered in South Africa. Quebec, Canada is known to produce prehnite with distinct, individual crystals.

Prehnite: Related or Similar Gemstones

There are no closely related gemstones, but there are several gemstones which can have a very similar appearance (colour and luster), including jade, apatite, serpentine, brazilianite, periclase, chrysoprase, peridot, smithsonite and hemimorphite. Prehnite is also often found and associated with many microporous, aluminosilicate zeolite minerals such as datolite, calcite, apophyllite, stilbite and heulandite.

Prehnite Healing properties

Prehnite is considered a stone of unconditional love and the crystal to heal the healer. It enhances precognition and inner knowing. Enables you always to be prepared. Prehnite calms the environment and brings peace and protection. It teaches how to be in harmony with nature and the elemental forces. Helpful for “decluttering” letting go of possessions you no longer need, aiding those who hoard possessions, or love, because of an inner lack. Prehnite alleviates nightmares, phobias and deep fears, uncovering and healing the dis-ease that creates them. It is a stone for dreaming and remembering. Beneficial for hyperactive children and the causes that underlie the condition.

Prehnite heals the kidneys and bladder, thymus gland, shoulders, chest and lungs. It treats gout and blood disorders. Prehnite repairs the connective tissue in the body and can stabilise malignancy.

Properties of Prehnite

Chemical Formula	Ca2Al2Si3O12(OH)
Colour	Green
Hardness	6 - 6.5
Crystal System	Orthorhombic
Refractive Index	1.61 - 1.64
SG	2.8 - 3.0
Transparency	Translucent
Double Refraction	.030
Luster	Vitreous, waxy
Cleavage	1,1;3,1
Mineral Class	Prehnite

Platinum

What is Platinum?

Platinum is the most valued precious metal; its value exceeds even that of Gold. It has a beautiful silver-white colour, and, unlike Silver, does not tarnish. It is unaffected by common household chemicals and will not get damaged or discoloured by chlorine, bleach, or detergents. It is tougher than all precious jewellery metals, though due to its flexible tenacity it still must be alloyed with other metals to prevent it from bending. Natural Platinum usually contains small amounts of the rare element iridium. In jewellery, iridium is alloyed with the Platinum to increase its toughness. Platinum jewellery is usually 90 to 95 percent pure.

Platinum is one of the least reactive metals. It has remarkable resistance to corrosion, even at high temperatures, and is therefore considered a noble metal. Consequently, platinum is often found chemically uncombined as native platinum. Because it occurs naturally in the alluvial sands of various rivers, it was first used by pre-Columbian South American natives to produce artefacts. It was referenced in European writings as early as 16th century, but it was not until Antonio de Ulloa published a report on a new metal of Colombian origin in 1748 that it began to be investigated by scientists.

Platinum is used in catalytic converters, laboratory equipment, electrical contacts and electrodes, platinum resistance thermometers, dentistry equipment, and jewellery. Being a heavy metal, it leads to health issues upon exposure to its salts; but due to its corrosion resistance, metallic platinum has not been linked to adverse health effects. Compounds containing platinum, such as cisplatin, oxaliplatin and carboplatin, are applied in chemotherapy against certain types of cancer.

About Platinum

• Atomic number (number of protons in the nucleus): 78
• Atomic symbol (on the periodic table of elements): Pt
• Atomic weight (average mass of the atom): 195.1
• Density: 12.4 ounces per cubic inch (21.45 grams per cubic cm)
• Phase at room temperature: solid
• Melting point: 3,215.1 degrees Fahrenheit (1,768.4 degrees Celsius)
• Boiling point: 6,917 F (3,825 C)
• Number of natural isotopes (atoms of the same element with a different number of neutrons): 6. There are also 37 artificial isotopes created in a lab.
• Most common isotopes: Pt-195 (33.83 percent of natural abundance), Pt-194 (32.97 percent of natural abundance), Pt-196 (25.24 percent of natural abundance), Pt-198 (7.16 percent of natural abundance), Pt-192 (0.78 percent of natural abundance), Pt-190 (0.01 percent of natural abundance)

Platinum History (The "unmeltable" metal)

In ancient times, people in Egypt and the Americas used platinum for jewellery and decorative pieces, often times mixed with gold. The first recorded reference to platinum was in 1557 when Julius Scaliger, an Italian physician, described a metal found in Central America that wouldn't melt and called it "platina," meaning "little silver." 

In 1741, British scientist Charles Wood published a study introducing platinum as a new metal and described some of its attributes and possible commercial applications, according to Peter van der Krogt a Dutch historian. Then, in 1748, Spanish scientist and naval officer Antonio de Ulloa published a description of a metal that was unworkable and unmeltable. (He originally wrote it in 1735, but his papers were confiscated by the British navy.) 

Back in the 18th century, platinum was the eighth known metal and was known as "white gold," according to van der Krogt. (Previously known metals included iron, copper, silver, tin, gold, mercury and lead.)

In the early 1800s, friends and colleagues William Hyde Wollaston and Smithson Tennant, both British chemists, produced and sold purified platinum that they isolated using a technique developed by Wollaston, according to van der Krogt This technique involves dissolving platinum ore in a mixture of nitric and hydrochloric acids (known as aqua regia). After the platinum was separated from the rest of the solution, palladium, rhodium, osmium, iridium, and later ruthenium were all discovered in the waste.

Today, platinum is still extracted using a technique similar to that developed by Wollaston. Samples containing platinum are dissolved in aqua regia, are separated from the rest of the solution and byproducts, and are melted at very high temperatures to produce the metal.

Occurrence of Platinum

Platinum is an extremely rare metal, occurring at a concentration of only 0.005 ppm in Earth's crust. It is sometimes mistaken for silver (Ag). Platinum is often found chemically uncombined as native platinum and as alloy with the other platinum-group metals and iron mostly. Most often the native platinum is found in secondary deposits in alluvial deposits. The alluvial deposits used by pre-Columbian people in the Chocó Department, Colombia are still a source for platinum-group metals. Another large alluvial deposit is in the Ural Mountains, Russia, and it is still mined.

In nickel and copper deposits, platinum-group metals occur as sulfides (e.g. (Pt,Pd)S), tellurides (e.g. PtBiTe), antimonides (PdSb), and arsenides (e.g. PtAs2), and as end alloys with nickel or copper. Platinum arsenide, sperrylite (PtAs2), is a major source of platinum associated with nickel ores in the Sudbury Basin deposit in Ontario, Canada. At Platinum, Alaska, about 17,000 kg (550,000 ozt) had been mined between 1927 and 1975. The mine ceased operations in 1990. The rare sulfide mineral cooperite, (Pt,Pd,Ni)S, contains platinum along with palladium and nickel. Cooperite occurs in the Merensky Reef within the Bushveld complex, Gauteng, South Africa.

In 1865, chromites were identified in the Bushveld region of South Africa, followed by the discovery of platinum in 1906. The largest known primary reserves are in the Bushveld complex in South Africa. The large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin, Canada, are the two other large deposits. In the Sudbury Basin, the huge quantities of nickel ore processed make up for the fact platinum is present as only 0.5 ppm in the ore. Smaller reserves can be found in the United States, for example in the Absaroka Range in Montana. In 2010, South Africa was the top producer of platinum, with an almost 77% share, followed by Russia at 13%; world production in 2010 was 192,000 kg (423,000 lb).

Platinum deposits are present in the state of Tamil Nadu, India.

Platinum exists in higher abundances on the Moon and in meteorites. Correspondingly, platinum is found in slightly higher abundances at sites of bolide impact on Earth that are associated with resulting post-impact volcanism, and can be mined economically; the Sudbury Basin is one such example.

Properties of Platinum

Chemical Formula	Pt
Colour	Metallic, White
Hardness	4 - 4.5
Crystal System	Isometric
SG	14 - 19
Transparency	Opaque
Double Refraction	None
Luster	Metallic
Cleavage	None
Mineral Class	Platinum

Peridot

What is Peridot?

Peridot is a well-known and ancient gemstone, with jewellery pieces dating all the way back to the Pharaohs in Egypt. The gem variety of the mineral Olivine, it makes a lovely light green to olive-green gemstone. The intensity of colour depends on the amount of iron present in a Peridot's chemical structure; the more iron it contains the deeper green it will be. The most desirable colour of Peridot is deep olive-green with a slight yellowish tint. Deeper olive-green tones tend to be more valuable than lighter coloured greens and yellowish-greens.

Peridot is the gem variety of the mineral olivine. Its chemical composition includes iron and magnesium, and iron is the cause of its attractive yellowish green colours. The gem often occurs in volcanic rocks called basalts, which are rich in these two elements.

The glorious yellow-green Peridot has been under-appreciated for years, overlooked as a lesser gem, small, easily obtained and relatively inexpensive, often considered as simply the birthstone for August. Its popularity has fallen in and out of vogue for centuries. However, a new resurgence is bringing to light what Peridot lovers have always known: this is a truly remarkable stone.

Called “the extreme gem” by the Gemological Institute of America, Peridot is born of fire and brought to light, one of only two gems (Diamond is the other) formed not in the Earth’s crust, but in molten rock of the upper mantle and brought to the surface by the tremendous forces of earthquakes and volcanoes. While these Peridots are born of Earth, other crystals of Peridot have extraterrestrial origins, found in rare pallasite meteorites (only 61 known to date) formed some 4.5 billion years ago, remnants of our solar system’s birth. Peridot in its basic form, Olivine, was also found in comet dust brought back from the Stardust robotic space probe in 2006, has been discovered on the moon, and detected by instrument on Mars by NASA’s Global Surveyor. Ancients believed, quite accurately, that Peridot was ejected to Earth by a sun’s explosion and carries its healing power.

History and Introduction

Peridot is a gem-quality variety of the mineral olivine. It belongs to the forsterite-fayalite mineral series. Some even refer to peridot as 'olivine', but when it comes to the gemstone, 'peridot' is the correct term. Peridot is an idiochromatic gem, meaning its colour comes from the basic chemical composition of the mineral itself and not from minor traces of impurities. Thus, peridot is found only in green. In fact, peridot is one of the few gemstones available that can be found only in one color, although the shades of green may vary from light yellowish to dark brownish-green.

The name 'peridot' was derived from the Arabic word for gem 'faridat'. It is sometimes referred to as 'the poor man's emerald' or as 'chrysolite', a word derived from the Greek word 'goldstone'. It is one of the oldest known gemstones, with records dating back as early as 1500 B.C. Historically, the volcanic island of Zabargad (St. John) in the Red Sea, east of Egypt, had the most important deposit that was exploited for over 3500 years. Today, the finest quality peridot comes from Mogok in Burma, although Pakistani peridot is now highly regarded as well. There are other very important deposits found in Arizona, China and Vietnam. Peridot has also been discovered in fallen meteors and it has also been discovered on Mars and the moon in olivine form.

Occurrence of Peridot

Olivine, of which peridot is a type, is a common mineral in mafic and ultramafic rocks, and it is often found in lavas and in peridotite xenoliths of the mantle, which lavas carry to the surface; but gem quality peridot only occurs in a fraction of these settings. Peridots can be also found in meteorites.

Olivine in general is a very abundant mineral, but gem quality peridot is rather rare. This is due to the mineral's chemical instability on the Earth's surface. Olivine is usually found as small grains, and tends to exist in a heavily weathered state, unsuitable for decorative use. Large crystals of forsterite, the variety most often used to cut peridot gems, are rare; as a result olivine is considered to be precious.

In meteorites

Peridot crystals have been collected from some pallasite meteorites.

Identifying Peridot

Chemically, peridot is an iron magnesium silicate and its intensity of colour depends on the amount of iron it contains. There may also be traces of nickel and chromium present. Peridot is not especially hard and it has no resistance to acid. On very rare occasions, peridot is known to form with cat's eye chatoyancy (asterism) in the form of four ray stars. Peridot can be mistaken for similar coloured gems, but its strong double refraction is often a very distinguishing trait. In thicker stones, the doubling of lower facet edges can be easily seen by looking down though the table without the need for magnification.

Peridot: Origin and Sources

Most gemstones are formed in earth's crust, but peridot is formed much deeper in the mantle region. Peridot crystals form in magma from the upper mantle and are brought to the surface by tectonic or volcanic activity where they are found in extrusive igneous rocks. Historically the volcanic island Zabargad (St. John) in the Red Sea was the location of the most important deposit. It was exploited for 3500 years before it was abandoned for many centuries; later, it was rediscovered around 1900 and has been heavily exploited ever since.

Today, the most important deposits are found in Pakistan (in the Kashmir region and the Pakistan-Afghanistan border region). Beautiful material is also found in upper Myanmar (Burma) and Vietnam. Other deposits are found in Australia (Queensland), Brazil (Minas Gerais), China, Kenya, Mexico, Norway (north of Bergen), South Africa, Sri Lanka, Tanzania and the United States (Arizona and Hawaii). Recently, China has become of the the largest producers of peridot.

Peridot: Related or Similar Gemstones

Peridot is a transparent gem variety of olivine. Olivine is not officially a mineral but is composed of two end-member minerals: fayalite and forsterite. Fayalite is iron rich olivine, while forsterite is magnesium rich olivine. Although iron is the colouring agent for peridot, it is technically closer to forsterite than fayalite with regard to chemical composition.

Peridot is sometimes referred to as 'chrysolite', a historical name which archaically refers to several green to yellow-green coloured gemstones. Other forms of 'chrysolite' include chrysoberyl, zircon, tourmaline, topaz and apatite.

Peridot Metaphysical and Healing properties

Peridot is highly beneficial for attuning to and regulating the cycles of one’s life, such as physical cycles, mental or emotional phases, as well as intellectual progression. It also helps dissipate negative patterns and old vibrations that play over and over, keeping one from realising they are deserving of success. By working with Peridot one can remove those blockages and move forward quickly, opening the heart and mind more fully to receive from the Universe with grace and gratitude.

A stone of transformation, Peridot is excellent for use in recovery from tobacco or inhalant dependencies, as well as other addictions. More importantly, it is a wounded healer stone, serving as a vital guide in facilitating healing processes that help others going through what you have already overcome. It is considered very effective in amplifying Reiki energies. Hold immediately after treatments using heat or warmth, such as sweat lodges, hot rocks or a sauna to continue the beneficial effects. 

Peridot is ideal for discharging emotional issues that affect the physical body. Place it over the Solar Plexus to relax and release nervous tension, known as “butterflies,” as well as to alleviate fear and guilt, anxiety or impatience. Place Peridot over the Heart Chakra to relieve heaviness of heart, empower forgiveness, or alleviate destructive jealousy or self-doubt caused by betrayal in past relationships. 

Use Peridot to gain results when seeking items that are lost or mislaid in the physical world, as well as in the quest for an enlightened state. 

Wear Peridot set in gold to bring peaceful sleep. It is especially effective for those who suffer from recurring nightmares about evil spirits, murders or sexual attacks. 

Wear or carry Peridot as a talisman of luck and as a sun stone to prevent personal darkness. It adds charm and eloquence to your presentations, evokes a positive, helpful response from normally unhelpful people, and increases profit in trades. It is naturally protective against envy, gossip behind your back, and people who would deceive you.

Properties of Peridot

Chemical Formula	(Mg,Fe)2SiO4
Colour	Green, Yellow
Hardness	6.5 - 7
Crystal System	Orthorhombic
Refractive Index	2.63 - 2.65
SG	1.54 - 1.55
Transparency	Transparent
Double Refraction	.009
Luster	Vitreous
Cleavage	2,1 ; 3,1
Mineral Class	Olivine