Saturday, 10 December 2016

Diopside

What is Diopside?

Diopside is a pyroxene mineral with a chemical composition of MgCaSi2O6. It occurs in igneous and metamorphic rocks at many locations around the world. Gem-quality crystals of diopside are faceted into attractive gemstones that are occasionally seen in commercial jewellery. Granular diopside can be easily cut and polished. When it has an attractive colour, it is sometimes used as an ornamental stone. Perhaps the most important use of diopside is its value as an indicator mineral in the search for diamonds. Trail-to-lode prospecting using diopside and other indicator minerals has found diamond deposits in Canada, the United States, Africa, and other locations. Diopside has potential uses in the glass and ceramics industries, but the mineral usually occurs in accumulations that are too small or impure for effective mining.

Geologic Occurrence of Diopside

The most common occurrence of diopside at Earth's surface is as a primary mineral in olivine-rich basalts and andesites. In these rocks it can be present in quantities of a few weight percent. Diopside also forms during contact metamorphism of limestones and dolomites. Most of the crystalline diopside used to cut faceted gems and the granular diopside used as ornamental stone occurs in these carbonate deposits. Diopside is much more abundant in Earth's mantle than at the surface. Evidence for this is diopside as a common mineral in ophiolites, and diopside as a common mineral in kimberlites and peridotites that were formed during deep-source volcanic eruptions.
A very nice specimen of Diopside from Minas Gerais Brazil
with a distinct crystal surface formation.

Diopside as a Diamond Indicator Mineral

Most diamonds found at or near Earth's surface were delivered from the mantle during deep-source volcanic eruptions. These diamonds occur in vertical igneous structures known as pipes, which are often composed of kimberlite or peridotite.
These pipes are difficult to locate. Their surface exposure is usually covered with soil and vegetation, and it might be only a few acres in size. The pipes are often found by searching soils and sediments for mineral grains that are characteristic of the pipe but absent in local surface materials. Small particles of chromium-rich diopside are bright green in colour, are often abundant in the pipes, and are easy to recognise in surface materials.
Geologists use these green diopside fragments to locate the pipes. They know that the fragments are liberated as the pipe weathers, then scattered by the actions of mass wasting, streams, and glaciers. When diopside fragments are discovered, the geologist knows that they originated up-slope, up-stream, or up-ice from the location in which they were found.
A trail of diopside fragments can lead the geologist to the pipe from which they were weathered. This activity, known as "trail-to-lode" prospecting, finds many diamond pipes and an even larger number of pipes without diamonds.
Note: It would be almost impossible to locate pipes by looking for diamonds. Diamonds make up such a small fraction of the overall rock in the pipe, and weathering debris from the pipe is then mixed into local rock debris. An exceptional pipe might contain a couple carats of diamond per ton!

Chrome Diopside

Some crystals of diopside contain enough chromium to give them a rich green colour. These can be cut into beautiful faceted stones, beads, and cabochons. The appearance of these stones is best when they are under two carats because the material is often dark or strongly saturated.
Chrome diopside is occasionally seen in commercial jewellery. It has a rich green colour that enables it to serve as an alternative gem for emerald at a significantly lower price. Diopside is rarely treated, unlike emerald which is often treated with various materials to seal and hide fractures.
One problem with chrome diopside is its durability. It has two directions of perfect cleavage and a Mohs hardness of only 5.5 to 6.5. This gives it a risk of being scratched or broken. The gem is best used in earrings, necklaces, brooches, and other items that will not be subjected to abrasion or impact.
Even though chrome diopside is very attractive, there are barriers to it becoming a popular gem that is widely seen in jewellery. First are the durability concerns described above; second is that the jewellery-buying public is not familiar with diopside; and, third is the fact that a reliable supply of commercial stones in calibrated sizes has not been developed.

Star Diopside

Some diopside crystals are filled with microscopic needle-shaped inclusions that occur in parallel alignment with the crystal structure of the mineral. This network of parallel inclusions is known as a "silk." When this diopside is cut en cabochon, the parallel needles of the silk can reflect light much like how light is reflected from a spool of silk thread.
A silk with one direction of needle alignment will produce chatoyance, also known as a cat's eye. Silk with two or three directions of needle alignment will produce asterism. Two directions produces a four-ray star, and three directions produces a six-ray star. For these phenomena to appear, the stone must be cut with the needles oriented parallel to the bottom of the stone, and the top of the cabochon must be symmetrically cut.
The mineral needles that form the silk are known in some instances to be magnetite. They are sometimes abundant enough to make the cut gems slightly magnetic. If you approach them slowly with a magnet, the gems will move before the magnet touches them. The needles in some non-magnetic gems are thought to be rutile or ilmenite.

Violane

Some diopside formed during the contact metamorphism of dolomite or limestone has a granular texture similar to marble. This material is known as "violane." It is often white, gray, light blue, lilac, or purple in colour. Violane accepts a bright polish and is sometimes used to make cabochons, beads, and ornamental items. Violane is a rare material in nature and almost never seen in commerce.

Diopside as an Industrial Mineral

Diopside has potential uses in ceramics, glass-making, biomaterials, nuclear waste immobilisation, and fuel cell technology. Unfortunately, natural diopside is rarely found in deposits that simultaneously have a size, purity, and location that allows economic mining. This makes synthetic diopside cost-competitive with diopside produced by mining.

Geographic Distribution of Diopside

Gem-quality chrome diopside and violane are mined in limited amounts in Siberia, Russia. Most of the chrome diopside used in jewellery today comes from a few locations in Siberia. Small occurrences of chrome diopside are also known in Austria, Brazil, Burma, Canada (Ontario and Quebec), Finland, India, Italy, Madagascar, Pakistan, South Africa, Sri Lanka, and the United States (New York), but none of them produce regularly or in significant quantities.

Properties of Diopside

Chemical FormulaCaMgSi2O6
CompositionCalcium magnesium silicate, often with some iron
Variable FormulaCa(Mg,Fe)Si2O6
ColourLight to dark green, gray, yellow, light blue, purple, and white. May also be green with thin white streaks running though a crystal. Rarely colorless or multicoloured.
StreakWhite to light green
Hardness5 - 6
Crystal SystemMonoclinic

Crystal Forms 
and Aggregates
Usually as single, short prismatic crystals. Crystals may also be somewhat elongated, and usually have good terminations. Also massive, grainy, columnar, bladed, radiating, fibrous, as cleavage fragments, and in disordered aggregates of elongated crystals. May also be in v-shaped penetration twins, and crystals from certain localities have partially hollow or dissolved etchings.
TransparencyTransparent to opaque
Specific Gravity3.3 - 3.6
LusterVitreous, dull
Cleavage1,2 - prismatic at cleavage angles of 87º and 93º (Characteristic of minerals in the pyroxene group). May also exhibit parting in one direction.
FractureUneven to splintery
TenacityBrittle
Other ID MarksWhite forms of Diopside are occasionally fluorescent.
In GroupSilicates; Inosilicates; Pyroxene Group
Striking FeaturesColour, cleavage angles, and localities
EnvironmentContact and regional metamorphic rocks in hornfels, and in skarn deposits of hydrothermal metamorphic rock.
Rock TypeMetamorphic
Popularity (1-4)2
Prevalence (1-3)2
Demand (1-3)1