Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

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)

ColourMetallic, Yellow, Gray
Hardness6 - 6.5
Crystal SystemIsometric
SG4.9 - 5.2
TransparencyOpaque
Double RefractionNone
LusterMetallic
CleavageNone
Mineral ClassPyrite

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 FormulaCa2Al2Si3O12(OH)
ColourGreen
Hardness6 - 6.5
Crystal SystemOrthorhombic
Refractive Index1.61 - 1.64
SG2.8 - 3.0
TransparencyTranslucent
Double Refraction.030
LusterVitreous, waxy
Cleavage1,1;3,1
Mineral ClassPrehnite

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 FormulaPt
ColourMetallic, White
Hardness4 - 4.5
Crystal SystemIsometric
SG14 - 19
TransparencyOpaque
Double RefractionNone
LusterMetallic
CleavageNone
Mineral ClassPlatinum

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
ColourGreen, Yellow
Hardness6.5 - 7
Crystal SystemOrthorhombic
Refractive Index2.63 - 2.65
SG1.54 - 1.55
TransparencyTransparent
Double Refraction.009
LusterVitreous
Cleavage2,1 ; 3,1
Mineral ClassOlivine

Banded-iron formations (BIFs) - Evidence of Oxygen in Early Atmosphere

Our knowledge about the rise of oxygen gas in Earth’s atmosphere comes from multiple lines of evidence in the rock record, including the age and distribution of banded iron formations, the presence of microfossils in oceanic rocks, and the isotopes of sulfur.
However, this article is just focus on Banded Iron Formation.

BIF (polished) from Hamersley Iron Formation, West Australia, Australia

Summary: Banded-iron formations (BIFs) are sedimentary mineral deposits consisting of alternating beds of iron-rich minerals (mostly hematite) and silica-rich layers (chert or quartz) formed about 3.0 to 1.8 billion years ago. Theory suggests BIFs are associated with the capture of oxygen released by photosynthetic processes by iron dissolved in ancient ocean water. Once nearly all the free iron was consumed in seawater, oxygen could gradually accumulate in the atmosphere, allowing an ozone layer to form. BIF deposits are extensive in many locations, occurring as deposits, hundreds to thousands of feet thick. During Precambrian time, BIF deposits probably extensively covered large parts of the global ocean basins. The BIFs we see today are only remnants of what were probably every extensive deposits. BIFs are the major source of the world's iron ore and are found preserved on all major continental shield regions. 

Banded-iron formation (BIF)
is 
consists of layers of iron oxides (typically either magnetite or hematite) separated by layers of chert (silica-rich sedimentary rock). Each layer is usually narrow (millimeters to few centimeters). The rock has a distinctively banded appearance because of differently colored lighter silica- and darker iron-rich layers. In some cases BIFs may contain siderite (carbonate iron-bearing mineral) or pyrite (sulfide) in place of iron oxides and instead of chert the rock may contain carbonaceous (rich in organic matter) shale.

It is a chemogenic sedimentary rock (material is believed to be chemically precipitated on the seafloor). Because of old age BIFs generally have been metamorphosed to a various degrees (especially older types), but the rock has largely retained its original appearance because its constituent minerals are fairly stable at higher temperatures and pressures. These rocks can be described as metasedimentary chemogenic rocks.



                     Jaspilite banded iron formation (Soudan Iron-Formation, Soudan, Minnesota, USA
Image Credits: James St. John



In the 1960s, Preston Cloud, a geology professor at the University of California, Santa Barbara, became interested in a particular kind of rock known as a Banded Iron Formation (or BIF). They provide an important source of iron for making automobiles, and provide evidence for the lack of oxygen gas on the early Earth.

Cloud realized that the widespread occurrence of BIFs meant that
the conditions needed to form them must have been common on the ancient Earth, and not common after 1.8 billion years ago. Shale and chert often form in ocean environments today, where sediments and silica-shelled microorganisms accumulate gradually on the seafloor and eventually turn into rock. But iron is less common in younger oceanic sedimentary rocks. This is partly because there are only a few sources of iron available to the ocean: isolated volcanic vents in the deep ocean and material weathered from continental rocks and carried to sea by rivers.


Banded iron-formation (10 cm), Northern Cape, South Africa.
Specimen and photograph: A. Fraser
Most importantly, it is difficult to transport iron very far from these sources today because when iron reacts with oxygen gas, it becomes insoluble (it cannot be dissolved in water) and forms a solidparticle. Cloud understood that for large deposits of iron to exist all over the world’s oceans, the iron must have existed in a dissolved form. This way, it could be transported long distances in seawater from its sources to the locations where BIFs formed. This would be possible only if there were little or no oxygen gas in the atmosphere and ocean at the time the BIFs were being deposited. Cloud recognized that since BIFs could not form in the presence of oxygen, the end of BIF deposition probably marked the first occurrence of abundant oxygen gas on Earth (Cloud, 1968).
Cloud further reasoned that, for dissolved iron to finally precipitate and be deposited, the iron would have had to react with small amounts of oxygen near the deposits. Small amounts of oxygen could have been produced by the first photosynthetic bacteria living in the open ocean. When the dissolved iron encountered the oxygen produced by the photosynthesizing bacteria, the iron would have precipitated out of seawater in the form of minerals that make up the iron-rich layers of BIFs: hematite (Fe2O3) and magnetite (Fe3O4), according to the following reactions:
4Fe3 + 2O2 → 2Fe2O3
6Fe2 + 4O2 → 2Fe3O4
The picture that emerged from Cloud’s studies of BIFs was that small amounts of oxygen gas, produced by photosynthesis, allowed BIFs to begin forming more than 3 billion years ago. The abrupt disappearance of BIFs around 1.8 billion years ago probably marked the time when oxygen gas became too abundant to allow dissolved iron to be transported in the oceans.
Banded Iron Formation
Source is unknown

It is interesting to note that BIFs reappeared briefly in a few places around 700 millionyears ago,during a period of extreme glaciation when evidence suggests that Earth’s oceans were entirely covered with sea ice. This would have essentially prevented the oceans from interacting with the atmosphere, limiting the supply of oxygen gas in the water and again allowing dissolved iron to be transported throughout the oceans. When the sea ice melted, the presence of oxygen would have again allowed the iron to precipitate.

References:

1. Misra, K. (1999). Understanding Mineral Deposits Springer.
2. 
Cloud, P. E. (1968). Atmospheric and hydrospheric evolution on the primitive Earth both secular accretion and biological and geochemical processes have affected Earth’s volatile envelope. Science, 160(3829), 729–736.
3. 
James,H.L. (1983). Distribution of banded iron-formation in space and time. Developments in Precambrian Geology, 6, 471–490.