carbon minerals

Carbon, C,

is present in the Earth’s crust with an average abundance of about 480 ppm. Its extraction is greater than any other element if we include the various rocks like limestone, dolomite and marble together with the many fossil fuels such as oil, gas and coal. World annual production of carbon in the form of graphite...

rockngem magazine issue 66
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is around 1.1 million tonnes with China, India, Brazil, North Korea and Canada with main suppliers. The chemistry of carbon is vast, particularly regarding organic compounds. The numbers of inorganic compounds is also very large. In industry no other element plays a role so diverse an array of useful solid, liquid and gaseous materials. Industrial uses are also immense of which in the form of coke for the iron and steel fabrication is the largest single use. It is essential to all food production, wood, paper, plants, hydrocarbons as fuels and compounds in the petrochemical industry, plastics, various alloys, abrasives, textiles, electronics, paints, pharmaceuticals, dyes, lubricants, ultra-strong and ultra soft materials, and in jewellery in the form of diamond. A great countless other uses and applications exist but space restricts us here. The main mineral groups in which carbon plays an essential role includes the native elements in its different polymorphs, carbides carbonates (the largest and most widespread group with over 300 members) and the varied organic minerals. It is very commonly present in many other groups as well.

Graphite, C,

is the hexagonal polymorph of carbon that forms platy crystals with triangular striations up about 20 cm in size. Twinned crystals also exist. Foliated, massive, scaly, columnar, granular, compact, earthy to globular aggregates are frequent. The cleavage is perfect and easy with a flexible tenacity and greasy feel. Hardness is low at 1-2. The specific gravity is 2.0. to 2.2. It is opaque and transparent only in very thin fragments. Its colour is iron-black to steel-grey with a black to steel-grey streak and glistening appearance. The lustre
is metallic, dull to earthy. The above properties and occurrence are sufficient for identification. Graphite is formed by the reduction of carbon compounds during metamorphism and occurs as a primary mineral in igneous rocks. It also is found in meteorites. It is associated with pyrite, calcite, quartz, marcasite, magnetite, phlogopite, sphalerite, albiite, arsenopyrite, diopside, molybdenite, willemite, ankerite, microcline, troilite, chlorite group, corundum, and aegirine etc. There are many locations known, but only a few supply well-crystallised specimens for collectors. Such occurrences are found in the USA, Canada, Russia, Sir Lanka, Germany, Finland, England, Czech Republic, Austria, France, Tanzania, Mexico, Australia, China and numerous other countries.

Diamond, C,

is the cubic polymorph of carbon that commonly forms octahedral, dodecahedral, tetrahedral and cubic crystals 6 cm or more in size. Contact and penetration twinning is abundant. Curved and striated faces are also common. Various polycrystalline, cryptocrystalline and granular aggregates exist. It is transparent, translucent to opaque with an adamantine to greasy lustre. It has a perfect cleavage, conchoidal fracture and is brittle. The hardness is the highest among minerals at 10 and is diagnostic. The specific gravity is 3.5. Colour is variable and is the only gem species that occurs naturally in all colour variations and intensities from colourless, white, grey, yellow, brown (the commonest colour), less frequently oranges, pinks, greens, blues, purples, violets, reds and black. The above properties like the supreme hardness, lustre, infusible, insoluble, crystal habit and its geological environment contribute to its identity. Diamond occurs primarily in pipes and dykes as xenocrysts of mantle origin within kimberlite, lamproite or lamprophyre, and in alluvial deposits derived from their weathering. It also is known from carbonaceous achondite and iron meteorites; and in some eclogite, peridotite schist and gneiss. This rare mineral is accompanied by fosterite, phlogopite, pyrope, diopside, imenite, amphibole group, rutile, augite, hematite, enstatite, magnetite, tourmaline group, zircon and many other minerals. Diamond is found in numerous deposits but only a few are of economic importance. Some of these include those in South Africa, Botswana, Democratic Republic of Congo Angola, Namibia, Russia, Australia, Canada, Sierra Leone, and Guinea etc. Small diamonds are usually to be found at dealer’s stands in the many Rock ’n’ Gem Shows across the UK. Diamond is famously cut into magnificent gems of many carets and commands the highest prices.

Lonsdaleite, C,

is a rarer hexagonal polymorph of carbon that forms small fine-grained aggregates consisting of microscopic crystals and as polycrystalline aggregates. It is brownish-yellow to grey in colour and is transparent with an adamantine lustre. The hardness is in the range between 7 and 8. The specific gravity is 3.5. The above cited properties and x-rays should be used for correct recognition. This mineral occurs in meteorites, kimberlites and in placers. It is associated with schreibersite, cohenite, taenite, graphite, chromite, kosmochlor, sphalerite, diamond and troilite. Lonsdaleite samples have been recorded from various meteorites that have fallen in Antarctica, Australia, China (in a kimberlite), Finland, India, Russia and the USA.

Moissanite, SiC,

is the commonest carbide and forms hexagonal platy crystals that are sometimes rounded. Syntactically intergrowths also are present. It is transparent with a metallic to adamantine lustre. Colour varies from green, emerald-green, blue-green, deep blue, blue-black, black, rarely pale green, yellow or colourless and may be colour zoned on occasions. It possesses a poor cleavage and a conchoidal fracture. The hardness is very high at 9.5 with a specific gravity of 3.1-3.2. The extreme hardness and other mentioned properties distinguish it from other mineral species. Moissanite is a characteristic mineral of diamondferous kimberlites and lamproities, in eclogites, volcanic breccias and rhyolites, in iron meteorites as inclusions in diamond. It is typically found with native iron, diamond, quartz, garnet group, clinopyroxenes, coesite, graphite, rutile, pyrite and pyrrhotite. Specimens have been derived from several scattered locations around the world and include those in China, Russia, USA and the Czech Republic. Other sites are single locations only.

Cohenite, (Fe,Ni,Co)3C,

has a tin-white
colour that oxidises to light bronze then to golden brown yellow with a metallic lustre. It is found as imperfect platy to needle-like orthorhombic crystals, and as eutectic dendritic intergrowths. The mineral is opaque with several cleavages and is very brittle. Its hardness is 5.5 to 6 and is strongly magnetic. The specific gravity is high and variable between 7.2 and 7.6. The chemistry, colour change and mode of occurrence help in its recognition. Cohenite is a mineral that is common in meteorites but is also present in terrestrial iron, mafic volcanic rocks reduced from the assimilation of graphite. Schreibersite, troilite and wustite occur with it. A number of sites are known for this rare species. Greenland, Antarctica, Germany, Poland, Australia, USA, Russia, South Africa, Brazil, Kazakhstan, Saudi Arabia, are some of those nations that have supplied samples.

Dolomite, CaMg(CO3)2,

occurs abundantly as a product of diagenesis or hydrothermal metasomatism of limestone, as a primary phase in hypersaline sediments, major component of contact metamorphic rocks, as the most important carbonate gangue mineral next to calcite in hydrothermal veins, in carbonatites and in some ultramafic rocks and rarely in pegmatites. This mineral is a major rock-forming mineral species in the rock type dolomite. It is associated with a great many species like calcite, fluorite, barite, siderite, quartz, pyrite, chalcopyrite, celestine, gypsum, talc, serpentine minerals, magnesite, magnetite, diopside, tremolite, forsterite, wollastonite, ankerite, fluorapatite and numerous others. The mineral crystallises in the trigonal system with rhombohedral or prismatic crystals terminated by rhombohedrons, and are repeatedly curved. Typically forms saddle-shaped crystal aggregates and is frequently twinned but less so than calcite. Columnar, stalactitic, granular and massive habits are common. It is transparent to translucent and ranges in colour from colourless, white, yellow, pink to brown with a white streak. The lustre is vitreous to pearly with a perfect rhombohedral cleavage and a parting. Dolomite is brittle and may fluoresce white to pink under UV light. The hardness is 3.5-4 and the specific gravity is 2.8. The above physical properties together with its slower solubility in cold dilute acids unless powdered first help distinguish this mineral from others. Various staining methods are also very useful here. It is very abundant and widespread worldwide with hundreds of localities for excellent crystallised specimens. Countries that have produced lovely specimens include: Spain, Austria, Germany, Switzerland, England, Russia, Brazil, Mexico, USA, Australia, Canada, China, Czech Republic, France and Italy.

Aragonite, CaCO3,

is a most appealing species that is a polymorph of calcium carbonate that comes in many different habits. It is orthorhombic forming pseudohexagonal crystals, which are short to long prismatic, acicular, dipyramidal and thick tabular with a variety of interesting terminations over 20 cm in length. Also frequently as columnar crystal aggregates, globular, reniform, pisolitic, coralloidal, stalactitic, fibrous and banded forms. Repeated contact twinning is very common in crystals along with trillings and recurring striations. It possesses both distinct and imperfect cleavages and is brittle. The hardness is 3.5 to 4 and the specific gravity is 2.9. It is fluorescent under UV light. Aragonite is transparent to translucent with a vitreous to resinous lustre. Its colour is quite variable from colourless, white, grey, and various shades of yellow, blue, violet, green, brown and red. The streak is white and dissolves in dilute HCl. The above physical and chemical properties contribute towards its recognition. The mineralogical environment for this mineral is varied from primary precipitates from sea waters, evaporitic deposits, in sinters, hot springs, dripstones, low temperature hydrothermal veins, characteristic component of high-pressure low temperature blueshists facies metamorphism, amygdules in basalts and as a secondary mineral in altered ultramaifc rocks. This mineral converts to calcite over geological time. It is usually occurs with gypsum, sulphur, celestine, calcite, dolomite, quartz, chalcopyrite, pyrite, pumpellyite, lawsonite, zeolite group, magnesite, brucite and numerous other species. Numerous sites are known but fine crystals are uncommon (being significantly less abundant than calcite). Some of the best nations for specimens are Spain, Italy, Slovakia, Poland, Austria, Germany, Czech Republic, England, France, Hungary, Morocco, Namibia, USA, Mexico, Australia and Canada etc.

Ankerite, Ca(Fe2+,Mg)(CO3)2,

is a brown, yellow to white mineral with a vitreous to pearly lustre. This species is transparent to translucent in character. It has a perfect cleavage with a subconchoidal fracture and is brittle. The hardness is 3.5-4 and the specific gravity is 2.9. It crystallises in the trigonal system with rhombohedral crystals with typically curved or saddle-shaped faces. Prismatic to tabular habits are also noted along with simple twins. Columnar, stalactitic, granular to massive forms also occur. The noted properties together with chemical and staining tests are useful.
Ankerite occurs in sedimentary rocks, low-grade metamorphic ironstones, banded iron formations, carbonatites, as a gangue mineral in hydrothermal sulphide vein deposits, some alpine fissures and a product of alteration in carbonate sediments etc.
This mineral is associated with siderite, dolomite, calcite, chaclopyrite, pyrite, galena, rutile, garnet group and cummingtonite. Noted countries for samples include USA, Czech Republic, Germany, Slovakia, Italy, Austria, England, Switzerland, Spain, Greece, Hungary, Norway, Russia, New Zealand, Canada, Australia, Bolivia, Brazil and China.
The organic class of minerals is a relatively small group with remarkable species with different and complex structures to add to your collection and study. Some general characteristics are given for interested readers. They tend to be fine-grained or poorly crystalline and colourless, although they can be dark due to mixtures with organic matter. Hardness and specific gravity tend to be low. Some of them occur as remains of biogenic activity in caves or as a result of diagenesis of lake sediments. Others are present in low-temperature hydrothermal deposits, coal seams, limestones, hot springs and an even a few in igneous rocks. Here we describe two of them.

Whewellite, Ca(C2O4)•H2O,

crystallises in the monoclinic system and occurs as equant to short prismatic crystals normally distorted with a variety of different faces over 23 cm in size. Twinned crystals are regularly found. Colour ranges from colourless, white with light shades of yellow and brown. It is transparent with a vitreous to pearly lustre. Several cleavages are present ranging in quality from very good, imperfect to indistinct together with a brittle character. This mineral has a conchoidal fracture and a hardness of 2.5-3. The specific gravity is 2.2. Above properties, chemistry and the mode of occurrence help in distinguishing it from similar species. Whewellite is an uncommon mineral of low-temperature, primary hydrothermal vein carbonate-sulphide deposits, geodes, septarian nodules, coal seams, created from oxidised organic matter and some uranium deposits. Calcite, barite, sphalerite, pyrite and weddellite are found with it. A scattered number of places are known for this species. The main sites are found in countries like Germany, Czech Republic, Romania, France, Russia, Hungary, USA and Australia.

Mellite, Al2[C6(COO)6]•16H2O,

is usually honey-yellow, various red shades, brown, grey, white or colourless in colour with a resinous to vitreous lustre. Its streak is white and is transparent to translucent. An indistinct cleavage is present with a conchoidal fracture and is slightly sectile. The hardness is low at 2-2.5 and may fluoresce pale yellow to blue under SW and LW UV light. The specific gravity is rather low at 1.6. This species crystallises in the tetragonal system forming dipyramidal, prismatic and elongated crystals with various faces. It also occurs as nodules, encrustations, vein-infillings and as fine-grained massive aggregates. The cited distinguishing features and chemical tested are required here. Mellite is an uncommon secondary mineral that occurs in brown coals and lignite deposits along with pyrite and various clay minerals. Suitable specimens have been acquired from the following locations in: Czech Republic, Hungary, Germany, Austria, France and Russia. Minerals containing the element sulphur will be studied next dolomite time.

 

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