Siderite is a carbonate mineral (FeCO₃). It is not rare but nowhere as common carbonate as calcite (CaCO₃) and dolomite (CaMg(CO₃)₂). Typically to carbonates, siderite also forms rhombohedral crystals. It shares the same crystal structure with calcite, but there is no solid solution between them because Ca and Fe ions are significantly different in size. There is, however, solid solution between siderite (FeCO₃), rhodochrosite (MnCO₃), and magnesite (MgCO₃). Note that similar replacements occur in other mineral groups as well. Garnet is a good example (almandine (Fe), spessartine (Mn), and pyrope (Mg)).

http://picasaweb.google.com/107509377372007544953/Beil#5749877231061090498
Concretionary siderite in a clay-ironstone from Farmsen, Germany. Width of sample 9 cm. TUG 1608-1779

Siderite is some shade of brown or brownish yellow. It is more dense (3.96) than other common carbonates because of iron content.

It occurs as an earthy mass or as concretionary nodules (clay ironstone) in iron-rich sedimentary rocks. Crystals form in hydrothermal veins and it may be found in banded iron formations from the Precambrian. Large masses of this mineral may form in hydrothermally altered limestone. It is also afraid of acids just as other carbonates, but it does not react as vigorously as calcite. It is commonly altered to iron oxides (hematite) and hydroxides (goethite). Metamorphism may oxidize it to magnetite.

Siderite is used as an iron ore in few places. It may also be used as a brown pigment.

Diamond is one of the most well known minerals although it is very rare. Perhaps this is the reason it is so well known? However, there are hundreds of exceedingly rare minerals that are virtually unknown, even for mineralogists. There has to be more good reasons to explain the fame of diamond.

Diamond is known for its extreme properties which mostly originate from the strong covalent bonding between the atoms forming the crystal lattice of diamond. These are all carbon atoms (almost), each one of them forming covalent bonds with four other carbon atoms. Hence, the chemical formula of diamond is very simple — C.

The extreme hardness is the most important reason diamond is important and extensively used industrial mineral. Most industrially used diamonds are artificial. The hardness of diamond seems to be universally known property but much less known is the fact that diamond also has very high thermal conductivity (highest of any material).

Another important property of diamond is its high optical dispersion. It means that light that penetrates diamond is split into components with different wavelengths. This is what gives “fire” to diamond (flashes of differently colored light).

Diamond is usually almost pure but 58 other elements have been found. These are mostly impurities outside the crystal lattice. Only two chemical elements, nitrogen and boron, can replace carbon in the lattice.

Natural diamonds form in the mantle at depths of approximately 150…200 km. Temperature range there in 900…1200 °C. It can not form in the crust because there are not enough pressure. Rare exceptions are impact events which create very high pressure for a very short time and may therefore result in the formation of many microscopic diamond crystals. The formation conditions of diamond are very different from the atmospheric conditions to which they are exposed at the surface. It should normally result in low resistance to weathering but diamond tolerates weathering very well because of its strong covalent bonds.

Diamonds are mostly found in ultramafic rocks named kimberlites and lamproites. It is usually kimberlite that hosts diamonds but the most productive diamond mine is Argyle in Australia where about 7,000 kg of diamonds a year is produced. These diamonds are extracted from lamproites. Kimberlite is a volcanic breccia that fills diatremes (pipe-like volcanic vents) that are formed by gas-charged explosions which transport kimberlitic material (and diamonds within it) from great depths up to the surface.

Most important diamond producing countries are South Africa (plus more countries in Southern Africa), Australia, Russia, Canada, Brazil, India, China, etc. Diamond-bearing rocks have been found on every continent except Antarctica. There is no reason to think that Antarctica does not have diamonds but we simply can not look below the thick ice shield and it would not make much sense economically because mining in Antarctica is currently not allowed anyway.

Diamond occurs as a detrital mineral but its concentration in sand is always very low. However, diamond-bearing kimberlite pipes are often found by working through many fluvial sand samples but it is not diamond that prospectors are looking for. These are mostly dark red Mg-rich garnet pyrope and bright green chromian diopside which occur in kimberlite in much higher concentration than diamond and they are also visually easier to spot in sand samples.

Celestine is a strontium sulfate mineral (SrSO₄). It is similar to another and more common sulfate mineral barite (BaSO₄). Their crystals may be very similar and there is a continuous solid solution between the two. Celestine is the principal source of strontium. The other major strontium-bearing mineral is strontianite (SrCO₃) which belongs to the carbonate group, but it isn’t usually exploited as a source of strontium.

http://picasaweb.google.com/107509377372007544953/Rocks#5805070870103057058
The druse from Yates in England. Strontium was released during the dolomitization of originally aragonitic sediments. It was released because dolomite can hold only 200…600 ppm of strontium while aragonite can hold up to 8000¹. Width of sample 10 cm.

Celestine is mostly found in cavities in dolomites and dolomitic limestone. Tabular crystals are found in hydrothermal veins. It also occurs in evaporite deposits.

Is there anything celestial about celestine as the name seems to suggest? Yes, its crystals may be bluish as the sky because of impurities but this color is actually not very typical. Perhaps the first crystals described were bluish but overall this name is a misnomer. It is usually colorless or white.

Strontium extracted from celestine is used in pyrotechnics. Beautiful bright red colors in the sky of New Years fireworks are caused by strontium compounds. Because of that, the term “celestine” may not be such a misnomer as it first seems to be.

http://picasaweb.google.com/107509377372007544953/2015#6221848100687513618
Celestine in a cavity in carbonate rocks in Cyprus. Width of view 25 cm.

References

1. Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.

Microcline is a very common rock-forming mineral. It belongs to the feldspar group. It is one of three K-feldspars (K means Potassium). The other two are orthoclase and sanidine. The chemical formula of K-feldspars: KAlSi₃O₈, but they usually contain sodium as well which replaces potassium.

http://picasaweb.google.com/107509377372007544953/Rocks#5851061963885050450
Microcline with perthitic exsolution. White veins are composed of albite. Width of sample 43 mm.

It is often very difficult to distinguish between the three, especially between orthoclase and microcline. Sanidine is not usually colored while microcline and orthoclase are often salmon red or yellow. Sanidine occurs in volcanic rocks, the other two occur more frequently in deep-seated rocks, especially microcline which is absent in volcanic rocks. These three form like a series with their typical occurrences — microcline in deep intrusions, orthoclase in shallower intrusions but also in volcanics, and sanidine in volcanic rocks.

This mineral is often perthitic. It means that there are narrow bands of albite (sodium-rich feldspar) in the K-feldspar host crystal. Perthitic texture is often visible to the naked eye. This texture is the result of exsolution which happened after the cooling of originally homogenous crystals which are not stable at lower temperatures. Perthitic feldspar is almost always microcline.

Some varieties (amazonite) are green or bluish green. The color of amazonite is caused by a small amounts of lead. Amazonite is a rare variety when compared to the total volume of all microcline in the crust, but it is used as a decorative stone and therefore pretty well known.

This mineral is a common constituent of granite and granitic pegmatites. It also occurs in other granitoids, syenite, and pelitic metamorphic rocks. It is a common mineral in detrital sediments. Its grains are commonly more fuzzy than generally clear quartz crystals and they tend to have rectangular blocky shape. This is the result of good cleavage in several directions. The cleavage planes are at right angles to each other.

http://picasaweb.google.com/107509377372007544953/2015#6190952781767212722
Perthitic exsolution lamellae of albite in microcline. Evje, Norway. Width of sample 6 cm.

Topaz is a silicate mineral. It belongs to the orthosilicate group. These are silicates that have isolated silica tetrahedra (isolated from each other like islands in a three-dimensional body). It contains volatile components (F^– and OH^–) in addition to common chemical elements of most silicate minerals. The chemical formula is Al₂SiO₄(F,OH)₂.

http://picasaweb.google.com/107509377372007544953/Coll#5850338932127576674
Crystals from Ouro Preto, Brazil. Width of view 30 mm.

Volatiles suggest that topaz must form in an environment that contains lots of them. Such an environment forms when granitic magma is almost completely crystallized. Residual fluids are rich in incompatible and volatile elements which favors the formation of minerals containing them. Such minerals which are frequently associated with topaz are cassiterite (contains tin which isn’t compatible in main magmatic minerals), tourmaline (contains boron and volatiles), apatite (volatiles), lepidolite (lithium), fluorite (fluorine), beryl (beryllium), etc. The rocks that host these sanctuaries for unpopular chemical elements are known as pegmatites. It may also form in cavities of rhyolite (volcanic equivalent of granite) and in greisen (granitic rock intruded by residual magmatic fluids). Common magmatic phases like quartz, feldspar, and mica are also usually present in topaz-bearing rocks.

Topaz is pretty famous mineral because it is extensively used as a gemstone. It could find use as an abrasive (its hardness in the Mohs scale is 8, it is one of the ten defining minerals of the scale) but there are enough much cheaper synthetic alternatives.

Topaz forms elongated prismatic crystals which may be striated (striations parallel to the elongation). It has one good cleavage perpendicular to the striations. It often determines the shape of the crystals which seem to be cut at right angles to the elongation of crystals. It is resistant to the weathering processes and may be therefore found in sand.

Molybdenite (MoS₂) is a principal source of molybdenum. It is a sulfide mineral (molybdenum sulfide). Molybdenite occurs in hydrothermal vein deposits and as disseminated grains in rocks which have been attacked by hydrothermal fluids (porphyry moly and copper deposits). It may also occur in pegmatites and skarns. Associated minerals are quartz, pyrite, chalcopyrite, fluorite, anhydrite, etc.

http://picasaweb.google.com/107509377372007544953/Coll#5851080512221163778
Metallic gray mineral with calcite (blocky gray), pyrite (greenish crystals in the upper part), and serpentinite (green in the background). Phillippsburg, New Yersey, USA. Width of sample 8 cm. TUG 1608-3216.

It has a metallic luster and steel gray color. It resembles graphite, both are very soft minerals and therefore have a greasy feel to the touch, but its crystals have a clear bluish tone. Molybdenite has a slightly greenish streak on an unglazed porcelain while graphite is black. It has one perfect cleavage and the folia are inelastic.

Molybdenite is the principal ore of molybdenum, which is mostly used to make high-strength steel. Molybdenum is also a micronutrient, so it is added to fertilizers and also to the dietary supplement pills. High doses of it are toxic.

http://picasaweb.google.com/107509377372007544953/Coll#5851080506572285922
With quartz from Altenberg, The Ore Mountains, Germany. Width of sample 8 cm. TUG 1608-6160.

Augite is a very common rock-forming mineral that gives black color to many igneous rocks. This mineral belongs to a group of silicate minerals known as pyroxenes. By the way, ‘pyroxene’ comes from the Greek language and means that these minerals have nothing to do with fire (i.e. igneous processes). This is, of course, especially unfortunate misnomer but the name has stuck and nowadays very few people associate it with its original meaning.

http://picasaweb.google.com/107509377372007544953/Rocks#5851064555789504994
It forms solid solution series with hedenbergite and diopside. There is no solid solution between augite and pigeonite. “Wollastonite” is written in italics because it isn’t a pyroxene. It is still included because of compositional considerations. Not all pyroxenes can be shown on this diagram. The same diagram is available in PDF version also.

Augite is the most common mineral of this group. It is usually black because of significant iron content, but small crystals (in sand, for example) tend to be dark green. It is actually a member of a group of closely related minerals. There are continuous solid solutions between augite, diopside, and hedenbergite. The latter two occur mostly in metamorphic or ultramafic rocks, while augite is usual component of mafic igneous rocks. The chemical formula is usually expressed the following way: (Ca,Mg,Fe²⁺,Fe³⁺,Al)₂(Si,Al)₂O₆ Augite contains less calcium but more aluminum, magnesium and iron than diopside-hedenbergite pair. These three minerals together are usually called calcic clinopyroxenes or Ca-rich clinopyroxenes. Clino- means that they are monoclinic (some pyroxenes are orthorhombic, they are called orthopyroxenes). These terms refer to the crystal systems which are defined based on the combinations of the elements of symmetry.

Augite occurs mostly in mafic igneous rocks (gabbro, basalt, andesite, pyroxenite, peridotite, etc.). It may also occur in metamorphic rocks (skarn, amphibolite, granulite) but in this case its composition is often fairly close to diopside. Its crystals are prismatic but not as slender as the crystals of another pyroxene aegirine which also is connected with augite chemically but doesn’t fit into the diagram below along with several other members of the pyroxene group.

Wollastonite (CaSiO₃) is a silicate mineral that occurs in metamorphosed carbonate rocks. This mineral belongs to a group of chain silicates named pyroxenoids. These are minerals that are similar to pyroxenes but their crystal chains are distorted (not straight). Compared to pyroxenes, these minerals are rare.

http://picasaweb.google.com/107509377372007544953/Rocks#5805071095321014514
Wollastonite with diopside (green) from a mine in Willsboro, New York State, USA. Width of rock 8 cm. TUG 1608-1654.

There are only three common minerals among pyroxenoids (wollastonite, rhodonite, and pectolite) but even these are found in few rock types that are not very voluminous. Wollastonite as a most important of these minerals occurs only in a handful of places where it is abundant enough to make mining worthwhile. One of them is Willsboro in New York State (two photos below).

It forms when limestone reacts with silicate fluids:

CaCO₃ (calcite) + SiO₂ (quartz) → CaSiO₃ (wollastonite) + CO₂ (carbon dioxide)

Wollastonite may form when impure (contains silica) limestone (or dolostone) gets buried deep enough for the necessary metamorphic reactions to take place (regional metamorphism) or when magmatic fluids intrude the limestone body (metasomatism which produces skarns). In either case many other minerals may form as well. Diopside, calcite, dolomite, tremolite, andradite, grossular, plagioclase, epidote, vesuvianite, etc. may be associated with wollastonite. It may be very rarely found in some igneous rocks.

Pure mineral is white. It is usually relatively pure and therefore white but gray and light green colors are common also. It is typically fibrous, columnar, or bladed. It may be very similar to tremolite (amphibole group mineral). Unfortunately, these two love to occur together which complicates the identification process. Tremolite is light green (but usually darker) in color and forms columnar or acicular crystals.

http://picasaweb.google.com/107509377372007544953/Rocks#5851066645843439458
Calc-silicate minerals diopside (green), andradite (brown), and wollastonite (white) in a skarn from a mine in Willsboro, New York State, USA. Width of view 5 cm. TUG 1608-4877.
http://picasaweb.google.com/107509377372007544953/Rocks#5851066640282650834
A sample from Lahore, Pakistan. Width of specimen 8 cm. TUG 1608-1675.
http://picasaweb.google.com/107509377372007544953/Rocks#5851066647397721138
Wollastonite, tremolite (green), and actinolite (black) from Bastnäs, Sweden. Width of specimen 9 cm.

Graphite is a mineral with a very simple composition — C (carbon).

Everyone is familiar with this mineral because pencil “leads” are made of it. It’s named “lead” because centuries ago people thought that this mineral is a form of lead. Real lead have never been used to make pencils. Early pencils were indeed made of a stick of pure graphite but such mineral deposits are very scarce. The best source at the beginning of the 19th century was in England. These deposits were unavailable to the French because of naval blockade during the Napoleonic Wars. So they had to start thinking and very soon one clever man discovered that you can use impure graphite powder (which is quite plentiful) to make pencil leads if you mix it with various amounts of clay and burn the rods in a kiln. The hardness of the lead depends on the amount of clay (more clay adds hardness).

This mineral occurs mostly in metamorphic rocks. It is basically metamorphosed organic matter. Most of the graphite occurs in slates, schists, carbonate rocks, and metamorphosed coal beds (anthracite). Sometimes it occurs in skarn and rarely in igneous rocks.

Graphite has a layered structure. Very strong separate layers of carbon atoms are held together by weak chemical bonds which means that graphite as a mineral is very soft (its hardness on Mohs scale is 1…2). That allows it to be used in pencil leads — we can scrape off layer after layer by only applying a moderate amount of force when writing on a paper. The softness gives a characteristic greasy feel to the mineral which somewhat resembles talc (softest mineral in the Mohs scale). It also conducts electricity which opens up many more industrial applications.

Graphite may be turned into diamond (which is also pure carbon) but it takes huge pressure to achieve that. Hence, natural diamonds form in the upper mantle and are very rarely brought up to the surface by violent gas-charged eruptions of kimberlite magma.

Further reading

Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.