Mohs scale of mineral hardness

The Mohs scale of mineral hardness is a qualitative ordinal scale characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. Created in 1812 by German geologist and mineralogist Friedrich Mohs, it is one of several definitions of hardness in materials science, some of which are more quantitative.[1] The method of comparing hardness by observing which minerals can scratch others is of great antiquity, having been mentioned by Theophrastus in his treatise On Stones, c. 300 BC, followed by Pliny the Elder in his Naturalis Historia, c. 77 AD.[2][3][4] While greatly facilitating the identification of minerals in the field, the Mohs scale does not show how well hard materials perform in an industrial setting.[5]

Mohssche-haerteskala hg
Mohs hardness kit, containing one specimen of each mineral on the ten-point hardness scale

Usage

Despite its lack of precision, the Mohs scale is relevant for field geologists, who use the scale to roughly identify minerals using scratch kits. The Mohs scale hardness of minerals can be commonly found in reference sheets.

Mohs hardness is useful in milling. It allows assessment of which kind of mill will best reduce a given product whose hardness is known.[6] The scale is used at electronic manufacturers for testing the resilience of flat panel display components (such as cover glass for LCDs or encapsulation for OLEDs).

Minerals

The Mohs scale of mineral hardness is based on the ability of one natural sample of mineral to scratch another mineral visibly. The samples of matter used by Mohs are all different minerals. Minerals are chemically pure solids found in nature. Rocks are made up of one or more minerals. As the hardest known naturally occurring substance when the scale was designed, diamonds are at the top of the scale. The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, or the softest material that can scratch the given material. For example, if some material is scratched by apatite but not by fluorite, its hardness on the Mohs scale would fall between 4 and 5.[7] "Scratching" a material for the purposes of the Mohs scale means creating non-elastic dislocations visible to the naked eye. Frequently, materials that are lower on the Mohs scale can create microscopic, non-elastic dislocations on materials that have a higher Mohs number. While these microscopic dislocations are permanent and sometimes detrimental to the harder material's structural integrity, they are not considered "scratches" for the determination of a Mohs scale number.[8]

The Mohs scale is a purely ordinal scale. For example, corundum (9) is twice as hard as topaz (8), but diamond (10) is four times as hard as corundum. The table below shows the comparison with the absolute hardness measured by a sclerometer, with pictorial examples.[9][10]

Mohs hardness Mineral Chemical formula Absolute hardness[11] Image
1 Talc Mg3Si4O10(OH)2 1 Talc block
2 Gypsum CaSO4·2H2O 3 Gypse Arignac
3 Calcite CaCO3 9 Calcite-sample2
4 Fluorite CaF2 21 Fluorite with Iron Pyrite
5 Apatite Ca5(PO4)3(OH,Cl,F) 48 Apatite Canada
6 Orthoclase feldspar KAlSi3O8 72 OrthoclaseBresil
7 Quartz SiO2 100 Quartz Brésil
8 Topaz Al2SiO4(OH,F)2 200 Topaz cut
9 Corundum Al2O3 400 Cut Ruby
10 Diamond C 1500 Rough diamond

On the Mohs scale, a streak plate (unglazed porcelain) has a hardness of approximately 7.0. Using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale.[1]

Intermediate hardness

The table below incorporates additional substances that may fall between levels:[12]

Hardness Substance or mineral
0.2–0.3 caesium, rubidium
0.5–0.6 lithium, sodium, potassium
1 talc
1.5 gallium, strontium, indium, tin, barium, thallium, lead, graphite, ice[13]
2 hexagonal boron nitride,[14] calcium, selenium, cadmium, sulfur, tellurium, bismuth, gypsum
2–2.5 halite (rock salt), fingernail[15]
2.5–3 gold, silver, aluminium, zinc, lanthanum, cerium, Jet (lignite)
3 calcite, copper, arsenic, antimony, thorium, dentin
3.5 platinum
4 fluorite, iron, nickel
4–4.5 steel
5 apatite (tooth enamel), zirconium, palladium, obsidian (volcanic glass)
5.5 beryllium, molybdenum, hafnium, glass, cobalt
6 orthoclase, titanium, manganese, germanium, niobium, rhodium, uranium
6–7 fused quartz, iron pyrite, silicon, ruthenium, iridium, tantalum, opal, peridot, tanzanite, jade
7 osmium, quartz, rhenium, vanadium
7.5–8 emerald, hardened steel, tungsten, spinel
8 topaz, cubic zirconia
8.5 chrysoberyl, chromium, silicon nitride, tantalum carbide
9 corundum (includes sapphire and ruby), tungsten carbide, titanium nitride
9–9.5 silicon carbide (carborundum), titanium carbide
9.5–10 boron, boron nitride, rhenium diboride (a-axis),[16] stishovite, titanium diboride
10 diamond, carbonado

Comparison with Vickers scale

Comparison between hardness (Mohs) and hardness (Vickers):[17]

Mineral
name
Hardness (Mohs) Hardness (Vickers)
kg/mm2
Graphite 1–2 VHN10=7–11
Tin VHN10=7–9
Bismuth 2–2½ VHN100=16–18
Gold VHN10=30–34
Silver VHN100=61–65
Chalcocite 2½–3 VHN100=84–87
Copper 2½–3 VHN100=77–99
Galena VHN100=79–104
Sphalerite 3½–4 VHN100=208–224
Heazlewoodite 4 VHN100=230–254
Carrollite 4½–5½ VHN100=507–586
Goethite 5–5½ VHN100=667
Hematite 5–6 VHN100=1,000–1,100
Chromite VHN100=1,278–1,456
Anatase 5½–6 VHN100=616–698
Rutile 6–6½ VHN100=894–974
Pyrite 6–6½ VHN100=1,505–1,520
Bowieite 7 VHN100=858–1,288
Euclase VHN100=1,310
Chromium VHN100=1,875–2,000

See also

References

  1. ^ a b "Mohs hardness" in Encyclopædia Britannica Online
  2. ^ Theophrastus on Stones. Farlang.com. Retrieved on 2011-12-10.
  3. ^ Pliny the Elder. Naturalis Historia. Book 37. Chap. 15. ADamas: six varieties of it. Two remedies.
  4. ^ Pliny the Elder. Naturalis Historia. Book 37. Chap. 76. The methods of testing precious stones.
  5. ^ Hardness. Non-Destructive Testing Resource Center
  6. ^ "Size reduction, comminution - grinding and milling". PowderProcess.net. Retrieved 27 October 2017.
  7. ^ American Federation of Mineralogical Societies. "Mohs Scale of Mineral Hardness". amfed.org
  8. ^ Geels, Kay. "The True Microstructure of Materials", pp. 5–13 in Materialographic Preparation from Sorby to the Present. Struers A/S, Copenhagen, Denmark
  9. ^ Amethyst Galleries' Mineral Gallery What is important about hardness?. galleries.com
  10. ^ Mineral Hardness and Hardness Scales Archived 2008-10-17 at the Wayback Machine. Inland Lapidary
  11. ^ Mukherjee, Swapna (2012). Applied Mineralogy: Applications in Industry and Environment. Springer Science & Business Media. pp. 373–. ISBN 978-94-007-1162-4.
  12. ^ Samsonov, G.V., ed. (1968). "Mechanical Properties of the Elements". Handbook of the Physicochemical Properties of the Elements. New York: IFI-Plenum. p. 432. doi:10.1007/978-1-4684-6066-7. ISBN 978-1-4684-6068-1.
  13. ^ "Ice is a mineral" in Exploring Ice in the Solar System. messenger-education.org
  14. ^ Berger, Lev I. (1996). Semiconductor Materials (First ed.). Boca Raton, FL: CRC Press. p. 126. ISBN 978-0849389122.
  15. ^ https://geology.com/minerals/mohs-hardness-scale.shtml
  16. ^ Levine, Jonathan B.; Tolbert, Sarah H.; Kaner, Richard B. (2009). "Advancements in the Search for Superhard Ultra-Incompressible Metal Borides" (PDF). Advanced Functional Materials. pp. 3526–3527. doi:10.1002/adfm.200901257.
  17. ^ Ralph, Jolyon. "Welcome to mindat.org". mindat.org. Hudson Institute of Mineralogy. Retrieved April 16, 2017.

Further reading

  • Mohs hardness of elements is taken from G.V. Samsonov (Ed.) in Handbook of the physicochemical properties of the elements, IFI-Plenum, New York, USA, 1968.
  • Cordua, William S. "The Hardness of Minerals and Rocks". Lapidary Digest, c. 1990.
Calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as "calcite".

Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite over timescales of days or less at temperatures exceeding 300 °C, and vaterite is even less stable.

Ceramic knife

A ceramic knife is a knife designed with a ceramic blade typically made from zirconium dioxide (ZrO2; also known as zirconia). These knife blades are usually produced through the dry-pressing and firing of powdered zirconia using solid-state sintering. It is 8.5 on the Mohs scale of mineral hardness, compared to 4.5 for normal steel and 7.5 to 8 for hardened steel and 10 for diamond. The resultant blade has a hard edge that stays sharper for longer when compared to conventional steel knives. While the edge is harder than steel knives, it is less tough than steel knives; no surprise that ceramic blades are more brittle than steel blades. The ceramic blade is sharpened by grinding the edges with a diamond-dust-coated grinding wheel.

Dentin

Dentin () (American English) or dentine ( or ) (British English) (Latin: substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by enamel on the crown and cementum on the root and surrounds the entire pulp. By volume, 45% of dentin consists of the mineral hydroxylapatite, 33% is organic material, and 22% is water. Yellow in appearance, it greatly affects the color of a tooth due to the translucency of enamel. Dentin, which is less mineralized and less brittle than enamel, is nesessary for the support of enamel. Dentin rates approximately 3 on the Mohs scale of mineral hardness.

There are two main characteristics which distinguish dentin from enamel: firstly, dentin forms throughout life; secondly, dentin is sensitive.Dentinal sclerosis/transparent dentin-sclerosis of primary dentin is regressive alteration in tooth characterized by calcification of dentinal tubules. It can occur as a result of injury to dentin by caries or abrasion, or as part of the normal aging process.

Faustite

The IMA approved mineral faustite, named after the American mineralogist and petrologist with the U.S. Geological Survey (USGS) Dr. George Tobias Faust, is a member of the triclinic turquoise group of hydrous phosphates with the following chemical composition:

ZnAl6(PO4)4(OH)8·4H2O

Some divalent copper generally replaces the zinc position. Faustite is the zinc rich analogue of turquoise having almost four times as much zinc than copper in its crystal structure. Trivalent (ferric) iron may replace some of the aluminum. Minor amounts of calcium may also be present. It has a hardness of 4.5 - 5.5 on the Mohs scale of mineral hardness and aside from having a slightly lower hardness, it may be difficult to distinguish it from turquoise in hand specimens.

Faustite has a blue-green to apple green color in polished cabochons, and may be presented as a turquoise imitation, and it may also be treated with stabilizers for jewelry making.

Friedrich Mohs

Carl Friedrich Christian Mohs (German: [moːs]; 29 January 1773 – 29 September 1839) was a German geologist and mineralogist. He was the creator of the Mohs scale of mineral hardness. Mohs also introduced a classification of the crystal forms in crystal systems independently of Christian Samuel Weiss.

Gypsum

Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard chalk and wallboard. A massive fine-grained white or lightly tinted variety of gypsum, called alabaster, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire and the Nottingham alabasters of Medieval England. Gypsum also crystallizes as beautiful translucent crystals of selenite (mineral). It also forms as an evaporite mineral and as a hydration product of anhydrite.

The Mohs scale of mineral hardness defines hardness value 2 as gypsum based on scratch hardness comparison.

Hardstone

Hardstone is an unscientific term, mostly encountered in the decorative arts or archaeology, that has a similar meaning to semi-precious stones, or gemstones. Very hard building stones, such as granite, are not included in the term in this sense; only stones which are fairly hard, and regarded as attractive - in effect ones which could be used in jewellery. Hardstone carving is the three-dimensional carving for artistic purposes of semi-precious stones such as jade, agate, onyx, rock crystal, sard or carnelian, and a general term for an object made in this way. Two-dimensional inlay techniques for floors, furniture and walls include pietre dure, opus sectile (Ancient Roman), and medieval Cosmatesque work - these typically inlay hardstone pieces into a background of marble or some other building stone.

The definition of "hardstone" is not very rigid, but excludes "soft" stones such as soapstone (steatite) and minerals such as alabaster, both widely used for carving. Hard organic minerals such as amber and jet are included, as well as the mineraloid obsidian. Geologically speaking, most of the gemstones carved in the West are varieties of quartz, including: chalcedony, agate, amethyst, sard, onyx, carnelion, heliotrope, jasper and quartz in its uncoloured form, known as rock crystal. On the Mohs scale of mineral hardness, quartz rates no higher than a 7, less for types with impurities. Stones typically used for buildings and large sculpture are not often used for small objects such as vessels, although this does occur.

The term is derived as a literal translation of the Italian plural pietre dure= "hard stones", which in Italian covers all hardstone carving. The semi-anglicized singular pietra dura is used in English for multi-coloured stone inlay work, in fact using marbles as much as semi-precious stones, which was a Florentine speciality from the Renaissance onwards. "Pietre dure" is sometimes used (normally italicized) for Italian or European vessels and small sculptures of the same period.

Knorringite

Knorringite is a mineral species belonging to the garnet group, and forms a series with the species pyrope. It was discovered in 1968 in the Kao kimberlite pipe in the Butha-Buthe District of Lesotho and is named after Oleg Von Knorring, a professor of mineralogy at the University of Leeds in England.Synthetic knorringite has the pure endmember formula Mg3Cr2(SiO4)3. As knorringite is a member of the knorringite - pyrope series, natural samples contain variable aluminium in the chromium site. Knorringite is a greenish blue color with a Mohs scale of mineral hardness of six to seven.

It occurs as a rare component within ultramafic nodules in kimberlites in association with olivine, enstatite, chrome diopside, chromian pyrope, chromian spinel, ilmenite, perovskite, zircon, diamond, omphacite, rutile, carbonates and micas. It has been reported

from the Red Ledge mine in Nevada County, California in addition to the type location in Lesotho.Knorringite is a tracer mineral in the search for diamonds in kimberlite pipes.

Lapis Lacedaemonius

Lapis Lacedaemonius (Latin: stone from Lacedaemon), also known as Spartan basalt, is a form of andesite or volcanic rock known today only from a single source in the village of Krokees on the Peloponnese in Greece. In addition, ancient sources mention a quarry of lapis Lacedaemonius in Taygetus. The stone has a dark green colour, speckled with elements shifting from yellow to light green. Occasionally, the speckles have crystallised in a way that creates rosette-like patterns. It is rated as having a hardness of six or higher on the Mohs scale of mineral hardness. It appears in comparatively small blocks. It is known in Italian as porfido verde antico and in German as Krokeischer Stein.

The stone is known to have been processed by Neanderthals. In slightly more recent times, lapis Lacedaemonius has been used for making Minoan sealstones and vases, both Minoan and Mycenean. It also was used as an element in decorative elements created with opus sectile technique (with examples known from Ostia Antica and Nemi) during Roman times, as well as in several churches in Rome (e.g. Santa Prassede) and in the pavement of St. Peter's Square. Its use is mentioned by Pausanias. During the Middle Ages it was employed in mosaics in e.g. Palermo, Constantinople, and Santiago de Compostela. Furthermore, Roman and Byzantine decorative stones frequently were reused up until the eighteenth century. For example, in St. Mark's Basilica, Venice and also as far away from the Mediterranean Sea as in Westminster Abbey, London.The stone from Krokees comes from a lava dome approximately 230 million years old. Its light spots are plagioclase, which over time has turned into albite and epidote. In places, iron minerals have oxidized into hematite, giving it a reddish tone. Geologically similar rock, equally exploited during antiquity, may be found on Samothrace.

List of physics mnemonics

This is a categorized list of physics mnemonics.

Mohs

Mohs or MoHS can refer to:

Friedrich Mohs, a 19th-century German geologist who developed:

Mohs scale of mineral hardness, a scale used in materials science to describe hardness

Frederic E. Mohs, an American doctor who developed:

Mohs surgery, a microscopically controlled surgery highly effective for common types of skin cancer

Erik Mohs, a German professional racing cyclist

Mohs Automobile, an automobile built by the American Mohs Seaplane Corporation

Moanalua High School, a public, co-educational college preparatory high school in Hawaiʻi

The Melancholy of Haruhi Suzumiya, a 2006 anime sci-fi television series

Mount Olive High School, a U.S. public high school in Flanders, New Jersey

Monazite

Monazite is a reddish-brown phosphate mineral containing rare-earth metals. It occurs usually in small isolated crystals. It has a hardness of 5.0 to 5.5 on the Mohs scale of mineral hardness and is relatively dense, about 4.6 to 5.7 g/cm3. There are at least four different kinds of monazite, depending on relative elemental composition of the mineral:

monazite-(Ce), (Ce, La, Nd, Th)PO4 (the most common member),

monazite-(La), (La, Ce, Nd)PO4,

monazite-(Nd), (Nd, La, Ce)PO4,

monazite-(Sm), (Sm, Gd, Ce, Th)PO4.The elements in parentheses are listed in the order of their relative proportion within the mineral: lanthanum is the most common rare-earth element in monazite-(La), and so forth. Silica (SiO2) is present in trace amounts, as well as small amounts of uranium and thorium. Due to the alpha decay of thorium and uranium, monazite contains a significant amount of helium, which can be extracted by heating.Monazite is an important ore for thorium, lanthanum, and cerium. It is often found in placer deposits. India, Madagascar, and South Africa have large deposits of monazite sands. The deposits in India are particularly rich in monazite.

Monazite is radioactive due to the presence of thorium and, less commonly, uranium. Because of its radioactive nature, monazite is used for monazite geochronology to study geological events, such as crystallization, heating, or deformation of the rocks containing monazite.

The name monazite comes from the Greek μονάζειν (to be solitary), via German Monazit, in allusion to its isolated crystals.

Porphyry (geology)

Porphyry is a textural term for an igneous rock consisting of large-grained crystals such as feldspar or quartz dispersed in a fine-grained silicate rich, generally aphanitic matrix or groundmass. The larger crystals are called phenocrysts. In its non-geologic, traditional use, the term porphyry refers to the purple-red form of this stone, valued for its appearance.

The term porphyry is from Ancient Greek (πορφύρα porphúra) and means "purple". Purple was the color of royalty, and the "imperial porphyry" was a deep purple igneous rock with large crystals of plagioclase. Some authors claimed the rock was the hardest known in antiquity. "Imperial" grade porphyry was thus prized for monuments and building projects in Imperial Rome and later. Porphyry typically has hardness 7 on the Mohs scale of mineral hardness, corresponding to steel and quartz.

Subsequently, the name was given to any igneous rocks with large crystals. The adjective porphyritic now refers to a certain texture of igneous rock regardless of its chemical and mineralogical composition. Its chief characteristic is a large difference in size between the tiny matrix crystals and the much larger phenocrysts. Porphyries may be aphanites or phanerites, that is, the groundmass may have invisibly small crystals as in basalt, or crystals easily distinguishable with the eye, as in granite. Most types of igneous rocks display some degree of porphyritic texture.

Recycled diamond

A recycled diamond is a diamond, which had a prior use and has re-entered the diamond supply chain.

Diamonds in recent times have become good candidates for recycling, due to a number of reasons including their hardness and the gemstone being a finite and valuable resource. Diamonds are the hardest known naturally occurring substance and rate a 10 on the Mohs scale of mineral hardness.

It has been speculated that the value of the market could be as high as $1 trillion US dollars.

Seraphinite

Seraphinite is a trade name for a particular form of clinochlore, a member of the chlorite group.

Seraphinite apparently acquired its name due to its resemblance to feathers due to its chatoyancy. Seraphinite is named after the biblical seraphs or seraphim angels. With some specimens the resemblance is quite strong, with shorter down-like feathery growths leading into longer "flight feathers"; the resemblance even spurs fanciful marketing phrases like "silver plume seraphinite." Seraphinite is generally dark green to gray in color, has chatoyancy, and has hardness between 2 and 4 on the Mohs scale of mineral hardness.

Seraphinite is mined in a limited area of eastern Siberia in Russia. Russian mineralogist Nikolay Koksharov (1818-1892 or 1893) is often credited with its discovery. It occurs in the Korshunovskoye iron skarn deposit in the Irkutskaya Oblast of Eastern Siberia.

Silicon boride

Silicon borides (also known as boron silicides) are lightweight ceramic compounds formed between silicon and boron. Several stoichiometric silicon boride compounds, SiBn, have been reported: silicon triboride, SiB3, silicon tetraboride, SiB4, silicon hexaboride, SiB6, as well as SiBn (n = 14, 15, 40, etc.). The n = 3 and n = 6 phases were reported as being co-produced together as a mixture for the first time by Henri Moissan and Alfred Stock in 1900 by briefly heating silicon and boron in a clay vessel. The tetraboride was first reported as being synthesized directly from the elements in 1960 by three independent groups: Carl Cline and Donald Sands; Ervin Colton; and Cyrill Brosset and Bengt Magnusson. It has been proposed that the triboride is a silicon-rich version of the tetraboride. Hence, the stoichiometry of either compound could be expressed as SiB4 - x where x = 0 or 1. All the silicon borides are black, crystalline materials of similar density: 2.52 and 2.47 g cm−3, respectively, for the n = 3(4) and 6 compounds. On the Mohs scale of mineral hardness, SiB4 - x and SiB6 are intermediate between diamond (10) and ruby (9). The silicon borides may be grown from boron-saturated silicon in either the solid or liquid state.

The SiB6 crystal structure contains interconnected icosahedra (polyhedra with 20 faces), icosihexahedra (polyhedra with 26 faces), as well as isolated silicon and boron atoms. Due to the size mismatch between the silicon and boron atoms, silicon can be substituted for boron in the B12 icosahedra up to a limiting stoichiometry corresponding to SiB2.89. The structure of the tetraboride SiB4 is isomorphous to that of boron carbide (B4C), B6P, and B6O. It is metastable with respect to the hexaboride. Nevertheless, it can be prepared due to the relative ease of crystal nucleation and growth.Both SiB4 - x and SiB6 become superficially oxidized when heated in air or oxygen and each is attacked by boiling sulfuric acid and by fluorine, chlorine, and bromine at high temperatures. The silicon borides are electrically conducting. The hexaboride has a low coefficient of thermal expansion and a high nuclear cross section for thermal neutrons.

The tetraboride was used in the black coating of some of the space shuttle heat shield tiles.

Streak (mineralogy)

The streak of a mineral is the color of the powder produced when it is dragged across an un-weathered surface. Unlike the apparent color of a mineral, which for most minerals can vary considerably, the trail of finely ground powder generally has a more consistent characteristic color, and is thus an important diagnostic tool in mineral identification. If no streak seems to be made, the mineral's streak is said to be white or colorless. Streak is particularly important as a diagnostic for opaque and colored materials. It is less useful for silicate minerals, most of which have a white streak or are too hard to powder easily.

The apparent color of a mineral can vary widely because of trace impurities or a disturbed macroscopic crystal structure. Small amounts of an impurity that strongly absorbs a particular wavelength can radically change the wavelengths of light that are reflected by the specimen, and thus change the apparent color. However, when the specimen is dragged to produce a streak, it is broken into randomly oriented microscopic crystals, and small impurities do not greatly affect the absorption of light.

The surface across which the mineral is dragged is called a "streak plate", and is generally made of unglazed porcelain tile. In the absence of a streak plate, the unglazed underside of a porcelain bowl or vase or the back of a glazed tile will work. Sometimes a streak is more easily or accurately described by comparing it with the "streak" made by another streak plate.

Because the trail left behind results from the mineral being crushed into powder, a streak can only be made of minerals softer than the streak plate, around 7 on the Mohs scale of mineral hardness. In case of harder minerals, the color of the powder can be determined by filing or crushing with a hammer a small sample, which is then usually rubbed on a streak plate. Most minerals that are harder have an unhelpful white streak.

Some minerals leave a streak similar to their natural color, such as cinnabar and lazurite. Other minerals leave surprising colors, such as fluorite, which always has a white streak, although it can appear in purple, blue, yellow, or green crystals. Hematite, which is black in appearance, leaves a red streak which accounts for its name, which comes from the Greek word "haima", meaning "blood." Galena, which can be similar in appearance to hematite, is easily distinguished by its gray streak.

Talc

Talc is a clay mineral composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Talc in powdered form, often in combination with corn starch, is a widely used substance known as baby powder. This mineral is used as a thickening agent and lubricant, is an ingredient in ceramics, paint and roofing material, and is also one of the main ingredients in many cosmetic products. It occurs as foliated to fibrous masses, and in an exceptionally rare crystal form. It has a perfect basal cleavage, uneven flat fracture and it is foliated with a two dimensional platy form.

The Mohs scale of mineral hardness is based on scratch hardness comparison, ranging from 1-10, a value of 10 being the hardest of minerals. Talc is the defining value 1 therefore the softest of minerals. Any mineral below a value of 2 on Mohs scale of mineral hardness can be scratched by a fingernail. When scraped on a streak plate it produces a white streak, this indicator is of little importance because most silicate minerals produce a white streak. Talc is translucent to opaque with colors ranging from whitish grey to green with a vitreous and pearly luster. Talc is not soluble in water, but is slightly soluble in dilute mineral acids. l.

Soapstone is a well known metamorphic rock composed predominantly of talc.

Åkermanite

Åkermanite (Ca2Mg[Si2O7]) is a melilite mineral of the sorosilicate group, containing calcium, magnesium, silicon, and oxygen. It is a product of contact metamorphism of siliceous limestones and dolostones, and rocks of sanidinite facies. Sanidinite facies represent the highest conditions of temperature of contact metamorphism and are characterized by the absence of hydrous minerals. It has a density of 2.944 g/cm3. Åkermanite ranks a 5 or 6 on the Mohs scale of mineral hardness, and can be found gray, green, brown, or colorless. It has a white streak and a vitreous or resinous luster. It has a tetragonal crystal system and a good, or distinct, cleavage. It is the end member in a solid solution series beginning with gehlenite (Ca2Al[AlSiO2]).The mineral is named for Anders Richard Åkerman (1837–1922), a Swedish metallurgist. It has been found at Monte Somma and Vesuvius, and Monte Cavalluccio near Rome. It was "grandfathered" in as a species of mineral because it was described prior to 1959, before the founding of the International Mineralogical Association.

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