Mineral: Difference between revisions

A mineral is a naturally occurring

.

There are currently more than 4,000 known minerals, according to the International Mineralogical Association (IMA), which is responsible for the approval of and naming of new mineral species found in nature. Of these, perhaps 100 can be called "common", 50 are "occasional", and the rest are "rare" to "extremely rare".

Mineral definition and classification

To be classified as a true mineral, a substance must be a

structure. It must also be a naturally occurring, homogeneous substance with a defined chemical composition.

The International Mineralogical Association approved the following definition in 1995:

"A mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes."^[2]

According to this definition and classification scheme, biogenic materials were excluded from the mineral kingdom:

"Biogenic substances are chemical compounds produced entirely by biological processes without a geological component (e.g., urinary calculi, oxalate crystals in plant tissues, shells of marine molluscs, etc.) and are not regarded as minerals. However, if geological processes were involved in the genesis of the compound, then the product can be accepted as a mineral."^[2]: 690

However, other researchers do not adhere to this exclusion rule. Lowenstam (1981), for example, states the following:^[3]

"Organisms are capable of forming a diverse array of minerals, some of which cannot be formed inorganically in the biosphere."^: 1126

The distinction is a matter of classification and less to do with the constituents of the minerals themselves. Skinner (2005) views all solids as potential minerals and includes biominerals in the mineral kingdom, which are those that are created by the metabolic activities of organisms. Inclusion of these biogenic minerals requires an expanded definition of a mineral as:

"An element or compound, amorphous or crystalline, formed through biogeochemical processes."^[4]: 621

Mineral classification schemes and their definitions are evolving to match recent advances in mineral science. More recent classifications, for example, include an organic class – in both the new Dana and the

^: 1073

Recent advances in high-resolution genetic and x-ray absorption spectroscopy is opening new revelations on the biogeochemical relations between microrganisms and minerals that may make Nickel's (1995)[2] biogenic mineral exclusion obsolete and Skinner's (2005) biogenic mineral inclusion a necessity.^[4] For example, the IMA commissioned 'Environmental Mineralogy and Geochemistry Working Group'^[9] deals with minerals in the hydrosphere, atmosphere, and biosphere. Mineral forming microorganisms inhabit the areas that this working group deals with. These organisms exist on nearly every rock, soil, and particle surface spanning the globe reaching depths at 1600 meters below the sea floor (possibly further) and 70 kilometers into the stratosphere (possibly entering the mesosphere).^[10][11][12] Biologists and geologists have recently started to research and appreciate the magnitude of mineral geoengineering that these creatures are capable of. Bacteria have contributed to the formation of minerals for billions of years and critically define the biogeochemical cycles on this planet. Microorganisms can precipitate metals from solution contributing to the formation of ore deposits in addition to their ability to catalyze mineral dissolution, to respire, precipitate, and form minerals.^[13][14][15]

Prior to the International Mineralogical Association's listing, over 60 biominerals had been discovered, named, and published.^[16] These minerals (a sub-set tabulated in Lowenstam (1981)^[3]) are considered minerals proper according to the Skinner (2005) definition.^[4] These biominerals are not listed in the International Mineral Association official list of mineral names,^[17] however, many of these biomineral representatives are distributed amongst the 78 mineral classes listed in the `Dana' classification scheme.^[4] Another rare class of minerals (primarily biological in origin) include the mineral liquid crystals that are crystalline and liquid at the same time. To date over 80,000 liquid crystalline compounds have been identified.^[18][19]

Concerning the use of the term “mineral” to name this family of liquid crystals, one can argue that the term inorganic would be more appropriate. However, inorganic liquid crystals have long been used for organometallic liquid crystals. Therefore in order to avoid any confusion between these fairly chemically different families, and taking into account that a large number of these liquid crystals occur naturally in nature, we think that the use of the old fashioned but adequate “mineral” adjective taken sensus largo is more specific that an alternative such as “purely inorganic”, to name this subclass of the inorganic liquid crystals family.^[19]

The Skinner (2005) definition^[4] of a mineral takes this matter into account by stating that a mineral can be crystalline or amorphous. Liquid mineral crystals are amorphous. Biominerals and liquid mineral crystals, however, are not the primary form of minerals, most are geological in origin,^[20] but these groups do help to identify at the margins of what constitutes a mineral proper.

Mineral chemistry and nomenclature

The

.

Minerals with the same structure and forming

classification of minerals (silicates)

].

Some ion groups with a similar radius can occupy the same structural site in the crystal cell:

• O^2- and OH^- with 1.32 and 1.33
  Å
  respectively.
• Si⁴⁺ and Al³⁺ with 0.42 and 0.51 Å respectively, the charge is neutralized through an exchange of the other cations:
  • Si⁴⁺ – (Al³⁺ and Na⁺) or (Si⁴⁺ and Na⁺) – (Al³⁺ and Ca²⁺).[21]
• Larger molecules may have an unoccupied structural site by occupying another unoccupied structural site or by using a divalent cation instead of two monovalent cations, for instance (amphibole family).
• By the end of the 18th century, the minerals were getting chemical formulas. There were some difficulties, as elements were being discovered and isolated for the first time. The minerals:
  gadolinite-(Y) (first publication: 1802, 09.AJ.20), aeschynite-(Ce) (first publication: 1830, 04.DF.05), vanadinite (first publication: 1838, 08.BN.05), aenigmatite (first publication: 1865, 09.DH.40), labyrinthite (IMA 2002-065, 09.CO.10), illustrate these difficulties.^[22]

More recent definitions:

• "A mineral group consists of two or more minerals with the same (isotypic) or essentially the same (homeotypic) structure, and composed of chemically similar elements" (IMA-CNMNC).
• "two structures are considered homeotypic if all essential features of topology are preserved between them" (IUCr).^[23]

Differences between minerals and rocks

A mineral is a naturally occurring solid with a definite chemical composition and a specific crystalline structure. A

Over half of the mineral species known are so rare that they have only been found in a handful of samples, and many are known from only one or two small grains.

Commercially valuable minerals and rocks are referred to as

).

Mineral composition of rocks

A main determining factor in the formation of minerals in a rock mass is the chemical composition of the mass, for a certain mineral can be formed only when the necessary elements are present in the rock. Calcite is most common in

silica

.

Other factors are of equal importance in determining the natural association or paragenesis of rock-forming minerals, principally the mode of origin of the rock and the stages through which it has passed in attaining its present condition. Two rock masses may have very much the same bulk composition and yet consist of entirely different assemblages of minerals. The tendency is always for those compounds to be formed which are stable under the conditions under which the rock mass originated. A granite arises by the consolidation of a molten magma at high temperatures and great pressures and its component minerals are those stable under such conditions. Exposed to moisture, carbonic acid and other subaerial agents at the ordinary temperatures of the Earth's surface, some of these original minerals, such as quartz and white mica are relatively stable and remain unaffected; others weather or decay and are replaced by new combinations. The feldspar passes into kaolinite, muscovite and quartz, and any mafic minerals such as pyroxenes, amphiboles or biotite have been present they are often altered to chlorite, epidote, rutile and other substances. These changes are accompanied by disintegration, and the rock falls into a loose, incoherent, earthy mass which may be regarded as a sand or soil. The materials thus formed may be washed away and deposited as sandstone or siltstone. The structure of the original rock is now replaced by a new one; the mineralogical constitution is profoundly altered; but the bulk chemical composition may not be very different. The sedimentary rock may again undergo metamorphism. If penetrated by igneous rocks it may be recrystallized or, if subjected to enormous pressures with heat and movement during mountain building, it may be converted into a gneiss not very different in mineralogical composition though radically different in structure to the granite which was its original state.^[24]

Physical properties of minerals

Classifying minerals can range from simple to very difficult. A mineral can be identified by several physical properties, some of them being sufficient for full identification without equivocation. In other cases, minerals can only be classified by more complex optical, chemical or X-ray diffraction analysis; these methods, however, can be costly and time-consuming.

Commonly used physical properties include crystal structure and habit, hardness, lustre, diapheniety, colour, streak, cleavage and fracture, and specific gravity. Other less general tests include

Crystal structure and habit

A mineral may show good crystal habit or form, or it may be massive, granular or compact with only microscopically visible crystals.

A

diffraction. Chemistry and crystal structure together define a mineral. In fact, two or more minerals may have the same chemical composition, but differ in crystal structure (these are known as polymorphs). For example, pyrite and marcasite are both iron sulfide, but their arrangement of atoms differs. Similarly, some minerals have different chemical compositions, but the same crystal structure: for example, halite (made from sodium and chlorine), galena (made from lead and sulfur) and periclase (made from magnesium and oxygen) all share the same cubic crystal structure.

Crystal structure greatly influences a mineral's physical properties. For example, though diamond and graphite have the same composition (both are pure carbon), graphite is very soft, while diamond is the hardest of all known minerals. This happens because the carbon atoms in graphite are arranged into sheets which can slide easily past each other, while the carbon atoms in diamond form a strong, interlocking three-dimensional network.

Hardness

The hardness of a mineral defines how much it can resist scratching. This physical property is controlled by the chemical composition and crystalline structure of a mineral. A mineral's hardness is not necessarily constant for all sides, which is a function of its structure; crystallographic weakness render some directions softer than others.^[25] An example of this property exists inf kyanite, which has a Mohs hardness of 5½ parallel to [001] but 7 parallel to [100].^[26]

The most common scale of measurement is the ordinal Mohs hardness scale. Defined by 10 indicators, a mineral with a higher index will scratch those below it. The scale ranges from talc, a

Mohs hardness	Mineral	Chemical formula	Image
1	Talc	Mg3Si4O10(OH)2
2	Gypsum	CaSO4·2H2O
3	Calcite	CaCO3
4	Fluorite	CaF2
5	Apatite	Ca5(PO4)3(OH,Cl,F)
6	Orthoclase Feldspar	KAlSi3O8
7	Quartz	SiO2
8	Topaz	Al2SiO4(OH,F)2
9	Corundum	Al2O3
10	Diamond	C

Lustre

Lustre indicates how light reflects from the mineral's surface, with regards to its quality and intensity. There are numerous qualitative terms used to describe this property:^[27]

• Metallic – high reflectivity like metal: galena and pyrite
• Sub-metallic – slightly less than metallic reflectivity: magnetite
• Non-metallic lusters:
  • Adamantine – brilliant, the luster of diamond also cerussite and anglesite
  • Vitreous – the luster of a broken glass: quartz
  • Pearly – iridescent and pearl-like: talc and apophyllite
  • Resinous – the luster of resin: sphalerite and sulfur
  • Silky – a soft light shown by fibrous materials: gypsum and chrysotile
  • Dull/earthy – shown by finely crystallized minerals: the kidney ore variety of hematite

Diaphaneity

describes how well light passes through a mineral; there are three basic degrees of transparency:

• Transparent objects can be seen through a transparent mineral, such as a clear quartz crystal
• Translucent light passes through the mineral but no objects can be seen
• Opaque no light passes through the mineral

Many minerals range from transparent to translucent or translucent to opaque. Calcite, for instance, can be translucent or opaque. Some minerals that are naturally translucent become opaque with weathering.

Colour and streak

Colour indicates the appearance of the mineral in reflected light or transmitted light for translucent minerals (i.e. what it looks like to the naked eye).

• Iridescence – the play of colors due to surface or internal interference. Labradorite exhibits internal iridescence whereas hematite and sphalerite often show the surface effect.

Streak refers to the color of the powder a mineral leaves after rubbing it on an unglazed porcelain streak plate. Note that this is not always the same color as the original mineral.

Cleavage and fracture

Cleavage describes the way a mineral may split apart along various planes. In thin sections, cleavage is visible as thin parallel lines across a mineral. Fracture describes how a mineral breaks when broken contrary to its natural cleavage planes.

• Chonchoidal fracture is a smooth curved fracture with concentric ridges of the type shown by glass.
• Hackley is jagged fracture with sharp edges.
• Fibrous
• Irregular

Specific gravity

Specific gravity relates the mineral mass to the mass of an equal volume of water, namely the density of the material. While most minerals, including all the common rock-forming minerals, have a specific gravity of 2.5–3.5, a few are noticeably more or less dense, e.g. several sulfide minerals have high specific gravity compared to the common rock-forming minerals.

Classification of minerals

There are two major systems of mineral classification: the

. The following list provides to the classes of minerals according to the Strunz classification.

Elements

The elemental group includes

).

Sulfides

Many

(sulfur and a second anion such as arsenic).

Halides

The

minerals.

Oxides and hydroxides

minerals.

Carbonates and nitrates

The

.

Borates

This section needs expansion. You can help by adding to it. (August 2012)

Sulfates

minerals.

Phosphates

The phosphate mineral group actually includes any mineral with a tetrahedral unit AO₄ where A can be phosphorus, antimony, arsenic or vanadium. By far the most common phosphate is apatite which is an important biological mineral found in teeth and bones of many animals. The phosphate class includes the phosphate, arsenate, vanadate, and antimonate minerals.

Silicates

The largest group of minerals by far are the

.

Organic compounds

The organic mineral class includes

Examples include whewellite, moolooite, mellite, fichtelite, carpathite, evenkite and abelsonite.

See also

Bibliography

• Busbey, A.B.; Coenraads, R.E.; Roots, D.; Willis, P. (2007). Rocks and Fossils. San Francisco: Fog City Press.
  ISBN 978-1-74089_632-0. {{cite book}}: Check |isbn= value: invalid character (help); Cite has empty unknown parameter: |1= (help
  )
• Dyar, M.D.; Gunter, M.E. (2008). Mineralogy and Optical Mineralogy. Chantilly, Virginia:
  ISBN 978-0-939950-81-2
  .

References

External links

Wikimedia Commons has media related to Minerals.

• Mindat mineralogical database, largest mineral database on the Internet
• "Mineralogy Database" by David Barthelmy (2009)
• "Vibrational Spectroscopy and Photo Atlas of Minerals", Mineral atlas with properties, photos, etc.
• Ontogeny of minerals in drawings. Drawings of crystals, druses, and mineral aggregates, showing genetic features indicative of their history, ontogenesis, and formative processes
• "Mineral Identification Key II" Mineralogical Society of America
• "The Mineral and Gemstone Kingdom" interactive reference guide to minerals and gemstones
• "American Mineralogist Crystal Structure Database"

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