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Geologists classify minerals based on various common characteristics depending on the sphere of geology in which the classification is used. This can include classification based on chemical composition (more specifically, based on the anion group), crystal lattice structure, or other physical properties.
In this article, we will discuss the most widespread classification used in mineralogy (the science of minerals), which is the classification based on chemical composition (anion group).
In this classification, the chemical composition of the mineral plays a key role. However, within this classification, there are typically groups based on the crystal lattice structure and other characteristics of minerals.
Why Do Geologists Classify Minerals Into Groups?
The classification of minerals is significant because it helps geologists with identification, organization, simplification, prediction, and effective communication within the field of mineralogy. It provides a systematic approach to studying and understanding the wide diversity of minerals found in nature.
First of all, the classification of minerals into groups allows for a systematic organization of the vast variety of minerals found in nature.
It helps geologists understand the relationships and similarities between different minerals and creates a framework for studying and cataloging them. This is really important because a large array of data is very difficult to use if it is not organized and divided into groups.
Secondly, classifying minerals into groups provides a common language and framework for geologists to communicate and share their findings.
It enables researchers to reference and discuss minerals in a standardized manner, facilitating collaboration and advancing scientific knowledge in the field of mineralogy.
Also, grouping minerals based on their properties and composition allows geologists to make predictions about the behavior and occurrence of minerals in different geological environments.
They can anticipate the presence or absence of certain minerals in specific rock types or geological settings based on their classification and known associations.
Of course, classification helps geologists identify minerals based on their properties and characteristics.
By grouping minerals with similar features together, geologists can use diagnostic properties such as color, hardness, crystal form, cleavage, and specific gravity to narrow down the possibilities and determine the mineral’s identity.
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Classification Based on Anion or Anionic Complex
Dividing minerals into classes based on anion or anionic complex is convenient because we can determine class from the chemical formula. Besides being convenient, however, this classification scheme makes sense in other ways.
Within each class, the type of structure and bonding are similar. This means that minerals within a class often have similar physical properties, making the classes useful in mineral identification. Such would not be the case if we divided minerals into groups based on cations.
For example, pyrite, and fayalite, both contain Fe but have few properties in common. Minerals within a single class are often found together.
There are currently around 4170 known mineral species. Among these minerals, about 50 are common rock-forming minerals. The common minerals of economic importance, forming the ores, are about 70 to 80.
Classically minerals are classified into 11 classes based on their anion or anionic complex:
- Silicate class
- Native Element Class
- Sulfide Class
- Halide Class
- Oxide Class
- Hydroxide Class
- Carbonate and Nitrate Class
- Borate Class
- Sulfate Class
- Tungstate, Molybdate, and Chromate Class
- Phosphate, Arsenate, and Vanadate Class
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Silicates
Silicates are the most abundant class of minerals. They constitute over 90% of the Earth’s crust. The fundamental structural unit of silicates is a tetrahedron (SiO4 ), where the center of the tetrahedron is occupied by a silicon atom and the 4 apexes by oxygen atoms.
Because there are so many important minerals in the silicate class, geologists divide them into subclasses according to the arrangement of these (SiO4 ) tetrahedra. Silicate subclasses are named according to how the tetrahedra are linked (polymerized) in the atomic structure. Here are the names of these subclasses:
- Isolated silicates (Nesosilicates): in this group, each individual silicate tetrahedron is isolated and does not share oxygen atoms with neighboring tetrahedra. Examples include garnets, olivines, and zircon.
- Paired tetrahedral silicates (Sorosilicates): have pairs of silicate tetrahedra that share oxygen atoms. The basic structural unit in this group is the double tetrahedra. Examples include epidote and vesuvianite.
- Ring silicates (Cyclosilicates): consist of rings of silicate tetrahedra. Each tetrahedron shares two oxygen atoms with adjacent tetrahedra, creating a cyclic structure. Well-known examples include beryl, tourmaline, and axinite.
- Chain silicates (Inosilicates): in this group, silicate tetrahedra form single or double chains by sharing oxygen atoms. Examples include pyroxenes, such as augite and diopside, as well as amphiboles like hornblende.
- Sheet silicates (Phyllosilicates): have tetrahedral sheets that share three oxygen atoms with adjacent sheets. The resulting structure forms layers. Examples are micas (e.g., muscovite and biotite) and clay minerals.
- Framework silicates (Tectosilicates): have a three-dimensional framework of silicate tetrahedra. Each tetrahedron shares all four oxygen atoms with adjacent tetrahedra. Quartz and feldspar minerals, including orthoclase and plagioclase, belong to this group.
The next table shows the examples of minerals in subclasses:
| Subclasses | Species |
|---|---|
| Isolated silicates | Forsterite, garnets, olivines, zircon. |
| Paired tetrahedral silicates | Akermanite, epidote, vesuvianite |
| Ring silicates | Tourmaline, beryl, axinite |
| 4. Chain silicates | Enstatite, tremolite, augite, diopside, hornblende |
| 5. Sheet silicates | Pyrophyllite, muscovite, biotite, talc, and clay minerals (e. g. kaolinite) |
| 6. Framework silicates | Quartz, albite, orthoclase, and plagioclase |
We further divide classes and subclasses of minerals into groups based on structural or chemical similarity.
For example, we group the feldspar minerals together because, although their compositions vary, they all have similar atomic structures. We group the serpentine minerals (antigorite, chrysotile, lizardite) together because they all have the same composition.
Groups may also be further divided into subgroups or series.
Subgroups are minerals that naturally group together for chemical or other reasons. The K-feldspars, for example, are a subgroup because they all have the composition KAlSi3O8.
Series involve minerals with compositions we can describe in terms of two end members. The plagioclase feldspar series consists of feldspars that are solutions of the two end members albite and anorthite.
Albite and anorthite form a solid solution series, and plagioclase can have any composition between pure albite and pure anorthite. Usually, atomic structure or composition is identical for all minerals in subgroups or series. They are always very similar.
Individual mineral species, which make up series and subgroups, are occasionally further divided into varieties if the varieties have some special properties. For example, rose quartz, smoky quartz, amethyst, and citrine are all varieties of quartz.
The scheme below presents how to connect subclasses with groups into “Silicate class”:
Silicate class
- Framework silicate subclass
- silica group
- feldspar group
- feldspathoid group
- scapolite series
- zeolite group
- other framework silicates
- Sheet silicate subclass
- serpentine group
- clay mineral group
- mica group
- chlorite group
- other sheet silicates
- Chain silicate subclass
- pyroxene group
- amphibole group
- pyroxenoid group
- Ring silicate subclass
- Isolated tetrahedral silicate subclass
- garnet group
- olivine group
- humite group
- aluminosilicate group
- other isolated tetrahedral silicates
- Paired tetrahedral silicate subclass
- lawsonite group
- epidote group
The table below shows a few of the most common minerals of various groups in the framework silicate subclass as an example:
| Group | Species |
|---|---|
| 1. Silica group | Quartz, cristobalite, tridymite, coesite. |
| 2. Feldspar group | Alkali feldspar: orthoclase, sanidine, microcline. Plagioclase feldspar series: albite, anorthite. |
| 3. Felspatoid group | Analcime, leucite, nepheline. |
| 4. Scapolite group | Marialite, meionite. |
| 5. Zeolite group | Natrolite, chabazite, heulandites, stilbite, sodalite. |
| 6. Other | Beryl, cordierite |
Native Element Class
The native element class refers to a group of minerals composed of a single chemical element. This class is divided into 3 subclasses. The scheme below shows these divisions into subclasses and examples of major minerals:
- Metals
- gold – Au
- silver – Ag
- platinum – Pt
- copper – Cu
- iron – Fe
- Semimetals
- arsenic – As
- bismuth – Bi
- antimony – Sb
- Nonmetals
- diamond – C
- graphite – C
- sulfur – S
Native Metals
Gold, silver, platinum, and copper are the most common of the native metals. Iron, zinc, nickel, lead, and indium have occasionally been reported from meteorites or altered igneous rocks.
All native metals have similar properties: metallic luster, high thermal and electrical conductivity, malleability, and opaqueness to visible light. Complex solid solutions are possible; many natural alloys have been given their own names. Kamacite and taenite, for example, are Fe-Ni alloys.
The native metals are relatively rare in their pure form in the Earth’s crust. However, they can be found in certain geological environments, such as placer deposits, where they have been concentrated by natural processes like weathering, erosion, and deposition.
Semimetals
The native semimetals, all rare, are found in hydrothermal deposits but rarely have economic importance.
Nonmetals
The native nonmetals are diverse in occurrence and properties. Graphite is common as an accessory mineral in many metamorphic rocks, sulfur exists in massive beds or as encrustations associated with fumaroles, and diamond is primarily restricted to kimberlite pipes and mantle nodules.
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Sulfides
The sulfide class of minerals comprises a group of minerals that are primarily composed of sulfide ions (S2-) combined with various metallic elements.
Sulfide minerals are commonly found in hydrothermal veins, sedimentary rocks, and volcanic deposits. They are characterized by their metallic luster, and opaque appearance, and often exhibit distinctive crystal forms.
The sulfide class is divided into 3 groups based on the mineral’s crystal structure.
- Tetrahedral sulfide group
This group has a crystal structure characterized by a tetrahedral arrangement of sulfide ions (S2-). In the tetrahedral sulfides, sulfur and arsenic are nearly closest packed and all metal atoms are in tetrahedral coordination.
These minerals, with their tetrahedral arrangement of sulfide ions, possess distinct physical and chemical properties.
- Octahedral sulfide group
Minerals in this group have a crystal structure characterized by an octahedral arrangement of sulfide ions (S2-). Galena and pyrrhotite are the two most important octahedral sulfides; niccolite, although not containing sulfur, is included in this group because of structural similarities.
In the octahedral sulfide structure, S and As are closest packed, and metal atoms occupy octahedral sites. Pyrrhotite is often slightly deficient in Fe, so its formula is written as, and the name troilite is given to end member FeS.
- Other sulfides
Minerals in this group have relatively simple structures based on closest packing and metal ions occupying either tetrahedral or octahedral sites, but not both.
Examples of minerals in these groups are presented in the table below:
| Groups | Minerals |
|---|---|
| Tetrahedral sulfide group | Sphalerite, Wurtzite, Chalcopyrite, Bornite, Enargite |
| Octahedral sulfide group | Galena, Pyrrhotite, Niccolite |
| Other sulfides | Molybdenite, Pentlandite, Millerite, Cinnabar, Covellite, Chalcocite, Argentite (Acanthite), Pyrite, Cobaltite, Marcasite, Arsenopyrite, Stibnite, Realgar, etc. |
Halides
Halides are a class of minerals that are composed primarily of halogen elements, such as fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).
Consequently, they bond with alkali and alkali earth elements to make chlorides, fluorides, bromides, and other salts referred to collectively as halides. Due to the generally large cation size and the nature of the bonding, halide structures tend to be simple with high symmetry.
At high temperatures, some halides exhibit mutual solubility, but under normal Earth surface conditions, most are usually close to end-member composition.
Example of halides:
| Class | Minerals |
|---|---|
| Halides | Halite, sylvite, chlorargerite, atacamite, fluorite, cryolite |
Oxides
Oxides are a class of minerals that are primarily composed of oxygen (O) combined with one or more metallic or semimetallic elements.
Similar to the sulfides, mineralogists divide oxide minerals into those having metal ions only in tetrahedral or only in octahedral coordination, and those in which the ions occupy sites with mixed or unusual coordination.
So, there are 3 groups that exist in the oxide class:
- Tetrahedral oxides: Zincite, a rare mineral, is the only known example of a purely tetrahedral oxide.
- Octahedral oxides: more than a dozen octahedral oxides are known – rutile, periclase, hematite, corundum, ilmenite, cassiterite, pyrolusite, columbite, tantalite.
- Spinels and other oxides with mixed or unusual coordinations: In spinel group minerals both tetrahedral and octahedral sites are occupied by metal ions.
In “normal” spinel minerals each metal species is found in either tetrahedral or octahedral coordination but not both. The main minerals are chromite and franklinite.
In “inverse” spinels one metal species occupies both coordinations. For example, these are spinel and magnetite.
Mineralogists have identified other oxides that do not fit into the tetrahedral, octahedral, or spinel groups. Examples of that minerals are perovskite, chrysoberyl, uraninite, thorianite, and cuprite.
Table of these groups and their minerals:
| Group | Minerals |
|---|---|
| Tetrahedral oxides | Zincite |
| Octahedral oxides | Rutile, periclase, hematite, corundum, ilmenite, cassiterite, pyrolusite, columbite, tantalite |
| Spinels and other oxides with mixed or unusual coordinations | Chromite, franklinite, spinel, magnetite, perovskite, chrysoberyl, uraninite, thorianite, cuprite |
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Hydroxides
The hydroxide class of minerals comprises minerals that are primarily composed of hydroxide (OH-) ions combined with one or more metallic elements.
These minerals typically form in environments rich in water or hydrothermal systems. Hydroxide minerals often have a characteristic non-metallic luster and can exhibit a range of colors.
An example of minerals in this class is presented in the table below:
| Class | Minerals |
|---|---|
| Hydroxides | gibbsite, brucite, manganite, goethite, diaspore, romanechite |
Carbonates and Nitrates
The carbonate class of minerals is composed primarily of minerals that contain the carbonate ion (CO3^2-).
Mineralogists divide carbonate minerals into three main groups based on the atomic arrangement:
- the calcite group,
- the dolomite group,
- and the aragonite group.
Several other species that have more complex structures and chemistries are classified separately.
Due to very high solubility in water, nitrate minerals are rare. They have structures similar to carbonates but contain monovalent rather than divalent cations because the anionic group is monovalent.
Over half a dozen nitrates are known, but nitratite and niter are the only common ones in more than just a few localities.
The table below shows the distribution of groups and their main minerals in this class:
| Group | Minerals |
|---|---|
| Groups | Minerals |
| Calcite Group | calcite, magnesite, siderite, rhodochrosite, smithsonite |
| Dolomite Group | dolomite, ankerite, kutnahorite |
| Aragonite Group | aragonite, witherite, strontianite, cerussite |
| Other Carbonates | malachite, azurite |
| Nitrate Group | nitratiteniter (saltpeter) |
Borates
Mineralogists have identified many borate minerals. Most, especially the anhydrous borates, are very rare. Borate minerals have complex structures and chemistries, due in large part to the small size and trivalent nature of ionic boron.
They have structural similarities to carbonates and nitrates because boron combines with oxygen to form anionic groups: (BO3)3- or (BO4)5-. Borates are commonly found in arid or desert regions where evaporation concentrates the minerals.
Borate class is divided into 2 groups:
- Anhydrous Borate Group – boracite, sinhalite.
- Hydrous Borate Group – borax, kernite, ulexite, colemanite, dumortierite.
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Sulfates
The sulfate class of minerals is composed of minerals that contain the sulfate ion (SO4)2-.
Mineralogists divide sulfate minerals into two groups: the anhydrous sulfates and the hydrous sulfates. The chemistries and structures of the anhydrous sulfates are related to the carbonates, with SO4 replacing CO3.
More than 100 sulfate minerals are known, and most are rare. Gypsum and anhydrite are the only rock-forming sulfates.
The distribution of minerals in the groups is represented in the table below:
| Group | Minerals |
|---|---|
| Anhydrous Sulfate | anhydrite, baritecelestite, anglesite |
| Hydrous Sulfate | gypsum, chalcanthite, epsomite, antlerite, alunite |
Tungstate, Molybdate, and Chromate Class
Tungstates, molybdates, and chromates are three classes of minerals that share similarities in their chemical compositions and crystal structures. They are all composed of elements combined with oxygen (O) and other elements, such as tungsten (W), molybdenum (Mo), and chromium (Cr).
Tungstates, molybdates, and chromates are chemically and structurally related to anhydrous sulfates. Generally rare minerals, they may be locally concentrated in ore deposits.
The table below represents some well-known minerals of this class:
| Group | Minerals |
|---|---|
| Tungstate Group | wolframite series, huebnerite, ferberite, scheelite |
| Molybdate Group | wulfenite |
| Chromate Group | crocoite |
Phosphates, Arsenartes and Vanadates
Phosphates, arsenates, and vanadates are three classes of minerals that share similarities in their chemical compositions and crystal structures. They are all composed of elements combined with oxygen (O) and other elements, such as phosphorus (P), arsenic (As), and vanadium (V).
The phosphate group contains many minerals, but most are extremely rare. Apatite is the only common example. Vanadates and arsenates, which are closely related to the phosphates in chemistry and structure, are also rare.
The table below represents some well-known minerals from the phosphate, arsenate, and vanadate classes:
| Group | Minerals |
|---|---|
| Phosphate Group | monazite, triphylite, apatite, pyromorphite, amblygonite, lazulite, wavellite, turquoise, autunite |
| Vanadate Group | vanadinite carnotite |
| Arsenate Group | erythrite |
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Conclusion
So, as we can see, the classification based on anion groups is quite extensive. Within each class, there are subclasses, groups, and subgroups that are distinguished by the crystal lattice structure of minerals and other shared characteristics.
Below, we will provide a simplified scheme of this classification:
1. Silicate Class
Framework silicate subclass
- silica group
- feldspar group
- feldspathoid group
- scapolite series
- zeolite group
- other framework silicates
Sheet silicate subclass
- serpentine group
- clay mineral group
- mica group
- chlorite group
- other sheet silicates
Chain silicate subclass
- pyroxene group
- amphibole group
- pyroxenoid group
Ring silicate subclass
Isolated tetrahedral silicate subclass
- garnet group
- olivine group
- humite group
- aluminosilicate group
- other isolated tetrahedral
- silicates
Paired tetrahedral silicate subclass
- lawsonite group
- epidote group
2. Native Element Class
- metals
- semimetals
- nonmetals
3. Sulfide Class
- tetrahedral sulfide group
- octahedral sulfide group
- other sulfides
4. Halide Class
5. Oxide Class
- tetrahedral and octahedral oxide group
- spinels and other oxides with mixed or unusual coordination
6. Hydroxide Class
7. Carbonate and Nitrate Class
- calcite group
- dolomite group
- aragonite group
- other carbonates
- nitrate group
8. Borate Class
- anhydrous borate group
- hydrous borate group
9. Sulfate Class
- anhydrous sulfate group
- hydrous sulfate group
10. Tungstate, Molybdate, and Chromate Class
- tungstate group
- molybdate group
- chromate group
11. Phosphate, Arsenate, and Vanadate Class
- phosphate group
- vanadate group
- arsenate group
Sources (the books are available on Amazon)
- “Mineralogy” – Dexter Perkins, Third Edition, 2014
- “Mineralogy for Petrologists” – Michel Demange, 2012
- “Dana’s Manual of Mineralogy” – seventeenth edition
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