All members of the group are isostructural, possessing unit cells which are completely analogous in structure and differ only in the identity of the constituent atoms. Because the calcite minerals are isostructural, they form solid solutions of varying proportions in which magnesium radius 0.
Calcium, however, does not substitute so readily for these elements because of its larger radius 0. Each of these species contains the semimetal calcium and a metal cation together with the carbonate anion. The structure of the dolomite minerals is composed of layers in which carbonate anions alternate with metal cations.
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In this way members of the dolomite group are similar in structure to the calcite group. However, the dolomites are more highly ordered than the calcites because their cation layers consist of layers of calcium alternating with layers of magnesium or iron. Due to its large atomic radius 0. Like the members of the calcite group, those of the dolomite group adhere to the rhombohedral crystal system. The members of this group are isostructural and may form a solid solution in which strontium, lead, and calcium may substitute for one another.
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Like the members of the calcite group, the aragonite minerals each contain the carbonate anion and ions of a single metal. However, the members of the aragonite group adhere to the orthorhombic crystal system rather than the rhombohedral crystal system preferred by the calcite and dolomite groups. Structurally calcite and aragonite are thus quite different. Polymorphism and Pseudomorphism in Calcite and Aragonite. Share to:. AX galena sphalerite chalcopyrite covellite cinnabar realgar niccolite breithauptite.
A 2 X chalcocite.
AX 2 pyrite cobaltite molybdenite calaverite. AX 3 skutterudite. CoAs 3. A 2 X 3 orpiment bismuthinite. A 4 S 3 cobaltite pyrargyrite proustite.
Minerals and Rocks - Geological Building Blocks - Hong Kong Geology
Minerals of the hydroxide class are typically softer than oxides and are of low to medium density. In terms of chemical composition, quartz SiO 2 is a member of the oxide class. However in terms of molecular structure, which in the Berzelian classification system is considered to be more fundamental than chemical composition, quartz is classified as a silicate. These elements form relatively large, lightly charged, easily polarized ions.
The halogen ions then bond to large, weakly polarized cations of sodium Na , potassium K , magnesium Mg or calcium Ca , and sometimes aluminum Al , copper Cu , or silver Ag. Halides are therefore constructed entirely of ionic bonds. Both cations and anions behave as almost spherical bodies which pack in a highly symmetric manner; most common halites possess isometric hexoctahedral crystalline structure. The halides tend to be soft, brittle, easily soluble in water, and possess medium to high melting points. They are poor conductors of heat and electricity when in solid state but good conductors when molten.
When molten, ions are liberated and may then transport charges. Molten halides thus conduct by electrolysis, or the transportation of charge by free-moving ions. Example members of the halide class are the mineral species halite NaCl ; also known as rock salt , sylvite KCl , fluorite CaF 2 , and chlorargyrite AgCl , also known as horn silver. In these substances plane triangular CO 3 2- anion groups are linked together by various cations. Each oxygen atom is bonded more strongly to its associated carbon than to any other atom of the structure, and oxygen atoms are not shared between the carbonate anions.
The plane triangular carbonate anions thus form the basic unit from which carbonate minerals are constructed. Minerals of the carbonate class thus react easily with acids such as hydrochloric acid HCl. Although chemically identical, these two species are structurally very different; calcite crystalizes according to the rhombohedral crystal system while aragonite crystallizes according to the orthorhombic crystal system. Aragonite is harder and has a higher specific gravity than calcite.
Two mineral species such as calcite and aragonite which are chemically identical and yet differ structurally and possess different physical properties are said to be polymorphs. The structure of a carbonate is determined by the radius of its constituent cation. Those carbonates which contain smaller cations form rhombohedral structures while those possessing larger cations occupy orthorhombic structures.
Magnesium, manganese, and iron are small cations, and the carbonates such as magnesite, rhodocrosite, and siderite which contain them adhere to the rhombohedral crystal system. Lead and strontium, on the other hand, are large cations, and species such as strontianite and cerussite which contain them belong to the orthorhombic crystal system. The radius of calcium occupies a unique position near the critical radius which distinguishes between the two possible crystal systems. Carbonate species which contain the calcium anion may thus crystallize according to either structural system.
For this reason the compound calcium carbonate CaCO 3 crystallizes with rhombohedral crystal structure as calcite and with orthorhombic structure as aragonite. The fact that the calcium cation is of approximately the critical radius leads to the ability of calcium carbonate to crystallize in either system.
Aragonite is less stable than calcite and therefore less common. In such cases the unit cells of the original aragonite have been gradually replaced by those of calcite while the original crystal shape remains intact. The element copper is a strong pigmenting agent and renders azurite a bright, distinctive azure blue, malachite a vivid green, and aurichalcite green to blue. Both azurite and malachite are of the monoclinic crystal system while aurichalcite is of the orthorhombic crystal system.
Malachite frequently forms a pseudomorph of azurite; azurite is less stable than malachite, and gradually inverts to it over long periods of time. Nitrogen bonds to three oxygen atoms in a structure very similar to the carbonate anion group CO 3 This nitrate radical, NO 3 - , forms the basic building block of the nitrates.
Mineral species of the nitrate class thus tend to occur less frequently in geologic formations than do the carbonates. The nitrates yield oxygen as the nitrate anion decomposes upon heating. Several mineral species of the nitrate class form structures analogous to certain of the carbonates. Nitratine is softer than calcite and melts ar a lower temperature, both of which are expected of a nitrate as compared to a carbonate. Niter KNO 3 , known colloquially as saltpeter is in turn isostructural with the orthorhombic carbonate aragonite CaCO 3.
The polymorph of rhombohedral nitratine is isostructural with orthorhombic niter KNO 3. The boron atom joins to three oxygen atoms to form the borate radical, BO 3 After this anion group has formed, each oxygen atom still has one electron available for bonding. Thus the oxygen atoms of the borate radical may, unlike those of the carbonate or nitrate radicals, be shared between anion groups.
The sulfur anion S 2- may bond to a positive metallic or semimetallic ion, receiving two electrons in order to fill its valence shell. The sulfur cation may then form very strong bonds with four oxygen atoms, producing the anion group SO 4 All members of the sulfate class contain the tetrahedral sulfate radical SO 4 These tightly bound anion groups do not share oxygen atoms and cannot polymerize.
These species are isostructural and may form a solid solution by exchanging atoms of strontium and lead for those of barium. The minerals of the barite group all possess orthorhombic symmetry, as does anhydrite. Anhydrite is not, however, isostructural with the barite minerals because the calcium ion possesses a radius smaller than that of barium, strontium, or lead. Barite BaSO 4 sometimes produces groups of divergent tabluar crystals known as "desert roses.
Specimens of concretionary habit have been found consisting of concentric layers often containing a core of unaltered galena. It possesses a sheetlike structure which consists of layers of calcium ions and sulfate anions separated by water molecules H 2 O.
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Gypsum demonstrates perfect cleavage along planes parallel to its sheetlike layers. If the water is driven out of the gypsum structure, the mineral will collapse into the configuration of anhydrite CaSO 4 with a marked decrease in volume and loss of the perfect cleavage. Learn more. General lattice vibrational properties of minerals, particularly those properties which strongly influence the thermodynamic behavior, are discussed in this paper the second of a series relating thermodynamic and lattice vibrational properties of minerals.
A method is given for enumeration of the fraction of total vibrational modes which are internal stretching modes. Inelastic neutron scattering data are reviewed to show the magnitude of anisotropy and dispersion of acoustic modes and of dispersion of optic modes in the minerals. Volume 17 , Issue 1. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account.
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Open access. Reviews of Geophysics Volume 17, Issue 1. Susan Werner Kieffer Search for more papers by this author. Tools Request permission Export citation Add to favorites Track citation. In general, mineral particles are small, having formed within confined areas such as lava flows or between grains of sediments.