Carbonate minerals orthorhombic

Figure 5.14. Chemical compositions of 106 samples of inorganic orthorhombic carbonate minerals. (After Speer, 1983.)...

Calcium carbonate minerals (CaCOs) are ubiquitous in biomineralizing systems, since they are in the skeletons of organisms ranging from bacteria and algae to humans, most commonly as calcite (rhomboedral) or aragonite (orthorhombic). Thus, their structures and compositions are highly diversified. Here, only some examples are discussed. 

Aragonite 1) Sensu stricto An orthorhombic carbonate with the formula CaC03, which is trimorphous with calcite and vaterite. Also known as Aragon spar it is a common matrix mineral in invertebrate shells. 2) Sensu lato A group of orthorhombic carbonate minerals, including aragonite, strontianite, cerussite, alstonite, and witherite. 

Formula Chemical class Chemical type BaCa(C03)2 Crystal system Anhydrous normal carbonate Mineral group AB(XOj)2 Space group Tridinic, pseudo orthorhombic Aragonite PI... 

Formula Chemical class Chemical type (La,Ce)2(C03)3-8H20 Crystal system Hydrated normal carbonate Mineral group A B (X03)p-xH20 where (m-l-n) p < 1 1 Space group Orthorhombic Lanthanite Pbnb... 

Formula Chemical class Chemical type SrCOj Crys tal s ys tern Anhydrous normal carbonate Mineral group ACXOj) Space group Orthorhombic Aragonite Pmcn... 

The most important carbonate mineral is calcium carbonate, CaCOs. This substance occurs in beautiful colorless hexagonal crystals as the mineral calcite. Marble is a microcrystalline form of calcium carbonate, and limestone is a rock composed mainly of this substance. Calcium carbonate is the principal constituent also of pearls, coral, and most sea shells. It also occurs in a second crystalline form, as the orthorhombic mineral aragonite. 

The known uranium(VI) carbonate soHds have empirical formulas, 1102(003), M2U02(C03)2, and M4U02(C03)3. The soHd of composition 1102(003) is a well-known mineral, mtherfordine, and its stmcture has been determined from crystals of both the natural mineral and synthetic samples. Rutherfordine is a layered soHd in which the local coordination environment of the uranyl ion consists of a hexagonal bipyramidal arrangement of oxygen atoms with the uranyl units perpendicular to the orthorhombic plane. Each uranium atom forms six equatorial bonds with the oxygen atoms of four carbonate ligands, two in a bidentate manner and two in a monodentate manner. 

Calcium carbonate occurs in two forms— hexagonal crystal known as cal-cite, and orthorhombic form, aragonite. Calcite decomposes on heating at 825°C, aragonite melts at 1,339°C (at 102.5 atm). Density 2.71 g/cm (calcite), 2.83 g/cm3 (aragonite) insoluble in water (15mg/L at 25°C) Ksp 4.8x10 soluble in dilute mineral acids. 

Spear J. A. (1983) Crystal chemistry and phase relations pf orthorhombic carbonates. In Carbonates Mineralogy, and Chemistry. Rev. Mineral, (ed. R. R. Reeder). Mineralogical Society of America, Washington, DC, vol. 11, pp. 145—225. 

Formula Chemical class Chemical type SrCe(C03)2(0H)H20 Hydrated carbonate with hydroxyl or halogen A B (X03)pZ,-xH20 with (m+n) p = 1 1 Crystal system Mineral group Space group Orthorhombic Ancylite Pmcn... 

Formula Chemical class Chemical type BajAl4(C03)4(0H)g-3Hj0 Crystal system Hydrated carbonate with hydroxyl or halogen Mineral group A B (X03)pZq-xH20 where (m+n) p = 3 2 Space group Orthorhombic Alumohydrocalcite Pbnm... 

Formula Chemical class Chemical type (Cu,Ni),(C03)(0H), Anhydrous carbonate with hydroxyl or halogen (AB)3(X03>Z, Crystal system Mineral group Space group Monoclinic, pseudo-orthorhombic. Malachite (rosasite) 9... 

Dal Negro et al. (1975) suggested that the crystal structure of ancylite-(Ce) can be compared to that of the orthorhombic carbonates, aragonite, strontianite, and wither-ite. These minerals belong to the same space group, Pmcn, and the cell dimensions of the a- and h-axis in ancylite-(Ce) are very close to the corresponding ones in strontianite a = 5.090(2), b = 8.358(2), c = 5.997(4) A). In addition, the mean M-O... 

Copper s relative abundance in the Earth s crust is about 50 mg/kg (i.e., ppm wt.), which is less than nickel and zinc. It occurs as a native element (4%), but the major part of its occurrence is as oxides minerals (10%) such as cuprite [Cu O, cubic] carbonates (5%) malachite [CuC03.Cu(OH)j, monoclinic] and azurite [2CuC03.Cu(0H)j, monoclinic] sulfide minerals (80%) chalcocite [Cu S, monoclinic], chalcopyrite [CuFeS, tetragonal], and bornite [CUjFeS, cubic] and in other rare minerals (1%) such as atacamite [CUjC OH), orthorhombic]. But only the oxide and sulfide minerals are used industrially as copper ores. Chile is the world s largest producer of copper, followed by the United States. 

The strontium content in the lithosphere is ca. 370 mg/kg (i.e., ppm wt.), but, owing to its chemical reactivity, the metal does not occur free in nature. The chief strontium containing minerals are the sulfate celestite or celestine [SrSO, orthorhombic] and the carbonate strontianite [SrCOj, orthorhombic], but strontium traces can also be found in calcium and barium-containing minerals. Nevertheless, strontium minerals rarely concentrate in large ore deposits, and the chief ore is only represented by celestite because there are no known economically workable strontianite deposits. However, strontium occurs widely dispersed in seawater and in igneous rocks as a minor constituent of rock-forming minerals. 

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