Orthorhombic carbonates-

The calcium ion is of such a size that it may enter 6-fold coordination to produce the rhombohedral carbonate, calcite, or it may enter 9-fold coordination to form the orthorhombic carbonate, aragonite. Cations larger than Ca2+, e.g., Sr2+, Ba2+, Pb2+, and Ra2 only form orthorhombic carbonates (at earth surface conditions) which are not, of course, isomorphous with calcite. Therefore these cations are incapable of isomorphous substitution in calcite, but may participate in isodimorphous or "forced isomorphous" substitution (21). Isodimorphous substitution occurs when an ion "adapts" to a crystal structure different from its own by occupying the lattice site of the appropriate major ion in that structure. For example, Sr2+ may substitute for Ca2 in the rhombohedral lattice of calcite even though SrC03, strontianite, forms an orthorhombic lattice. Note that the coordination of Sr2 to the carbonate groups in each of these structures is quite different. Very limited miscibility normally characterizes such substitution. 

Table 2.2. Anhydrous rhombohedral and orthorhombic carbonates. (After Speer, 1983.)...

Speer J.A. (1983) Crystal chemistry and phase relations of orthorhombic carbonates. In Reviews in Mineralogy Carbonates - Mineralogy and Chemistry (ed. R. J. Reeder), pp. 145-189. Mineralogical Society of America. Bookcrafters, Inc., Chelse, MI. 

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. 

Typical simple orthorhombic carbonate spectrum, distinct from calcite group spectra. 2854 268 ... 

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... 

For a free energy of fonnation, the preferred standard state of the element should be the thennodynamically stable (lowest chemical potential) fonn of it e.g. at room temperature, graphite for carbon, the orthorhombic crystal for sulfiir. 

In general, the chemistry of inorganic lead compounds is similar to that of the alkaline-earth elements. Thus the carbonate, nitrate, and sulfate of lead are isomorphous with the corresponding compounds of calcium, barium, and strontium. In addition, many inorganic lead compounds possess two or more crystalline forms having different properties. For example, the oxides and the sulfide of bivalent lead are frequendy colored as a result of their state of crystallisation. Pure, tetragonal a-PbO is red pure, orthorhombic P PbO is yeUow and crystals of lead sulfide, PbS, have a black, metallic luster. 

Lead Chloride. Lead dichloride, PbCl2, forms white, orthorhombic needles some physical properties are given in Table 1. Lead chloride is slightly soluble in dilute hydrochloric acid and ammonia and insoluble in alcohol. It is prepared by the reaction of lead monoxide or basic lead carbonate with hydrochloric acid, or by treating a solution of lead acetate with hydrochloric acid and allowing the precipitate to settle. It easily forms basic chlorides, such as PbCl Pb(OH)2 [15887-88 ] which is known as Pattinson s lead white, an artist s pigment. 

Lead Carbonate. Lead carbonate [598-63-0] PbCO, mol wt 267.22, d = 6.6g/cm, forms colorless orthorhombic crystals it decomposes at about 315°C. It is nearly insoluble in cold water (0.00011 g/100 mL at 20°C), but is transformed in hot water to the basic carbonate, 2PbC03 Pb(OH)2. Lead carbonate is soluble in acids and alkalies, but insoluble in alcohol and ammonia. It is prepared by passing CO2 iuto a cold dilute solution of lead acetate, or by shaking a suspension of a lead salt less soluble than the carbonate with ammonium carbonate at a low temperature to avoid formation of basic lead carbonate. 

Strontium Carbonate. Strontium carbonate, SrCO, occurs naturally as strontianite in orthorhombic crystals and as isomorphs with aragonite, CaCO, and witherite, BaCO. There are deposits in the United States in Schoharie County, New York in WestphaUa, Germany and smaller deposits in many other areas. None is economically workable. Strontianite has a specific gravity of 3.7, a Mohs hardness of 3.5, and it is colorless, gray, or reddish in color. 

Ferrous orthotitanate [12160-20-2] Fe2Ti04, is orthorhombic and opaque. It has been prepared by heating a mixture of ferrous oxide and titanium dioxide. Ferrous dititanate [12160-10-0] FeTi20, is orthorhombic and has been prepared by reducing ilmenite with carbon at 1000°C. The metallic ion formed in the reaction is removed, leaving a composition that is essentially the dititanate. Ferric titanate [1310-39-0] (pseudobrookite), Fe2TiO, is orthorhombic and occurs to a limited state in nature. It has been prepared by heating a mixture of ferric oxide and titanium dioxide in a sealed quartz tube at 1000°C. 

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. 

Antimony Trioxide. Antimony(III) oxide (antimony sesquioxide) [1309-64-4] Sb203, is dimorphic, existing in an orthorhombic modification valentinite [1317-98-2] is colorless (sp gr 5.67) and exists in a cubic form and senarmontite [12412-52-17, Sb O, is also colorless (sp gr 5.2). The cubic modification is stable at temperatures below 570°C and consists of discrete Sb O molecules. The molecule is similar to that of P40 and As O and consists of a bowed tetrahedron having antimony atoms at each corner united by oxygen atoms lying in front of the edges. This solid crystallizes in a diamond lattice with an Sb O molecule at each carbon position. 

The iron—carbon system contains the orthorhombic iron carbide (3 1) [12011 -67-5] which melts congmendy and represents the cementite in... 

The iron-carbon solid alloy which results from the solidification of non blastfurnace metal is saturated with carbon at the metal-slag temperature of about 2000 K, which is subsequendy refined by the oxidation of carbon to produce steel containing less than 1 wt% carbon, die level depending on the application. The first solid phases to separate from liquid steel at the eutectic temperature, 1408 K, are the (f.c.c) y-phase Austenite together with cementite, Fe3C, which has an orthorhombic sttiicture, and not die dieniiodynamically stable carbon phase which is to be expected from die equilibrium diagram. Cementite is thermodynamically unstable with respect to decomposition to h on and carbon from room temperature up to 1130 K... 

Calcium hydroxide leached from incompletely cured concrete causes serious corrosion of lead (see Section 9.3). This is because carbon dioxide reacts with the lime solution to form calcium carbonate, which is practically insoluble. Carbonate ions are therefore not available to form a passive film on the surface of the lead . Typically, thick layers of PbO are formed, which may show seasonal rings of litharge (tetragonal PbO) and massicot (orthorhombic PbO) . 

Crystal Structure and Lattice Parameters (nm) Orthorhombic, a = 0.283, b = 0.554, c = 1.1470 Cr3C2 is an intermediate carbide having carbon chains with C-C distance approximately 0.165 nm running through distorted metal lattice where the Cr atoms are at the corners of trigonal prisms and the carbon atoms in the center of the prisms.i li" ... 

Figure 3. Relation between the orthorhombic structure types of CrB and UBC (projected along [100]) as well as between the tetragonal structure types of a-MoB and ThBC (projected along [100]). Boron atoms are at the centers of trigonal metal prisms, carbon atoms at the centers of MjB-Oj,. The numbers given indicate the heights in projection.

Battery applications Titanium containing y-Mn02 (TM) hollow spheres synthesis and catalytic activities in Li-air batteries [123] Orthorhombic LiMn02 nanorods for lithium ion battery application [124] Electrochemical characterization of MnOOH-carbon nanocomposite cathodes for metal—air batteries [125] Electrocatalytic activity of nanosized manganite [126]... 

FIGURE 4.2 Representation of different carbon types on cobalt, (a) Atomic carbon/ surface carbide in a threefold hollow site, (b) CHX species located in threefold hollow sites, (c) Subsurface carbon lying in octahedral positions below the first layer of cobalt, (d) Cobalt carbide (Co2C) with an orthorhombic structure, (e) Polymeric carbon on a cobalt surface, (f) A sheet of graphene lying on a cobalt surface. The darker spheres represent carbon atoms in all the figures. 

Tanaka, S. Katayama, Y. Tate, M. R Hillhouse, H. W. Miyake, Y. 2007. Fabrication of continuous mesoporous carbon films with face-centered orthorhombic symmetry through a soft templating pathway. J. Mater. Chem. 17 3639-3645. 

Single crystal x-ray diffraction studies showed that the crystals of halcinonide recrystallized from tz-propyl alcohol-water azeotrope (79 22) are orthorhombic and belong to the gpace group P2 2 2, with unit ell constants of a = 10.007 A, = 11.875 A ana Q = 19.460 A. Density is 1.330 gm/cm2 3, as measured by flotation in a hexane-carbon tetrachloride gradient. The molecular weight calculated from the unit cell volume and density is 461 daltons (theoretical is 455 daltons). 

---