Mineral orthorhombic

Lead zirconate [12060-01 -4] PbZrO, mol wt 346.41, has two colorless crystal stmctures a cubic perovskite form above 230°C (Curie point) and a pseudotetragonal or orthorhombic form below 230°C. It is insoluble in water and aqueous alkaUes, but soluble in strong mineral acids. Lead zirconate is usually prepared by heating together the oxides of lead and zirconium in the proper proportion. It readily forms soHd solutions with other compounds with the ABO stmcture, such as barium zirconate or lead titanate. Mixed lead titanate-zirconates have particularly high piezoelectric properties. They are used in high power acoustic-radiating transducers, hydrophones, and specialty instmments (146). 

Texture. All limestones are crystalline, but there is tremendous variance in the size, uniformity, and arrangement of their crystal lattices. The crystals of the minerals calcite, magnesite, and dolomite are rhombohedral those of aragonite are orthorhombic. The crystals of chalk and of most quick and hydrated limes are so minute that these products appear amorphous, but high powered microscopy proves them to be cryptocrystalline. Hydrated lime is invariably a white, fluffy powder of micrometer and submicrometer particle size. Commercial quicklime is used in lump, pebble, ground, and pulverized forms. 

Occurrence. The principal strontium mineral is celestite, naturally occurring strontium sulfate. Celestite and celestine [7759-02-6] both describe this mineral. However, celestite is the form most widely used in Knglish-speaking countries. Celestite has a theoretical strontium oxide content of 56.4 wt %, a hardness of 3—3.5 on Mohs scale, and a specific gravity of 3.96. It is usually white or bluish white and has an orthorhombic crystal form. 

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. 

At higher temperatures the stable form is valentinite, which consists of infinite double chains. The orthorhombic modification is metastable below 570 °C however, it is sufficientiy stable to exist as a mineral. Antimony trioxide melts in the absence of oxygen at 656°C and partially sublimes before reaching the boiling temperature, 1425°C. The vapor at 1500°C consists largely of Sb O molecules, but these dissociate at higher temperatures to form Sb202 molecules. 

In the vapour phase As is known to exist as tetrahedral Asa molecules with (As-As 243.5 pm) and when the element is sublimed, a yellow, cubic modification is obtained which probably also contains Asa units though the structure has not yet been determined because the crystals decompose in the X-ray beam. The mineral arsenolamprite is another polymorph, e-As it is possibly isostructural with metallic orthorhombic P. 

Commercial elemental sulfur is usually of bright-yellow color at 20 °C [36]. Pure orthorhombic a-Ss is, however, of greenish-yellow color at 20 °C but totally colorless at 77 K while commercial sulfur often remains pale-yellow at this temperature [59]. The reasons for this different behavior are twofold. Commercial samples are never pure Ss but besides traces of organic impurities they always contain Sy in concentrations of between 0.1 and 0.5% [59]. Sulfur found as a mineral in Nature sometimes also contains Sy but in addition traces of selenium are quite often present (up to 680 ppm Se, probably as SySe molecules) [60]. These minor components influence the color of the samples at ambient and low temperatures in the sense that a more orange-type of yellow ( egg-yellow ) is recognized. 

Cupric sulfide, CuS, occurring as the mineral covellite (also known as covelline), exhibits a very unusual structure, in which the Cu is again partly 3-coordinate and partly 4-coordinate, with two-thirds of the sulfur atoms existing as S2 groups like those in pyrites. The low-temperature form of CuSe has also a covellite structure, the high-temperature modification (P-CuSe) being orthorhombic. All CuX2 compounds assume pyrite-type structures. 

Yellow orthorhombic crystal density 4.50 g/cm darkens on heating insoluble in water and organic solvents dissolves in mineral acid with decomposition. 

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. 

Tellurium dioxide in its orthorhombic form occurs in nature as mineral tellurite. It is mined from natural deposits. Also, tellurium dioxide is produced as an intermediate during recovery of tellurium metal from anode shmes of electrolytic copper refining (See Tellurium, Production). The dioxide also is prepared by treating tellurium metal with hot nitric acid to form 2Te02 HNO3. The product then is heated to drive off nitric acid. 

Barite is a barium sulfate, BaS04, with orthorhombic structure 2jm2jm2jm) where the sulfur is situated in tetrahedral coordination with oxygen, and barium in twelve-fold coordination with oxygen. The mineral barite is one of the first luminescent materials from which the famous Bologna stone was obtained. Nevertheless, up to today understanding of natural barite luminescence... 

The familiar cubic perovskite structure of ABO3 has of course just one structural parameter, the unit cell edge a. This requires the ratio of the A-O to the B-O bond lengths to be equal to y/2. When this condition cannot be met, the structme distorts in (one of) a number of well-documented ways . By far the largest of the families of derivative structures that arise when A is too small [/(A-0)//(B-0) < /2] is that of the orthorhombic perovskites (GdFeOa type) exemplified by the mineral perovskite (CaTi03) itself. 

The Perovskite Structure, ABXS Systems. Cubic Pm3m (Space Group 221) A cubic structure was assigned to the mineral perovskite, CaTiOj, but this particular compound was later found to actually possess orthorhombic symmetry. Today, however, we refer to the perovskite structure in its idealized form as having cubic symmetry and it is normally represented by a simple unit cell (Figure 10). 

We shall now present studies relating to the micro-twin structure performed by B. G. Hyde, who is a main proposer of the concepts of shear and micro-twin structures. One study is on the mineral humite family,the composition of which is generally expressed by nMg2SiO4-Mg(OH)2 (this has to be altered to nMg2SiO4-Mg(OH, F)2, because (OH) is often replaced by F in the minerals). In Table 2.4, the mineral name, together with the compositions and micro-twin structures proposed by the authors, are collected, the space group is Pmcn (orthorhombic) for n = odd and... 

In renal stones, apatite (Ca5(P04)3 OH, 1/2 C03) with a hexagonal shape has been observed however, this mineral is difficult to identify because of its cryptocrystalline appearance. Struvite (MgNH4P04 6 H20) orthorhombic, newberyte (MgHP04 3 H20) orthorhombic, whit-ockite (Ca3(P04)2) hexagonal, and also brushite have been reported to occur in renal stones82. ... 

Sulfides and Sulfates. Pyrite is the dominant sulfide mineral in coal. Marcasite has also been reported from many different coals. Pyrite and marcasite are dimorphs, minerals that are identical in chemical composition (FeS2) but differ in crystalline form pyrite is cubic while marcasite is orthorhombic. Other sulfide minerals that have been found in coals, and sometimes in significant amounts, are sphalerite (ZnS) and galena (PbS). 

T1he zeolite minerals mordenite (1), ptilolite (2), arduinite (3), flokite (4), and ashtonite (5) have been considered identical on the basis of x-ray diffraction studies and are now named mordenite (6, 7, 8,9,10). Meier (11) determined the structure on a sample of sodium-exchanged ptilolite (Challis Valley, Idaho). The symmetry was orthorhombic with space group either Cmcm or Cmc2i. 

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