Allotropes solid amorphous modifications

IR-3.4.5 Solid amorphous modifications and commonly recognized allotropes of indefinite structure... 

Solid amorphous modifications and commonly recognized allotropes of indefinite structure are distinguished by customary descriptors such as a Greek letter, names based on physical properties, or mineral names. 

Trigonal crystalline solid or amorphous powder mineral millerite has a yellow metallic luster color varies from yellow to brownish black density 5.30 to 6.65 g/cm3 exhibits three allotropic modifications (1) the acid-soluble amorphous alpha form obtained from nickel salt solution by precipitation with ammonium sulfide, (2) the alpha form rapidly transforms to a crystalline beta form as a brown colloidal dispersion upon exposure to air, and (3) a rhombo-hedral gamma modification found native as mineral millerite, which also can be prepared artificially under certain conditions. 

Allotropic forms of carbon. In the solid state, the element carbon exists in three different allotropic modifications—amorphous carbon and the two crystalline forms known as diamond and graphite. Amorphous carbon includes numerous common products such as wood charcoal, bone black, coke, lamp black, and carbon black. Each of these varieties of crystalline and amorphous carbon possesses properties that render it useful for a variety of purposes. 

Allotropic forms of sulfur. Solid sulfur exists in two crystalline modifications. Rhombic sulfur consists of S8 molecules, is stable at temperatures below 95.5°C, has a specific gravity of 1.96, and is soluble in carbon disulfide. At 95.5°C, rhombic sulfur changes slowly, with absorption of heat, into the monoclinic form. Molten rhombic sulfur consists of S8 molecules and exists as a pale-yellow, thin, and limpid liquid known as X sulfur. When the temperature is raised, X sulfur is slowly converted to dark and viscous p sulfur, which consists of Ss and S4 molecules and which is considered to be the amorphous variety of... 

Yellow forms of arsenic and antimony (the latter very unstable) have been described. These are presumably the nonmetallic modifications of these elements, analogous to white phosphorus, and also consisting of discrete molecules (tetrahedral quartets) in the solid state. The grey or metallic forms of arsenic and antimony are the most stable. They are far denser than the yellow forms, are insoluble in organic solvents, and have appreciable electrical conductivities. Black amorphous forms of arsenic and antimony are also known, and an additional allotrope of antimony, explosive (but always impure), has been described. 

The values reported vary by six orders of magnitude. The more recently reported spectrophotometrically determined constant gives values between 17 and 19. Such values imply that S ", similar to hardly occurs in aqueous solution. In the past, however, many solubility products have been determined by assuming a pA 2 14. Furthermore, solid sulfides occur often in different allotropic modifications for example, FeS occurs as troilite, mackinawite, pyr-rhotite, and amorphous FeS then there is greigite (Fe3S4) and different modifications of FeS2 (pyrite, marcasite). A further complication is that HS can form poly sulfides in reactions such as 3 S(s) -f HS = HS4. ... 

The particular advantage of diffraction analysis is that it discloses the presence of a substance as that substance actually exists in the sample, and not in terms of its constituent chemical elements. For example, if a sample contains the compound A By, the diffraction method will disclose the presence of A B as such, whereas ordinary chemical analysis would show only the presence of elements A and B. Furthermore, if the sample contained both A B, and Aj Bjy, both of these compounds would be disclosed by the diffraction method, but chemical analysis would again indicate only the presence of A and B. To consider another example, chemical analysis of a plain carbon steel reveals only the amounts of iron, carbon, manganese, etc., which the steel contains, but gives no information regarding the phases present. Is the steel in question wholly martensitic, does it contain both martensite and austenite, or is it composed only of ferrite and cementite Questions such as these can be answered by the diffraction method. Another rather obvious application of diffraction analysis is in distinguishing between different allotropic modifications of the same substance solid silica, for example, exists in one amorphous and six crystalline modifications, and the diffraction patterns of these seven forms are all different. 

AixotrOiw,—Dimorq hism apart, a few substances are known to exist in more than one solid form. These varieties of the same substance exhibit different physical properties, while their chemical qualities are the same in kind Such m ifications are said to be affoiropic. One or more allotropic modifications of a substance are usually crtjifidliyiet the other or others amorphous or vitreous. Sulplmr, for example, exists not only in two dimorphous varieties of ciy stols, but also in a third, allotropic form, in which it is flexible, amorphous, and transparent. Carbon exists in three allotropic forms two crystaUine, the diamond and graphite the Ihirtl amorphous. 

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