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AND CHEMICAL COMPOSITION
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AND CHEMICAL See also:COMPOSITION . That the See also:general and See also:physical characters of a chemical substance are profoundly modified by crystalline structure is strikingly illustrated by the two crystalline modifications of the See also:element carbon—namely, See also:diamond and See also:graphite. The former crystallizes in the cubic See also:system, possesses four directions of perfect cleavage, is extremely hard and transparent, is a non-conductor of See also:heat and See also:electricity, and has a specific gravity of 3.5; whilst graphite crystallizes in the hexagonal system, cleaves in a single direction, is very soft and opaque, is a See also:good conductor of heat and electricity, and has a specific gravity of 2.2. Such substances, which are identical in chemical composition, but different in crystalline See also:form and consequently in their physical properties, are said to be " dimorphous." Numerous examples of dimorphous sub-stances are known; for instance, See also:calcium carbonate occurs in nature either as See also:calcite or as See also:aragonite, the former being See also:rhombohedral and the latter orthorhombic; mercuric iodide crystallizes from See also:solution as red tetragonal crystals, and by sublimation as yellow orthorhombic crystals. Some substances crystallize in three different modifications, and these are said to be " tri morphous "; for example, See also:titanium dioxide is met with as the minerals See also:rutile,•anatase and See also:brookite (q.v.). In general, or in cases where more than three crystalline modifications are known (e.g. in See also:sulphur no less than six have been described), the See also:term " polymorphism " is applied. On the other See also:hand, substances which are chemically quite distinct may exhibit similarity of crystalline form. For example, the minerals iodyrite (AgI), See also:greenockite (CdS), and See also:zincite (ZnO) are practically identical in crystalline form; calcite (CaCO3) and See also:sodium nitrate (NaNO3); celestite (SrSO)4 and See also:marcasite (FeS2); See also:epidote and See also:azurite; and many others, some of which are no doubt only accidental coincidences. Such substances are said to be " homoeomorphous " (Gr. 6/.Lows, like, and µop4, form). Similarity of crystalline form in substances which are chemically related is frequently met with and is a relation of much importance: such substances are described as being " isomorphous." Amongst minerals there are many examples of isomorphous See also:groups, e.g. the rhombohedral See also:carbonates, See also:garnet (q.v.), See also:plagioclase (q.v.); and amongst crystals of artificially prepared salts isomorphism is equally See also:common, e.g. the sulphates and selenates of See also:potassium, See also:rubidium and See also:caesium. The rhombohedral carbonates have the general See also:formula R"CO3, where R" represents calcium, See also:magnesium, See also:iron, See also:manganese, See also:zinc, See also:cobalt or See also:lead, and the different minerals (calcite, See also:ankerite, See also:magnesite, See also:chalybite, See also:rhodochrosite and See also:calamine (q.v.)) of the See also:group are not only similar in crystalline form, cleavage, See also:optical and other characters, but the angles between corresponding faces do not differ by more than 1° or 2°. Further, See also:equivalent amounts of the different chemical elements represented by R" are mutually replaceable, and two or more of these elements may be See also:present together in the same crystal, which is then spoken of as a " mixed crystal " or isomorphous mixture. In another isomorphous See also:series of carbonates with the same general formula R" CO3, where R" represents calcium, See also:strontium, See also:barium, lead or zinc, the. crystals are orthorhombic in form, and are thus dimorphous with those of the previous group (e.g. calcite and aragonite, the other members being only represented by isomorphous replacements). Such a relation is known as " isodimorphism." An even better example of this is presented by the See also:arsenic and See also:antimony trioxides, each of which occurs as two distinct minerals: As2Os, Arsenolite (cubic) ; Claudetite (See also:monoclinic). Sb20,, Senarmontite (cubic) ; Valentinite (orthorhombic). Claudetite and valentinite though crystallizing in different systems have the same cleavages and very nearly the same angles, and are strictly isomorphous. Substances which form isodimorphous groups also frequently crystallize as See also:double salts. For instance, amongst the carbonates quoted above are the minerals See also:dolomite (CaMg(See also:COs)2) and See also:barytocalcite (CaBa(CO3)2). Crystals of barytocalcite (q.v.) are monoclinic; and those of dolomite (q.v.), though closely related to calcite in angles and cleavage, possess a different degree of symmetry, and the specific gravity is not such as would result by a See also:simple isomorphous mixture of the two carbonates. A similar See also:case is presented by artificial crystals of See also:silver nitrate, and potassium nitrate. Somewhat analogous to double salts are the molecular compounds formed by the introduction of " See also:water of See also:crystallization," " See also:alcohol of crystallization," &c. Thus sodium sulphate may crystallize alone or with either seven or ten molecules of water, giving rise to three crystallographically distinct substances. A relation of another See also:kind is the alteration in crystalline form resulting from the replacement in the chemical See also:molecule of one or more atoms by atoms or radicles of a different kind. This is known as a " morphotropic " relation (Gr. ,uop¢ii, form, Tp67ros, See also:habit). Thus when some of the See also:hydrogen atoms of See also:benzene are replaced by (OH) and (NO2) groups the orthorhombic system of crystallization remains the same as before, and the crystallographic See also:axis a is not much affected, but the axis c varies considerably: Benzene, See also:C6H6 See also:Resorcin, See also:C6H,(OH)2 Picric See also:acid ,C6H2(OH)(NO2)s A striking example of morphotropy is shown by the See also:humite (q.v.) group of. minerals: successive additions of the group Mg2SiO4 to the molecule produce successive increases in the length of the See also:vertical crystallographic axis. In some instances the replacement of one See also:atom by another produces little or no See also:influence on the crystalline form; this ,.happens in complex molecules of high molecular See also:weight, the " See also:mass effect " of which has a controlling influence on the isomorphism. An example of this is seen in the replacement of sodium or potassium by lead in the See also:alunite (q.v.) group of minerals, or again in such a complex See also:mineral as See also:tourmaline, which, though varying widely in chemical composition, exhibits no variation 'in crystalline form. For the purpose of comparing the crystalline forms of isomorphous and morphotropic substances it is usual to quote the angles or the axial ratios of the crystal, as in the table of benzene derivatives quoted above. A more accurate comparison is, how-ever, given by the " topic axes," which are calculated from the axial ratios and the molecular See also:volume; they See also:express the relative distances apart of the crystal molecules in the axial directions. The two isomerides of substances, such as tartaric acid, which in solution rotate the See also:plane of polarized See also:light either to the right or to the See also:left, crystallize in related but enantiomorphous forms. For geometrical See also:crystallography, dealing exclusively with the See also:external form of crystals, reference may be made to N. See also:Story-See also:Maskelyne, Crystallography, a See also:Treatise on the See also:Morphology of Crystals (See also:Oxford, 1895) and W. J. See also:Lewis, A Treatise on Crystallography (See also:Cambridge, 1899). Theories of crystal structure are discussed by L. Sohncke, Entwickelung einer Theorie der Krystallstruktur (See also:Leipzig, 1879) ; A. Schoenflies, Krystallsysteme and Krystallstructur (Leipzig, 1891) ; and H. See also:Hilton, Mathematical Crystallography and the Theory of Groups of Movements (Oxford, 1903). The physical properties of crystals are treated by T. Liebisch, Physikalische Krystallographie (Leipzig, 1891), and in a more elementary form in his Grundriss der physikalischen Krystallographie (Leipzig, 1896) ; E. Mallard, Traite de cristallographie, Cristallographie physique (See also:Paris, 1884) ; C. Soret, Elements de cristallographie physique (See also:Geneva and Paris, 1893). For an See also:account of the relations between crystalline form and chemical composition, see A. Arzruni, Physikalische Chemie der Krystalle (Braunschweig, 1893) ; A. Fock, An Introduction to Chemical Crystallography, translated by W. J. See also:Pope (Oxford, 1895) ; P. See also:Groth, An Introduction to Chemical Crystallography, translated by H. See also:Marshall (See also:London, 1906) ; A. E. H. Tutton, Crystalline Structure and Chemical Constitution, 1910. Descriptive See also:works giving the crystallographic constants of different substances are C. F. See also:Rammelsberg, Handbuch der krystallographisch-physikalischen Chemie (Leipzig, 1881-1882) ; P. Groth, Chemische Krystallographie (Leipzig, 1906) ; and of minerals the See also:treatises of J. D. See also:Dana and C. Hintze. (L. J. Additional information and CommentsThere are no comments yet for this article.
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