Online Encyclopedia

ZEOLITES

Online Encyclopedia
Originally appearing in Volume V28, Page 973 of the 1911 Encyclopedia Britannica.
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ZEOLITES, a family of minerals consisting of hydrated silicates of alumina with alkalis or alkaline earths or both. The water they contain is readily lost, and before the blowpipe it is expelled with intumescence; hence the name zeolite, from the Greek Eiv (to boil) and M Oos (a stone), given by A. Cronstedt in 1758. In some other characters, as well as in their origin and mode of occurrence, they have points in common. Several species have been distinguished, of which the following are the more important. Apophyllite (q.v.) and pectolite (see PYROXENE) are also sometimes included. Some of fhe chemical formulae given above are only approximate, since in some species the composition varies between certain limits and can be best expressed by the isomorphous mixing of different molecules (see, for example, CHABAZITE). They are all readily decomposed by hydrochloric acid, 'usually with the separation of gelatinous silica. By the action of various reagents several substitution products have been prepared artificially: thus, crystallized products, in which the alkalis or alkaline earths are replaced by ammonium or silver, &c., have been obtained. The zeolites are often beautifully crystallized, and belong to several crystal-systems. The crystals usually show evidences of twinning, and when examined in polarized light they frequently exhibit optical anomalies and a complex structure. The hardness (H.=3 -52) and specific gravity (2.0–2.4) are comparatively low, and so are the indices of refraction and the double refraction. The water of zeolites presents many points of interest. Laumontite loses water on exposure to air, and the crystals soon crumble to powder unless they are kept in a moist atmosphere. All the zeolites lose a portion of their " water of crystallization " in dry air (over sulphuric acid), and a considerable portion at a temperature of loo° C., increasing in amount to zoo° or 30o the actual amount lost depending not only on the temperature, but also on the tension of aqueous vapour in the surrounding atmosphere. In some species the remaining water is expelled only at a red heat, and is therefore to be regarded as " water of constitution." With the progressive loss of water there is a progressive change in the optical characters of the crystals. When a partially dehydrated and opaque crystal is exposed to moist air the water is reabsorbed, the crystal becoming again transparent and regaining its original optical characters. Not only may water be reabsorbed, but such substances as ammonia, hydrogen sulphide and alcohol may be absorbed in definite amounts and with an evolution of heat. The water of zeolites may therefore be partly driven off and reabsorbedor replaced by other substances without destroying the crystalline structure of the material, and it would thus seem„to differ from the water of crystallization of most other hydrated salts. Zeolites are minerals of secondary origin and in most cases have resulted by the decomposition of the felspars of basic igneous rocks: in fact their chemical composition is somewhat analogous to that of the felspars with the- addition of water. Nepheline and sodalite are often altered to zeolites. They usually occur as crystals lining the amygdaloidal and other cavities of basalt, melaphyre, &c. Usually two or more species are associated together, and often with agate, calcite and some other minerals. Less frequently they occur in cavities in granite and gneiss, and in metalliferous veins (e.g. harmotome) ; while only exceptionally are they primary constituents (e.g. analcite) of igneous rocks. Several species have been observed in the Roman masonry at the hot springs of Bourbonneles-Bains in France: and phillipsite has been dredged from the floor of the deep sea. See ANALCITE, CHABAZITE, HARMOTOME, HEULANDITE, NATROLITE, PHILLIPSITE, SCOLECITE, STILBITE. (L. J. S.)
End of Article: ZEOLITES
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