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ZEOLITES

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Originally appearing in Volume V28, Page 973 of the 1911 Encyclopedia Britannica.
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ZEOLITES , a See also:

family of minerals consisting of hydrated silicates of alumina with alkalis or alkaline earths or both. The See also:water they contain is readily lost, and before the See also:blowpipe it is expelled with intumescence; hence the name zeolite, from the See also:Greek Eiv (to See also:boil) and M Oos (a See also: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 See also:common. Several See also:species have been distinguished, of which the following are the more important. See also:Apophyllite (q.v.) and pectolite (see See also:PYROXENE) are also sometimes included. Some of fhe chemical formulae given above are only approximate, since in some species the See also:composition varies between certain limits and can be best expressed by the isomorphous mixing of different molecules (see, for example, See also:CHABAZITE). They are all readily decomposed by hydrochloric See also:acid, 'usually with the separation of gelatinous See also:silica. By the See also: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 See also: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 See also:light they frequently exhibit See also:optical anomalies and a complex structure. The hardness (H.=3 -52) and specific gravity (2.0–2.4) are comparatively See also:low, and so are the indices of See also:refraction and the See also:double refraction.

The water of zeolites presents many points of See also:

interest. Laumontite loses water on exposure to See also:air, and the crystals soon crumble to See also:powder unless they are kept in a moist See also:atmosphere. All the zeolites lose a portion of their " water of See also:crystallization " in dry air (over sulphuric acid), and a considerable portion at a temperature of See also: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 See also:heat, and is therefore to be regarded as " water of constitution." With the progressive loss of water there is a progressive See also: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 See also:original optical characters. Not only may water be reabsorbed, but such substances as See also:ammonia, See also:hydrogen sulphide and See also:alcohol may be absorbed in definite amounts and with an See also: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. See also:Nepheline and See also:sodalite are often altered to zeolites. They usually occur as crystals lining the amygdaloidal and other cavities of See also:basalt, melaphyre, &c. Usually two or more species are associated together, and often with See also:agate, See also:calcite and some other minerals. Less frequently they occur in cavities in See also:granite and See also:gneiss, and in metalliferous See also:veins (e.g. See also:harmotome) ; while only exceptionally are they See also:primary constituents (e.g. See also:analcite) of igneous rocks.

Several species have been observed in the See also:

Roman See also:masonry at the hot springs of Bourbonneles-Bains in See also:France: and See also:phillipsite has been dredged from the See also:floor of the deep See also:sea. See ANALCITE, CHABAZITE, HARMOTOME, See also:HEULANDITE, See also:NATROLITE, PHILLIPSITE, See also:SCOLECITE, STILBITE. (L. J.

End of Article: ZEOLITES

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