Agricultural geology . ,most usually in the form of a six-sided prism surmounted by a six-sidedpyramid. It is also frequently foundas irregular masses, or as aggregatesof small crystals and crystalline possesses no regular cleavage, butbreaks with an irregular and oftencurved fracture. The colour is veryvariable; some specimens are perfectlycolourless, clear and transparent, whileother varieties are opaque and milkyin appearance, or show various shadesof brown, yellow, pink, or purple; thevariations in colour are due to thepresence of minute quantities of someimpurity. Quartz is very
Agricultural geology . ,most usually in the form of a six-sided prism surmounted by a six-sidedpyramid. It is also frequently foundas irregular masses, or as aggregatesof small crystals and crystalline possesses no regular cleavage, butbreaks with an irregular and oftencurved fracture. The colour is veryvariable; some specimens are perfectlycolourless, clear and transparent, whileother varieties are opaque and milkyin appearance, or show various shadesof brown, yellow, pink, or purple; thevariations in colour are due to thepresence of minute quantities of someimpurity. Quartz is very hard, scratch-ing glass easily, and its specific gravityis about 2-65. It is totally unaffectedby any acids, except hydrofluoric acid,and is one of the most stable and resistant of all minerals. Note. Silica occurs widely in nature in other forms, differ-ing from quartz in their physical properties. They are eitherobscurely crystalline or amorphous, forming such substances Hatch, Mineralogy, 4th edition, London, Fig. 1. A simple crystal ofquartz, consisting of a hexa-gonal prism, terminatedby hexagonal pyramids. INTKODUCTION [CH. as opal, chalcedony, jasper and flint. Most of these possessa lower specific gravity than quartz (opal, 2-1) and are moreeasily attacked by acids and especially by alkalies. Felspar. Under this heading are comprised a large numberof minerals to which systematic mineralogists apply different names. Their chemical composi-tion is rather complex, but allcontain silica, alumina, and eitherpotash, soda or lime, or somemixture of the three latter convenience they may bedivided into the potash-felspar ororihoclase group, in which potashis dominant, and the soda-hme orplagioclase group, containing sodaor lime, or a mixture of the two groups differ slightlyin physical properties, though theirgeneral appearance is very chemical composition oforthoclase may be expressed by either of the formulae, KAlSijOg,or KjO . AlgOj. 6S
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectgeology
