Agricultural geology . nder 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 c
Agricultural geology . nder 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. 6Si02. The latter form is to be preferred. Thiscorresponds to Per cent. 64-7 18-4 100-0Hence the percentage of potash is seen to be high, and in factorthoclase is one of the most important sources of potassiumin soils. The felspars of the plagioclase group may be regarded asmixtures in any proportion of the two ideal compounds:. 2. A typical simple crystalof orthoclase felspar. ... albite, NagO . AlgOg . SSiOg, and anorthite, CaO . AUOgAlbite Anorthite 2Si02 Silica 68-7 43-2 Alumina 19-5 36-7 Soda 11-8 — Lime — 20-1100-0 100-0 I] MINERALS AND EOCKS 7 Special names have been applied to many intermediate varieties,but for our purpose it is unnecessary to enumerate these. Orthoclase crystalKzes in the obhque, and plagioclase in theanorthic system. The general forms of the crystals are howeververy similar. All felspars possess two very perfect cleavages,which are at right angles in orthoclase and nearly so in plagio-clase. The cleavage faces generally show a conspicuous pearlylustre. The colour is variable; generally white or grey, or apale shade of pink or red; green varieties are less a peculiar bluish iridescence is noticeable. Themineral is hard, though softer than quartz, and the specificgravity varies from 2-6 to 2-7, the varieties rich i
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectgeology
