When
required, the analysis is best made on the ignited residue, and
separately reported as "analysis of the insoluble portion."
Silicon only occurs in nature in the oxidised state; but the oxide
generally known as silica (SiO_{2}) is common, being represented by the
abundant minerals--quartz, flint, &c. Silica, combined with alumina,
lime, oxide of iron, magnesia and the alkalies, forms a large number of
rock-forming minerals. Most rock masses, other than limestones, contain
over 50 per cent. of silica. The following are analyses of some of the
commoner silicates; but it must be noted that these minerals often show
great variation in composition. This is more especially true of
chlorite, schorl, hornblende and augite.
[Table has been split into two because of its width--Transcriber]
------------------+--------+------------+------------+-------+----------------
| | | Ferric |Ferrous|
| Silica | Alumina | Oxide, | Oxide,| Fluorine,
|SiO_{2}.|Al_{2}O_{3}.|Fe_{2}O_{3}.| FeO. | Water &c.
------------------+--------+------------+------------+-------+----------------
Potash-felspar | 65.2 | 18.2 | 0.2 | -- |
Soda-felspar | 67.0 | 19.2 | -- | 0.3 |
Lime-felspar | 43.
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