If we again examine our piece of granite we shall see that besides the grains of glassy quartz there are several kinds of minerals which enter into its composition. Further examination will show us that these are all simple or compound silicates of the above-mentioned substances and so we believe that they were all formed when the mass was highly heated and subjected to great pressure. All granitoid, i. e. granitelike, rocks are believed to have been formed under these conditions.
§ 3. In the preceding paragraphs the term silicic acid has been used as though it referred to a single acid, and the word silicates as though the salts referred to were derived by the union of various bases with this acid. While this use is sanctioned by custom, chemists know that the term silicic acid refers not to one but to a series of acids, and that the salts of these acids differ as widely in their properties as do those of other similar series. It has been stated above that silicic acid is formed by the union of silica (SiO2) with water (H=O). SiOa 11=0 = H2SiOa, or silicic acid. This union actually takes place indicated and salts of the acid so formed are among the silicates most commonly met with; but we also know that one part of silica may and does unite with two parts of water to form another acid, Si02 + 21120 H4SiO4, with another well-known series of salts whose properties differ from those of the first-mentioned acid. In the same way a third acid, SiO2 + 3H=0 = H6SiO6, formed by the combination of one molecule of silica with three of water, and its union with bases, makes a series of known salts with properties quite different from those of either of the others. Theoretically, it would be possible to carry the series on in definitely, each added molecule of water making a new acid whose salts would have properties differing from those belonging to other members of the series. We do not yet know what the limits of the series really are, but feel certain that they are much wider than those indicated in most text-books on chemistry.
Formation of Silicates.—§ 4. When these acids unite with bases it is the hydrogen atoms which are crowded out by the base, H:SiO. + K20 = K2SiO8 + H.O. That is, if silicic acid (H2SiO8) is united with potash (K20) the K2 will replace the 112 and 'potassium silicate K2SiOa with water (H20) will be formed. Consequently the larger the number of replacable hydrogen atoms the acid carries, the larger number of atoms of base will it unite with and the larger will be the proportion of base to silica.
and so on.
It will be noticed that the proportion of base to silica increases regu larly as the series advances.
Other series of acids are formed by the union of two or more mole cules of silica to form a compound molecule which then combines with one or more molecules of water to form acids which, by their union with bases, for'hi series of salts containing a greater proportion of silica than those noted above. Examples are : 5. The last three paragraphs explain how it is that in the natural silicates the ratio of bases to silica varies indefinitely in both directions. The silicates so formed are not stable. Changing conditions of heat, pressure, mechanical force, etc., as well as variations in the dissolved material carried by earthwater, cause certain bases to be' replaced by others and even effect changes_in the composition of silicic acid itself. In this.way orthoclase feldspar (KA1Si3O8) is known to have been transform ed into albite (NaAlSi.02) through the replacement of potash by soda, and into anorthite (CaAl2Si2O8) through the replacement of potash by lime with a change of acid from 21120 + 3SiO2 = H•Si8O8 to 21120 + SiO2 = H+SiO4, as well as into a large number of minerals less closely related (§14). A glance at the table of analyses of any silicate given by Dana or Hintze will give evidence of such replacements, and will show the regular gradation from one mineral into another. For example, in the table of analyses of orthoclase we find the potash and soda content vary ing from 15.21% K-.65% Na to 2.62% K-10.52% Na with the greatest variety of intermediate forms showing the gradual passage from ortho clase to albite. It is for this reason that we often see the chemical formula for a mineral written in this way (Mg Fe) SiO., which means that a variable quantity of magnesia and iron are combined with the silica. This mineral (MgFe) SiO•, forms the intermediate member of a series of which MgSiO• and FeSiO4 are the extremes, and the formula covers all the forms which are produced as the composition is gradually changed from that of a silicate of magnesia to that of a silicate of iron or the reverse. Changes like these are continually taking place in the crystalline rocks, some minerals being removed or transferred into others while entirely new minerals are being introduced. .