More reliable figures for the relative abundance of the minerals are available for each of the two classes of rocks, igneous and sedimentary. The igneous rocks contain minerals in about the following proportions:
PERCENTAGE OF COMMON MINERALS IN IGNEOUS ROCKS
Feldspar 50 Quartz 21 Augite, hornblende, olivine, etc. 17 Mica 8 Magnetite 3 Titanite and ilmenite 1 --- 100
The sedimentary rocks contain minerals in about the following proportions:
PERCENTAGE OF COMMON MINERALS IN SEDIMENTARY ROCKS
Quartz 35 Feldspar 16 White mica 15 Kaolin (clay) 9 Dolomite 9 Chlorite 5 Calcite 4 Limonite 4 Gypsum, carbon, rutile, apatite, magnetite, etc. 3 --- 100
The sedimentary rocks comprise three main divisions: (1) The muds and clays, with their altered equivalents, shale, slate, etc.; (2) the sands, with their altered equivalents, sandstone, quartzite, quartz-schist, etc.; (3) the marls, limestones, and dolomites, with their altered equivalents, marble, talc-schist, etc. For brevity these groups are referred to respectively as shale, sandstone, and limestone. The proportions of minerals in each of these groups of rocks are as follows:
PERCENTAGE OF COMMON MINERALS IN SHALE, SANDSTONE, AND LIMESTONE
-----------+----------+-----------+----------- | Average | Average | Average | shale | sandstone | limestone -----------+----------+-----------+----------- Quartz | 31.91 | 69.76 | 3.71 Kaolin | 10.00 | 7.98 | 1.03 White mica | 18.40 | | Chlorite | 6.40 | 1.15 | Limonite | 4.75 | .80 | Dolomite | 7.90 | 3.44 | 36.25[1] Calcite | | 7.21 | 56.56 Gypsum | 1.17 | .12 | .10 Feldspar | 17.60 | 8.41 | 2.20 Magnetite | | .58 | Rutile | .66 | .12 | .06 Ilmenite | | .25 | Apatite | .40 | .18 | .09 Carbon | .81 | | -----------+----------+-----------+----------- Total | 100.00 | 100.00 | 100.00 -----------+----------+-----------+----------- 1: Includes small amount of FeCO_{ 3}.
In comparing the mineral composition of igneous and sedimentary rocks, it will be noted that the most abundant single mineral of the igneous rocks, and the most abundant mineral of the lithosphere as a whole, is _feldspar_; that next in order is _quartz_; and that third comes a group of dark green minerals typified by augite and hornblende, commonly called _ferro-magnesian silicates_ because they consist of iron and magnesia, with other bases, in combination with silica. The sedimentary rocks, which are ultimately derived from the destruction of the igneous rocks, contrast with the igneous rocks mainly in their smaller proportions of feldspars and ferro-magnesian minerals, their higher proportions of quartz and white mica (sericite or muscovite), and their content of kaolin, dolomite, calcite, chlorite, limonite, etc., which are nearly absent from the unaltered igneous rocks. Evidently the development of sediments from igneous rocks has involved the destruction of much of the feldspars and ferro-magnesian silicates, and the building from the elements of these destroyed minerals of more quartz, white mica, clay, dolomite, calcite, chlorite and limonite. The composition of the minerals of the sedimentary rocks is such as to indicate that the constituents of the air and water have been added in important amounts to accomplish this change of mineral character. For instance, carbon dioxide of the atmosphere has been added to lime and magnesia of the igneous rocks to make calcite and dolomite, water has been added to some of the alumina and silica of the igneous rocks to make kaolin or clay, and both oxygen and water have been added to the iron of the igneous rocks to make limonite.