The chemical composition of a few granites from different parts of the world is given below:—
| SiO2. | Al2O3. | Fe2O3. | FeO. | MgO. | CaO. | Na2O. | K2O. | |
| I. | 74.69 | 16.21 | .. | 1.16 | 0.48 | 0.28 | 1.18 | 3.64 |
| II. | 71.33 | 11.18 | 3.96 | 1.45 | 0.88 | 2.10 | 3.51 | 3.49 |
| III. | 72.93 | 13.87 | 1.94 | 0.79 | 0.51 | 0.74 | 3.68 | 3.74 |
| IV. | 76.12 | 12.18 | 1.21 | 0.72 | 1.12 | 1.54 | 2.55 | 3.21 |
| V. | 73.90 | 13.65 | 0.28 | 0.42 | 0.14 | 0.23 | 2.53 | 7.99 |
| VI. | 68.87 | 16.62 | 0.43 | 2.72 | 1.60 | 0.71 | 1.80 | 6.48 |
I. Carn Brea, Cornwall (Phillips); II. Mazaruni, Brit. Guiana (Harrison); III. Rödö, near Alnö, Vesternorrland, Sweden (Holmquist); IV. Abruzzen, a group of hills in the Riesengebirge (Milch); V. Pikes Peak, Colorado (Matthews); VI. Wilson’s Creek, near Omeo, Victoria (Howitt).
Only the most important components are shown in the table, but all granites contain also small amounts of zirconia, titanium oxide, phosphoric acid, sulphur, oxides of barium, strontium, manganese and water. These are in all cases less than 1%, and usually much less than this, except the water, which may be 2 or 3% in weathered rocks. From the chemical composition it may be computed that granites contain, on an average, 35 to 55% of quartz, 20 to 30% of orthoclase, 20 to 30% of plagioclase felspar (including the albite of microperthite) and 5 to 10% of ferromagnesian silicates and minor accessories such as apatite, zircon, sphene and iron oxides. The aplites, pegmatites, graphic granites and muscovite granites are usually richest in silica, while with increase of biotite and hornblende, augite and enstatite the analyses show the presence of more magnesia, iron and lime.
In the weathering of granite the quartz suffers little change; the felspar passes into dull cloudy, soft aggregates of kaolin, muscovite and secondary quartz, while chlorite, quartz and calcite replace the biotite, hornblende and augite. The rock often assumes a rusty brown colour from the liberation of the oxides of iron, and the decomposed mass is friable and can easily be dug with a spade; where the granite has been cut by joint planes not too close together weathering proceeds from their surfaces and large rounded blocks may be left embedded in rotted materials. The amount of water in the rock increases and part of the alkalis is carried away in solution; they form valuable sources of mineral food to plants. The chemical changes are shown by the following analyses:
| H2O. | SiO2. | TiO2. | Al2O3. | FeO. | Fe2O3. | CaO. | MgO. | Na2O. | K2O. | P2O5. | |
| I. | 1.22 | 69.33 | n.d. | 14.33 | 3.60 | .. | 3.21 | 2.44 | 2.70 | 2.67 | 0.10 |
| II. | 3.27 | 66.82 | n.d. | 15.62 | 1.69 | 1.88 | 3.13 | 2.76 | 2.58 | 2.44 | n.d. |
| III. | 4.70 | 65.69 | 0.31 | 15.23 | .. | 4.39 | 2.63 | 2.64 | 2.12 | 2.00 | 0.06 |
Analyses of I., fresh grey granite; II. brown moderately firm granite; III. residual sand, produced by the weathering of the same mass (anal. G. P. Merrill).
The differences are surprisingly small and are principally an increase in the water and a diminution in the amount of alkalis and lime together with the oxidation of the ferrous oxide.
(J. S. F.)