Porphyritic—magmas from which, in solidifying, one mineral has crystallized out first and the crystals have grown to considerable size, while the rest have remained small.
Granitoid—magmas which have solidified slowly, so that all the minerals have crystallized completely, and the component crystals are large enough to be recognized readily, as in granite.
Fragmental—a term applied to the fragments which have resulted from explosive eruptions of igneous rocks. These fragments may be loose or consolidated. Volcanic ashes are typical.
Porous—a term applied to the lava near the upper surface, which is filled with gas cavities, such as pumice.
Amygdoloidal—is the term applied to porous lavas, when the cavities have been filled by other minerals, such as calcite or some of the zeolites.
In determining a rock, first decide whether it is igneous, sedimentary or metamorphic. The igneous character is recognized by its being either glassy, or composed of masses of crystals irregularly arranged, there being neither layering nor bedding.
CLASSIFICATION OF IGNEOUS ROCKS
| Texture | Excess of light colored minerals | Excess of dark colored minerals | ||||||
|---|---|---|---|---|---|---|---|---|
| Glassy | [obsidian], [perlite], [pumice], [pitchstone] | [scorias], [trachylyte], [basalt-obsidian] | ||||||
| Feldspar orthoclase | Feldspar Plagioclase | No feldspar | ||||||
| Mica and/or hornblende and/or augite | Mica and/or hornblende | with pyroxene | augite and/or hornblende and/or mica | |||||
| +quartz | -quartz | +quartz | -quartz | +olivine | -olivine | +olivine | -olivine | |
| Dense | [rhyolite] | [trachite] | [dacite] (felsite) | [andesite] (felsite) | [basalt] | [augitite] or [hornblendite] | ||
| Porphyritic | [rhyolite-porphyry] | [trachite-porphyry] | [dacite-porphyry] | [andesite-porphyry] | [basalt-porphyry] | [augitite-porphyry] | ||
| Granitoid | [granite] | [syenite] | [quartz-diorite] | [diorite] | [olivine-gabbro] | [gabbro] | [peridotite] | [pyroxenite] |
| Fragmental | [rhyolite], [tuff] or [breccia] | [trachite], [tuff] or [breccia] | [Dacite], [tuff] or [breccia] | andesite [tuff] or [breccia] | Basalt [tuffs] and [breccias] | |||
When it is located as igneous, turn to the key on [page 177] and decide as to which type of texture is present. If glassy, the color, luster and type of construction will place it. If the rock is crystalline, first decide whether feldspar is present, and if present, what type: then determine the dark mineral, and lastly whether quartz or olivine is present. In dense rocks the presence of quartz may be determined by trying the hardness, for none of the other constituents of igneous rocks have so great hardness. For example, if it is found that a rock is composed of orthoclase hornblende and quartz, and the texture is granitoid, it is granite: or if the rock is plagioclase feldspar and pyroxene of any sort, it is gabbro, etc.