[Hornblende]
(CaMgFe)₃(SiO₃)₄CaMgAl₂(SiO₄)₃
[Pl. 37]
Occurs in well-defined crystals, in grains and in masses; hardness, 5.5; specific gravity 3.2; color black, dark-green, or dark-brown; luster vitreous; translucent on thin edges.
In composition hornblende corresponds to augite, but occurs in long slender, six-sided crystals with cleavage planes intersecting at 55°, so that it is a typical amphibole. It occurs in a very wide range of rocks, such as granite, syenite, diabase, and gabbro; and in such metamorphic rocks as schists and gneisses; and sometimes igneous rocks are made up almost entirely of hornblende, when they are known as amphibolites or hornblendite. It is found all through the New England States, down along the Piedmont Plateau, through the Blue Ridge Mountains, and in many of the western mountainous areas.
The [Garnet] Group
The garnets are a series of double silicates, which occur with surprisingly uniform characters. They are all isometric, and occur either as dodecahedrons, or as the 24-sided figure (the trapezohedron), which is formed by the beveling of the edges of the dodecahedron, and developing these new faces to the exclusion of the dodecahedron faces. Combinations of the dodecahedron and trapezohedron (36 faces) may occur. All the garnets have a hardness of 7 to 7.5, and the specific gravity runs from 3.2 to 4.3, according to the composition. In size they run from as small as a grain of sand up to as large as a boy’s marble, and occasionally even to four inches in diameter. The color varies with the composition, from colorless to yellow, red, violet, or green. There is no cleavage, and the luster is always vitreous.
Garnets are usually accessory minerals, found in metamorphic rocks, though they are sometimes also present in granites and lavas. They are always segregations which have taken place in the presence of high temperatures. When clear and perfect several of the garnets are used as gems. On the other hand some of the common garnets occur in such quantities that they are crushed and used as abrasives, for such work as dental polishes, or for leather and wood polishing.
The following is the composition of some of the commoner garnets.
| Ca₃Al₂(SiO₄)₃ | = grossularite |
| Mg₃Al₂(SiO₄)₃ | = pyrope |
| Fe₃Al₂(SiO₄)₃ | = almandite |
| Mn₃Al₂(SiO₄)₃ | = spessartite |
| Ca₃Fe₂(SiO₄)₃ | = andradite |
| Ca₃Cr₂(SiO₄)₃ | = uvarovite |
Grossularite is chiefly found in crystalline limestones, which have resulted either from contact with lavas, or from general metamorphism of impure limestones. These garnets are colorless to white, or more often shades of yellow, orange, pink, green or brown, according to traces of impurity which they may contain. The cinnamon-colored variety from Ceylon is termed cinnamon stone, and is a fairly popular gem.
Pyrope is a deep-red color and when perfect is highly prized as a gem. It is found in dark-colored igneous rocks, like lavas, or serpentines. Some of the finest come from South Africa, where they are found in company with the diamond.