Zircon refracts and disperses light to a degree second only to the diamond, so that clear crystals are sought as gems. They are often called “Matura diamonds” because of their abundance at Matura, Ceylon. When the crystals are colorless or smoky they are termed jargons or jargoons; when of a red-orange hue, they are hyacinth or jacinth. Most of the zircon of gem-quality comes from Ceylon, where it is picked up as rolled-pebbles from the beds of brooks.
The most remarkable American locality for zircon is near Green River, in Henderson Co., N. C., where it is found abundantly in a decomposed pegmatite dike, from which many tons have been obtained. It is also found at Moriah, Warwick, Amity and Diana, N. Y., at Franklin Furnace, and Trenton, N. J., in the gold-bearing sands of California, etc.
[Cyanite]
Al₂SiO₅
[Pl. 40]
Occurs in long blade-like crystals in gneisses and schists; hardness, 7 at right angles to the length, and 4.5 parallel to the length; specific gravity, 3.6; color blue; luster vitreous; translucent on thin edges.
There are only a few blue minerals, and the way in which cyanite occurs in long thin blade-like crystals is entirely characteristic. If more is still wanted to determine this mineral, its unique character in having the great hardness 7 when scratched parallel to the length, and only 4.5 when scratched crossways, will settle any doubts.
The mineral sillimanite has the same composition as cyanite, but is fibrous in habit and has the hardness 6.5. If cyanite is heated to 1350° C. it changes its character and becomes sillimanite.
Cyanite is found as an accessory mineral in metamorphic rocks, such as gneiss and schist, at Chesterfield, Mass., Litchfield and Oxford, Conn., in Chester Co., Penn., in North Carolina, etc.
The [Mica] Group
The micas are very common minerals, easily recognized by their very perfect basal cleavage, as a result of which thin sheets, often less than a thousandth of an inch in thickness, readily split off. These are tough and elastic, which distinguishes mica from the chlorite group in which there is similar basal cleavage, but the sheets are not elastic.
Micas are complex silicates of aluminum, with potassium, iron, lithium, magnesium and hydrogen. They are one of the principle components of many granites, gneisses, and schists. This mineral is always crystalline, being in the monoclinic system, but occurring in six-sided prisms. The cleavage is so dominant a character that the crystal form is usually overlooked, as it is seldom requisite in determining this mineral. The size of the sheets of mica depend on the size of the crystals, the larger sheets expressing great slowness in cooling from the original magmas. Sometimes the crystals may be two or even three feet in diameter. The hardness is not great, ranging between 2 and 3. The specific gravity lies between 2.7 and 3.2. The color varies according to the composition, from silvery-white, through gray, pink, and green to black. The luster is vitreous to pearly, sometimes gleaming in the darker-colored varieties. The commoner types of mica are as follows: