Chapter III: Section II: Simple Minerals

All substances found in the earth or upon its surface, which are not the products of art or of organic life, are regarded by the mineralogist as _simple minerals_. About four hundred mineral species are known, and the varieties are much more numerous; but only a small number of them are so abundant as to claim the attention of the geologist. An acquaintance with the following species is, however, necessary.

_Quartz_ is probably the most abundant mineral in nature. It is composed wholly of silex. Its specific gravity is 2.65. It is the hardest of the common minerals, gives sparks with steel, scratches glass, and breaks into irregular angular fragments under the hammer. When crystallized, its most common form is that of a six-sided prism, terminated by six-sided pyramids. When pure, it is transparent or translucent, and its lustre is highly vitreous. The transparent variety is called _rock crystal_. When purple, it is _amethyst_. When faint red, it is _rose quartz_. When its color is dark brown, or gray, and it has a conchoidal fracture, it is _flint_. When quartz occurs in white, tuberous masses, of a resinous lustre and conchoidal fracture, it is _opal_. The precious opal is distinguished by its lively play of colors. _Jasper_ is opaque, and contains a small per cent, of oxide of iron, by which it is colored dull red, yellowish red or brown. The light-colored, massive, translucent variety is _chalcedony_. The flesh-colored specimens are _carnelian_. When composed of layers of chalcedony of different colors, it becomes _agate_. Several of the varieties of quartz, such as amethyst, opal, carnelian and agate, are used to considerable extent in jewelry.

_Felspar_ is composed of silex, alumina and potassa. It resembles quartz, but it is not as hard, cleaves more readily, and is not generally transparent. Its specific gravity is 2.47. Its lustre is feebly vitreous, but pearly on its cleavage faces. Its color is sometimes green, but generally dull white, and often inclined to red or flesh-color.

_Mica_ is composed of the same ingredients as felspar, together with oxide of iron. Its specific gravity is nearly three. It is often colorless, but frequently green, smoky, or black. It may be known by its capability of division into exceedingly thin, transparent, elastic plates.

_Hornblende_ is composed of silex, alumina and magnesia. Its specific gravity is a little above three. Its color is generally some shade of green. When dark green or black, whether in a massive or crystalline state, it is _common hornblende_. When light green, it is _actinolite_. The white variety is _tremolite_. When it is composed of flexible fibres, it is _asbestus_; and when the fibres have also a silky lustre, it is _amianthus_.

_Augite_ or _Pyroxene_ has, till recently, been considered as a variety of hornblende. Its specific gravity is slightly different; its composition is the same, and in general appearance it is not easily distinguished from hornblende. It has, however, been made a distinct species, because its crystalline form is different.

_Hypersthene_ is composed of silex, magnesia and oxide of iron. Its specific gravity is 3.38. It closely resembles hornblende. The lustre of its cleavage faces is metallic pearly. Its color is grayish or greenish black.

_Talc_ is composed of silex and magnesia. Its specific gravity is 2.7. It resembles mica in its general appearance and in its lamellar structure, but it is easily distinguished from it by its plates being not elastic, and by its soapy feel. Its color is generally some shade of green. _Soapstone_ is an impure variety of talc, of a light gray color, earthy texture, and is unctuous to the touch. _Chlorite_, another impure variety, is a dark green rock, massive, easily cut with a knife, and unctuous to the touch.

_Serpentine_ is composed of silex and magnesia. Its specific gravity is 2.55. It is generally massive, unctuous to the touch, and of a green color. It is often variegated with spots of green of different shades. With a mixture of carbonate of lime it forms the _verd antique marble_.

_Carbonate of Lime_, or common limestone, is composed of carbonic acid and lime. Its specific gravity is 2.65. It presents a great variety of forms. In a crystalline state it is generally transparent, and when so, possesses the property of double refraction. It may be distinguished from every other common species by its rapid effervescence with acids. It readily cleaves parallel to all the faces of the primary form, which is a rhombohedron.

_Sulphate of Lime_, or Gypsum, is composed of sulphuric acid and lime. Its specific gravity is 2.32. When crystalline, it has a pearly lustre, is transparent, and goes under the name of _Selenite_. _Common Gypsum_ resembles the other earthy limestones, but it is softer, and may be readily distinguished by its not effervescing with acids.

To the minerals now enumerated may be added the following, which are of frequent occurrence, but not in great quantities; namely, carbonate of magnesia, oxide of iron, iron pyrites, rock-salt, coal, bitumen, schorl and garnet.

These simple minerals, either in separate masses or mingled more or less intimately together, compose almost wholly the earth's crust.