Fig. 458.—Crystals of carbon.
Silicon. Silica occurs generally in combination with alumina, and never in a free state. In combination with oxygen it is called silicic acid. Silica, when crystallized, is usually called quartz.
Quartz has several varieties. We need only enumerate them, they will all be immediately recognized. We give illustrations of the crystals of quartz (fig. 459):—
1. Rock crystal appears in beautiful six-sided prisms.
2. Amethyst is coloured by protoxide of manganese, supposed by the ancients to be a charm against drunkenness.
3. Common quartz, or quartz rock, forms granite in combination, and is also known as “cat’s-eye,” “rose” quartz, etc.
4. Chalcedony, sometimes termed cornelian: used for seals, etc.
5. Flint: much used in potteries. “Flint and steel” have been superseded by phosphorus.
6. Hornstone: something like flint, resembling horn.
7. Jasper: of various colours; opaque and dull in appearance.
8. Silicious slate: a combination; used as a whetstone.
9. Agate: a mixture of quartz, amethyst, jasper, and cornelian; very ornamental.
10. Opal: a peculiar variety, containing water. It is not found in the form of crystal, but in vitreous masses. Its changeableness of hue is proverbial. The “noble” opal is much prized.
11. Smoky quartz, or cairngorm.
12. Onyx and Sardonyx.
Fig. 459.—Quartz crystals in various forms.
We now arrive at some minerals which contain metals.
Potassium. This metal is so frequently combined in minerals with alumina that we may refer to it with the latter in sequel. There are two natural potassa salts—nitre, and sulphate of potassa. Nitre is known as saltpetre, and is of great use in medicine. It is the chief ingredient in the composition of gunpowder.
Sodium. We have a number of minerals in this group—viz., nitrate of soda (nitratine), which occurs in large quantities in Peru; rock salt, chloride of sodium, known as salt. It crystallizes in the cubic system. Colour usually white, but it occurs in secondary rocks in company with gypsum, etc. It is sometimes of a reddish colour, or even green and yellow. Biborate of soda is borax, and is found in and on the borders of a Thibetian lake. There are several other combinations with soda: the sulphates of soda—viz., thenardite and glauberite, anhydrous and hydrated respectively, carbonate of soda, and so on.
Ammonia combinations occur in lava fissures, and are not often met with in consequence of their volatile nature.