Aluminum, or Aluminium, gives us a large class of minerals. It is the metallic basis of alumina, which, combined with silica, is the chief component of our clay. Silicic acid and this base combine to form many minerals, and contains nearly all the precious stones. Corundums consist of pure alumina, and crystallize in the hexagonal system. The following stones are varieties of this mineral:—Sapphire, a beautiful blue; ruby, a red oriental; topaz, yellow oriental; amethyst, violet; all being sapphires more or less. The finest crystals are found in the East Indies in the sands of rivers and diluvial soils. The common corundum is very hard, and is used for polishing. Emery is well known, and is found in mica-slate. It is of a bluish-grey colour, and is also a polisher.
Alum forms another family, of which we may first mention aluminite, a “basic sulphate” of alumina and found in small quantities. Alum-stone is found in Italy. Alum occurs in large crystallized masses. (See illustration, fig. 464.) There are different minerals with a composition very similar to alum, in which the potassa base of alum is supplied by others. Thus we have the potassa alum, soda alum, manganese alum, ammonia alum—all being very nearly of the same constituents, and having similar crystals in the regular system, and are thus termed isomorphous, or similarly-formed. The potassa, or potash alum, is the commonest form, and is found abundantly in England, on the Continent of Europe, and the United States. Soda alum is called salfatarite, and magnesia alum pickeringite; manganese alum is apjohnite; phosphate of alumina is wavellite.
There are compounds of alumina and magnesia called Spinels. They are hard minerals, and the same isomorphous changes take place with them as are observable with the bases of alum. There are therefore varieties such as the spinel ruby found in the East Indies, very red in colour; the balas ruby not so red, and the orange-red, termed rubicelle. Ceylon is remarkable for some fine specimens of spinels. Chromite is like the spinel, but is known as chrome iron.
Zeolites are principally compositions of silica and alumina. They contain water, and are white, vitreous, and transparent. There are several varieties of them—natrolite, stilbite, etc. We will now pass on to the Clays, which are a very important family of the aluminum group.
There are a number of hard minerals which, when disintegrated, form certain earthy masses. These we term clay, or clays, which possess various colours and receive certain names, according to the proportion of metallic oxides they contain. All clays have an affinity for water, and retain it to a very great extent. The earth has also a peculiar smell. Clay is used in various ways; pottery, for instance, we read in the Bible as having been an employment from very ancient times. One attribute of clays, the retention of water, is of the greatest use to the world in providing moisture for plants and seeds. We may mention other characteristics of clay. It absorbs oil very quickly, and therefore is useful for removing grease-spots. It cannot be burned, so we have fire-bricks and fire-clay in our stoves and furnaces. There are various clays—pipe-clay, for instance, which is white; potters’ clay is coarser. There is porcelain clay as well as porcelain earth, of which more below. Yellow ochre and sienna are clays used by artists. Bole is a reddish clay; and tripoli is employed for polishing. There are, besides, andalusite, or chiastolite and disthene, crystalline forms of clay.
Porcelain has been known to the Chinese for centuries. In 1701 it was discovered in Germany by Böttcher, a chemist, who while endeavouring to make gold by Royal command, found the porcelain, and was thereby enriched. Porcelain earth is frequently found; is known in many places as kaolin, and usually comes from the decomposition of felspar. But in Cornwall we find it as decomposed granite, and the filtering process can be viewed from the railway, while both gneiss and granulite have been known to yield kaolin. It is also found in China, Saxony, and France. It is free from iron, and when ground and mixed with silicic acid, it is handed to the potter or moulder. After the vessels have been dried in the air they are put into the kiln, and then become white and hard. After that they are glazed in a mixture of porcelain earth and gypsum, or ground flints and oxide of lead, made fluid with water in the glazing of earthenware. The vessel is then put into the furnace again, or “fired,” as the process is called, and then comes out white, hard, and partly transparent.
Fig. 465.—Porcelain furnace.
