Fig. 462.—Tabular form of heavy spar.
Strontium is the metallic basis of strontia. Sulphate of strontium is celestine, the mineral which colours the blow-pipe flame a fine crimson. There are certain varieties. Strontia salts are chemical preparations. A beautiful pyrotechnic “red fire” is produced by mixing nitrate of strontia with sulphur, antimony, charcoal, and chlorate of potassia.[24] There is a carbonate of strontia in the same crystalline system.
Magnesium. With this metal we have a large group of minerals. Magnesite is carbonate of magnesia, and occurs as talc-spar. The magnesium limestone crystallizes as bitter spar. This dolomite is like marble or common limestone, according to colour. Talc is a combination of magnesia with silicic acid. The hydrated carbonate is termed “white magnesia.” The sulphate of magnesia is found in Siberia, and we have boracite, and native magnesia called periclase. The sulphate is generally present in mineral waters, such as the Seidlitz and Epsom Springs. Large masses have been found in the extensive caverns of Kentucky and Tennessee, etc.
Fig. 463.—Crystal of augite.
Meerschaum is a hydrated silicate of magnesia. It is found in Anatolia and Negropont, also in France and Australasia. Serpentine is another similar composition. It is found in Cornwall, where it is carved into various ornaments. It is sometimes called snakestone. There are many other hydrated silicates of magnesia—viz., gymnite, picrosmine, pycrophyll, etc.
Fig. 464.—Alum crystals.
There is another family allied to magnesia, called Augites. These minerals are black or dark-green, and are contained in lava and basalt: Augite and Hornblende are the chief representatives of this family. The former crystallizes in the fourth system (see fig. 463), and there are several varieties—diallage, bronzite, diopside, etc. Hornblende belongs to the same system, and is a large factor in the composition of gneiss, syenite, and porphyry. Tremolite is a hornblende, and asbestos (amianthos), and mountain-cork are also varieties. The attribute of asbestos for sustaining heat is well known, and may be usefully employed for fire-proof purposes. The well-known jade-stone of China and calamite are other varieties.