The Magnesium Group
Magnesium is a silvery-white metal, easily tarnished by exposure to moist air. Because of its light weight, less than twice the weight of water, and strength, it is being substituted for aluminum, especially in airplanes, where the question of weight is crucial. It is also used in automobile and ship production and other machine industries, and in the manufacture of flares and incendiary bombs. Magnesium is obtained chiefly from magnesite, dolomite, and in the United States as a result of a recently developed process, from sea water. Magnesium has a considerable number of minerals, of which three are taken up here and several more under the head of silicates, where both magnesium and silicon are combined in a mineral.
[Spinel]
MgAlO₄
[Pl. 29]
Occurs mostly as isometric octahedrons; hardness 8; specific gravity 3.5; color, red, yellow, green, or black; streak white; luster vitreous; transparent on thin edges.
This is a rather rare mineral, but, when in clear crystals is considered one of the gems. It was early confused with corundum, and the red variety called ruby, as it was found in the same gem-bearing sands in Ceylon, Burma, and Siam. However the form of the isometric octahedron as compared with the hexagonal prism of the corundum, together with the lesser hardness are sufficient to distinguish the two easily. The crystals are usually octahedrons, but may have the corners cut or the edges beveled. Twins are not uncommon.
The standard color is a clear deep-red, and such a spinel is known in the gem trade as a spinel-ruby. If the color is rose-red, it is a Balas ruby; if orange, it is rubicelle, if of a violet tinge, almandine. When small quantities of other elements replace the magnesium, the color is greatly changed. For example a little iron present gives the crystals a dark-green to black color, and the spinel is known as ceylonite. If there is both iron and chromium present, the color becomes yellowish or greenish-brown, and this variety is picotite. When the impurities are iron and copper, the color becomes grass-green, and it is called chlorospinel. A form, in which the magnesium is completely replaced by iron, is black in color and termed hercynite, and occurs fairly abundantly in Westchester Co., N. Y. From Amity, N. Y., to Andover, N. J., there is a belt of granular limestone in which spinel of all colors is found. St. Lawrence Co., N. Y., is also a rich locality. Bolton, Mass., Newton, Sterling, and Sparta, N. J., North Carolina, Alabama, and California all yield spinel.
[Magnesite]
MgCO₃
Occurs in cleavable or compact porcelain-like masses; hardness 4; specific gravity 3.1; color white to gray; luster vitreous; translucent on thin edges.
Magnesite is white and brittle, and cleaves perfectly parallel to the faces of the rhombohedron, but it seldom occurs in crystals. It will effervesce in warm hydrochloric acid and has some resemblance to calcite, but can be distinguished by the greater hardness. It is still more like dolomite, both having the same color and cleavage, both effervescing in warm hydrochloric acid; but the magnesite has half a point greater hardness and the porcelainous appearance. Magnesite is used in toilet preparations, paper making, and mixed with asbestos, as a covering for heating pipes.
Magnesite is found in Massachusetts, Pennsylvania, Texas, and in large deposits in California and Washington.
[Dolomite]
(MgCa)CO₃
Pl. [19] & [29]
Occurs in crystals, or in cleavable or granular masses; hardness 3.5; specific gravity 2.8; color white to pink or gray; streak white; luster vitreous; transparent on thin edges.
Dolomite crystallizes in the hexagonal system, in rhombohedrons (hemihedral form), which are more or less modified by faces on the corners or edges. The cleavage is parallel to the rhombohedron, and it will effervesce in warm hydrochloric acid. Sometimes the crystal faces are curved, and when this is the case, dolomite is easily determined. Usually however dolomite resembles both calcite and magnesite. From the calcite it is distinguished by the greater hardness, and from magnesite by lesser hardness and not being porcelainous in appearance. Some of the commoner forms are shown on [Plate 29], crystals like C being found embedded in anhydrite and gypsum.
Magnesium is a common element and is likely to be present wherever lime is being deposited, so dolomite crystals are common, and much of the limestone is dolomitic.
It may be found in almost any limestone section of the country. Some of the finest crystals of dolomite however come from Roxbury, Vt., Smithfield, R. I., Hoboken, N. J., Lockport, Rochester, and Niagara Falls, N. Y., etc.