[152] Small unrecorded amount.
[153] Nine months, Jan.-June and Oct.-Dec.
CHAPTER XXI
MAGNESITE
By R. W. Stone
USES OF MAGNESITE
Magnesite and its derived products are used in a variety of industries, the most essential of which, beyond doubt, is metallurgy. Owing to the high fusion point and chemical inertness of the oxide of magnesium, magnesite is one of the principal minerals used in the metallurgical and other industries where highly refractory material is required. For this purpose dead-burned magnesite is used in the form of brick or of grains. Brick and shapes are employed for lining open-hearth steel furnaces, welding, heating, and melting furnaces, reverberatories, settlers, and furnaces for refining lead, copper converters, and electrical furnaces. Crushed or granular magnesite is used for lining the bottoms of open-hearth steel furnaces, and in making crucibles and cupels.
In the manufacture of the cement known as oxychloride or Sorel cement the quantity of magnesite used is exceeded only by that used for refractory purposes. This cement is employed largely for sanitary flooring, and to a less extent for wall plaster, both interior and exterior. It is used also instead of Portland cement for some forms of exterior construction where quick and strong set is required. Magnesite is used in the manufacture of wood-pulp paper on the Pacific Coast, in fire-resisting paint, as a non-conductor of heat in pipe and furnace coverings, and in the manufacture of magnesium chloride, light carbonate, and other products, including metallic magnesium.
GEOLOGICAL OCCURRENCE
Deposits of magnesite are widely distributed throughout the world and occur in two distinct forms, amorphous and crystalline. Amorphous magnesite, the most common form, is fine-grained, and compact; it is usually found in veins or masses in serpentine resulting from the alteration of magnesia-rich rocks of the peridotite family. To this group belong the Grecian deposits, nearly all the California deposits, and those in Mexico, Venezuela, and other parts of the world. Crystalline magnesite is medium to coarse grained, and occurs as masses in limestone, dolomite or associated sediments which have been metamorphosed. The principal deposits of this class are those in Austria, Hungary, Quebec, and Washington.
Deposits of magnesite are regarded as having originated in three ways. The massive non-crystalline variety, such as that in California and Greece, is believed to have been formed by the decomposition of serpentine. Magnesite deposits near Bissel, California, and on Muddy River, near St. Thomas, Nevada, are said to be of sedimentary origin. The Austro-Hungarian, Washington, and Quebec deposits are regarded as resulting from the replacement of calcareous sedimentary rocks by magnesian-bearing solutions.
Magnesite deposits that occur as veins in connection with serpentinized magnesian rocks probably are formed both from the breaking down of the serpentine-making minerals and from the serpentine itself. It seems probable that usually both serpentine and magnesite are formed in the process of decay of the original minerals in peridotite and the allied basic rocks, and that during the decay of the serpentine the formation of magnesite continues. In any case the magnesia or magnesian mineral is changed to carbonate, dissolved by percolating water charged with carbon dioxide, and precipitated in cracks and crevices as veins. When formed in this way the magnesite occurs in large and small veins, lenses, and stockwork, and its distribution and extent are erratic. It seems fair to assume that these deposits may extend to the limit of depth of easily circulating surface waters, which in favorable conditions may be several hundred feet. Faulting, on the other hand, is as likely to cut the veins off in depth as in length. Any estimate of available tonnage of magnesite in deposits of this type therefore is unwarranted in advance of development work.