MAGNESIAN REFRACTORIES 



381 



tons, and that of plants recovering magnesia from 

 inland lakes, 200,000 tons (Industrial Minerals, 

 1970). 



GEOLOGY 



Magnesium is the eighth most abundant of the 

 elements in the earth's crust; it averages 2.09 per- 

 cent by weight of all igneous rocks. Magnesium 

 makes up about 0.13 percent of sea water; during 

 evaporation, it precipitates at an early stage, largely 

 with carbonate minerals, but it also precipitates at 

 the end stages, in bittern minerals. Magnesium car- 

 bonate also precipitates during evaporation from 

 certain inland lakes. 



MINERAL DEPOSITS 



This chapter deals largely with resources of mag- 

 nesite, brucite, and olivine; resources of dolomite, 

 evaporite minerals, and brines are described else- 

 where in this volume. 



MAGNESITE 



Magnesite occurs mainly in four types of deposits : 

 as crystalline masses replacing dolomite, as impure 

 crystalline masses replacing ultramafic rocks, as 

 cryptocrystalline masses in ultramafic rocks, and as 

 sedimentary beds and lenses. 



Crystalline magnesite deposits in dolomite. — Ac- 

 cumulations of crystalline magnesite occur as mas- 

 sive lenses, stockworks, or disseminations in dolo- 

 mite or in limestone locally altered to dolomite. 

 Crystalline magnesite is light gray, gray, brown, 

 pink, or red, and its grain size ranges from 0.5 to 

 20.0 mm. Its chief impurities are calcium, contained 

 within the lattice of magnesite grains or within in- 

 cluded dolomite grains; iron, contained either in 

 breunnerite (ferroan magnesite) or as disseminated 

 specularite; silica, as quartz veins or as impregna- 

 tions of quartz and chalcedony; and silicate min- 

 erals such as talc, tremolite, anthophyllite, or en- 

 statite, occurring in individual grains or in veins 

 or lenses. Igneous dikes or stocks commonly cut 

 magnesite lenses. 



Crystalline magnesite deposits range in size from 

 pockets containing a few tens of thousands of tons 

 to irregular to lenticular bodies containing more 

 than 100 million tons. Impurities may range from 

 about 2 to 20 percent, excluding blocks rendered 

 unminable by veins of silica or silicate minerals or 

 by igneous dikes. Grade is as critical as size in de- 

 termining the worth of any given deposit, particu- 

 larly for magnesite used in the manufacture of high- 

 purity refractories. There is little market for mag- 

 nesite containing more than 4 percent CaO. 



Only two districts in the United States contain 



large deposits of crystalline magnesite: one at 

 Gabbs, Nev., and the other in Stevens County, Wash. 

 Small occurrences are known in New Mexico and 

 Texas (Vitaliano and Callaghan, 1956; Vitaliano 

 and Cleveland, 1966; Schilling, 1968; and Bennett, 

 1941, 1943). 



Crystalline magnesite deposits in ultramafic 

 rocks. — Magnesite mixed with talc and with or 

 without quartz occurs as lenses replacing dunite or 

 serpentinized dunite. Deposits of this type are as 

 variable in size as those in dolomite. They form 

 massive lenses or occur as shells around ultramafic 

 bodies. The only crystalline magnesite deposits in 

 ultramafic rocks in the United States are found in 

 north-central Vermont. These deposits consist of 

 gray to faintly greenish gray rock, in which the 

 magnesite occurs as grains or clusters of grains 

 ranging from 0.01 to 30.0 mm in diameter in a 

 matrix of fine-grained talc. Proportions of magne- 

 site to talc in these deposits range from 38:62 to 

 50:50; the magnesite is low in lime but contains 

 several percent iron; the rock as a whole also con- 

 tains magnetite as an accessory mineral. A deposit 

 near Timmins, in Ontario, Canada, contains 48-72 

 percent magnesite, 6-20 percent talc, and 11-21 

 percent quartz (Chidester, 1962; and GriflSs, 1972). 



Cryptocrystalline magnesite deposits. — Crypto- 

 crystalline magnesite, also known as bone magnesite, 

 occurs as nodules, veins, and stockworks in ser- 

 pentinized zones of ultramafic rocks; it also is 

 found as small deposits in tuffs. Bone magnesite 

 resembles unglazed porcelain and generally is dead 

 white but locally is tinted gray, green, or pink. 

 Impurities include serpentine, enclosed in or bor- 

 dering nodules and associated vein minerals, as well 

 as opal, chalcedony, several hydrated magnesium 

 silicate minerals, and rarely calcite or dolomite. 

 Bone magnesite itself may contain traces to small 

 percentages of lime and iron. 



Bone-magnesite deposits tend to be much smaller 

 than crystalline deposits ; the largest group of veins 

 in the United States, at Red Mountain, Calif., origi- 

 nally contained about 1 million tons of magnesite. 

 Individual veins range from a few feet to hundreds 

 of feet in length and depth, and from fractions of 

 an inch to tens of feet in width. The wider veins 

 tend to consist of separate magnesite nodules in 

 serpentine. 



Most bone-magnesite deposits occur in ultramafic 

 rocks in California, but several are known in Ore- 

 gon and Pennsylvania. Deposits of bone magnesite 

 replace bedded rhyolitic tuff in eastern Nevada 

 (Davis, 1957; Bodenlos, 1950a; and Vitaliano, 

 1951). 



