382 



"UNITED STATES MINERAL RESOURCES 



Sedimentary magnesite. — Sedimentary magnesite 

 is a brown to gray carbonate rock which probably 

 formed by evaporation. Individual beds may cover 

 several tens of acres and range in thickness from 

 inches to several feet. Such magnesite is inter- 

 bedded with dolomite, clastic rocks, or strata of vol- 

 canic origin. Although the grade of some sedi- 

 mentary magnesite is high, the thin beds cannot be 

 mined economically. Deposits are limited to several 

 States in the Southwest (Rubey and Callaghan, 

 1936; and Vitaliano, 1950). 



PROSPECTING 



Deposits of crystalline magnesite may be over- 

 looked because magnesite differs from crystalline 

 dolomite only slightly in luster and specific gravity. 

 In some deposits, magnesite is more resistant to 

 weathering than dolomite and hence forms bolder 

 outcrops. Weathered surfaces may differ slightly in 

 color from the weathered surfaces of dolomite. Talc- 

 bearing crystalline carbonate rocks should be 

 checked for magnesite, although talc also occurs in 

 dolomite. Chemical or mineralogical tests thus may 

 be needed to identify new prospects. 



Crystalline carbonate masses in ultramafic rocks, 

 particularly if mixed with talc, probably consist of 

 magnesite; such material weathers shades of rusty 

 brown owing to its contained iron. 



Bone magnesite forms highly distinctive dead- 

 white outcrops and float, perhaps associated with 

 chalcedony and opal, which contrast like whitewash 

 against the somber shades of the host ultramafic 

 terrane. 



Sedimentary magnesite perhaps is the most diffi- 

 cult type of magnesite deposit to recognize, because 

 it has no clearly distinguishing physical features or 

 characteristic geological associations. 



Most carbon dioxide needed to form magnesite 

 deposits probably was derived from deep-seated ig- 

 neous sources, although some may have been fur- 

 nished through thermal dissociation of underlying 

 carbonate rocks; a very small amount may have 

 been carried by ground water. Ultramafic rocks 

 furnished the magnesia now found in their con- 

 tained magnesite deposits, but the source of mag- 

 nesia in crystalline deposits in dolomite is less 

 certain. One hypothesis proposes that the magnesia 

 was produced during thermal dissociation of under- 

 lying dolomite. Another hypothesis suggests that 

 crystalline magnesite deposits are metamorphosed, 

 recrystallized lenses of sedimentary magnesite, but 



this hypothesis does not explain the obvious replace- 

 ment nature of many if not most crystalline de- 

 posits (Faust and Callaghan, 1948; Schilling, 1968; 

 Vitaliano and Cleveland, 1966; Bodenlos, 1950a, 

 1954; and Lovering, 1969). 



Sedimentary magnesite in the southwestern States 

 formed in closed basins and therefore is of evapor- 

 ite origin. 



Brucite [Mg(0H2)] in minable concentrations 

 rarely is found. Two such deposits are associated 

 with magnesite at Gabbs, Nev. ; there the mineral 

 occurs as lenses of massive white to light-brown 

 material resembling old-fashioned laundry soap, 

 which is soft enough to scratch with the fingernail. 

 Impurities include veinlets and grains of dolomite, 

 magnesite, talc, forsterite, and periclase; the de- 

 posits also are cut by igneous dikes. The deposits 

 lie close to a stock of granodiorite postdating mag- 

 nesite deposition, suggesting that the intrusive 

 locally drove off carbon dioxide in magnesite and 

 formed magnesium hydroxide. 



Besides the Gabbs occurrence, brucite has been 

 found within the United States, in small amounts 

 only in New Mexico (Schilling, 1968; Davis, 1957) 

 and Arizona (Ericksen, 1969). 



OLIVINE 



Olivine [(MgFe)2 SiO*] is a common mineral in 

 quartz-free igneous rocks; its magnesium-rich va- 

 riety, forsterite, forms the rock called dunite. For- 

 sterite alters readily to serpentine minerals; thus, 

 masses of fresh dunite are not common. Within the 

 United States, fresh dunite makes up most of the 

 Twin Sisters Mountains, east of Bellingham, Wash., 

 and it occurs as smaller masses in North Carolina 

 and Georgia (Stuckey, 1965; Moen, 1969). 



OTHER SOURCES 



Deep-well brines in Michigan and in other States 

 have provided recoverable magnesium compounds, 

 and the brines of Great Salt Lake are under devel- 

 opment for the same purpose. (See "Evaporites and 

 Brines.") Magnesium compounds are extracted from 

 sea water in recovery plants on the Atlantic, Pacific, 

 and Gulf coasts, the annual rated capacity of which 

 in 1970 was 570,000 tons MgO. Dolomite is quarried 

 in various States. Dolomite used to recover mag- 

 nesium metal formerly was quarried in Connecticut, 

 New York, Ohio, Michigan, Mississippi, and Wash- 

 ington. Deposits of evaporite minerals in New 

 Mexico remain largely untapped as a source of 

 magnesium-bearing compounds. 



