64 FIELD MUSEUM OF NATURAL HISTORY 
value. This mineral is mined in large quantities for use as a 
flux and in the compounding of glazes, enamels, opaque glass, 
and similar materials. Minor uses and the chemical indus- 
tries also consume considerable quantities. 
Magnesite, the carbonate of magnesium, is represented by a 
small number of specimens, as it occurs in but few places, in 
which, however, it is extensively mined for its use as fire-resist- 
ing material, tiles, artificial marble, and even carbonic acid. 
Native epsom salt, sulphate of magnesia, from several localities, 
is also shown. Barite, the sulphate of barium, is represented 
by a series of specimens from many localities. The large series 
of crystals of this mineral shown is exceptional, as the massive 
form is usual in deposits of economic value. This material is 
largely ground for paint. The carbonate of barium, witherite, 
and the sulphate of strontia, strontianite, useful for colored fire 
and for sugar^ refining, are included here. 
Common salt is represented by a large series of specimens 
both of the crude salt and of the forms in which it is prepared 
for the market in different parts of the world. A large series of 
African salt is included here. 
Lithia compounds are represented by a large group of its ore, 
lepidolite, with an associated bright red tourmaline, rubellite, 
and also by its other common ore, spodumene, from various 
localities. 
Miscellaneous soda-bearing minerals are represented by a 
series of specimens including natural carbonate of soda of sev- 
eral varieties from different localities, and natural sulphate of 
soda. The fluoride of soda and aluminum, cryolite, and its 
uses, is represented by a complete series of specimens showing 
all stages of manufacture from the crude mineral to ordinary 
and caustic soda, alumina and alum. 
An unusually large and complete collection is that of the 
“Stassfurt salts” which are soluble compounds of potash and 
magnesia from which nearly the whole of the world’s supply of 
potash is drawn. With these are shown the more important 
potash compounds which are made from them. 
Native alum from various localities and alunite from which 
alum is extracted represent another class of minerals of some 
economic importance. 
