192 



UNITED STATES MINERAL RESOURCES 



The most conspicuous features of diatomite are 

 the light weight, high porosity, and whiteness of 

 the material. Scanning electron microscope photo- 

 graphs of diatomite show plainly the line aggregate 

 structure responsible for many remarkable prop- 

 erties of the rock (Gillott, 1969, p. 104). Dry blocks 

 of crude diatomite have an apparent density of 

 20-40 pounds per cubic foot, only about half that 

 of water, although the specific gravity of the opaline 

 silica of diatom frustules is about twice that of 

 water. Dry diatomite powder has an apparent 

 density of only 5-16 pounds per cubic foot. Diatom- 

 ite is highly porous, and the porosity increases with 

 increasing temperature up to about 700° C. It has 

 low thermal conductivity and the melting point 

 ranges from about 1,400° to 1,750°C; certain im- 

 purities lower the melting point considerably. The 

 rock can absorb 1V2-3 times its weight of water; 

 the absorptive capacity is increased when the com- 

 bined water of the opaline silica is driven off by 

 calcination. Diatom products also have useful ad- 

 sorptive properties (Anderson and others, 1947). 

 Diatomite has a dull earthy luster and generally is 

 white or nearly white, although both impurities and 

 water content affect the color. Diatomite ranges 

 from massive to finely laminated and, ordinarily, 

 it is easily friable. The opaline silica of diatoms has 

 a hardness of 4-6 on the Mohs scale, but the rock 

 itself has an apparent hardness of only 1.5 



Chemical analyses of diatomite generally show 70- 

 90 percent SiOa, 2-10 percent AI2O3, and variable 

 but small percentages of FejOa, FeO, TiOa, P2O5, 

 CaO, MgO, NasO, and K2O (Cressman, 1962, table 

 3; Cummins, 1960, table 1). 



The physical properties of diatomite vary con- 

 siderably from one deposit to another and may vary 

 from bed to bed in a single deposit. The varieties 

 and commercial grades of diatomite depend mainly 

 on (1) the kind and amount of impurities in the 

 crude material, and (2) the aggregate effect of the 

 shape, size, arrangement, and relative proportions 

 of the constituent diatoms. The commercial value 

 of diatomite generally is affected adversely by im- 

 purities such as clay, volcanic ash, sand, carbonate, 

 and organic matter. Also, the commercial quality 

 of the material is lessened if the constituent diatom 

 frustules are crushed or broken. 



USES 



Diatomite has hundreds of specific uses in 10 

 principal categories: (1) filters, (2) fillers (see 

 "Pigments and Fillers"), (3) insulating materials, 

 (4) mild abrasives, (5) absorbents, (6) catalyst 

 carriers, (7) reactive-silica source, (8) structural 



materials (see "Lightweight Aggregates"), (9) 

 additive or pozzolan for concrete, and (10) condi- 

 tioner or anticaking agent. The first category, filters, 

 accounts for more than half of the diatomite con- 

 sumed in the United States — 58 percent in 1969 

 (West, 1971, table 2). Processed diatomite is an 

 especially effective filter aid, or fine powder, that is 

 mixed with a liquid to be filtered; the filter aid 

 forms a continuously building porous cake on the 

 filter medium and retards clogging of the filter 

 pores by trapping impurities. The suitability of 

 diatomite for filtration depends largely on the shapes 

 and sizes of the constituent diatoms. Diatomite that 

 contains a substantial percentage of long hairlike 

 diatoms together with fragments of discoid diatoms 

 generally is best for retention of fine impurities 

 while allowing a high rate of flow; rock of this 

 type is mainly of marine origin. Diatomite has a 

 role in the purification and clarification of a host of 

 chemicals, beverages, foods, pharmaceuticals, and 

 other liquids. Its use in water purification expanded 

 greatly following development of diatomite filters 

 for military use during World War II. 



The chemical inertness of diatomite (readily solu- 

 ble only in hydrofluoric acid and strongly alkaline 

 solutions) makes the material especially useful as 

 a filler, extender, or diluent in many products, in- 

 cluding paints, papers, insecticides, plastics, and 

 fertilizers. This second category of uses accounts 

 for about 20 percent of the diatomite consumed in 

 the United States (West, 1971, table 2). 



PRODUCTION 



The United States leads the world in diatomite 

 production — followed by the U.S.S.R., Denmark, 

 France, West Germany, and Italy — and is the domi- 

 nant producer of filtration-quality material (West, 

 1971). Both the United States and world production 

 increased during the 20-year period 1949-68 (fig. 

 23). The United States exports substantial amounts 

 of prepared diatomite — 176,000 short tons in 1969, 

 valued at $13,510,000 (West, 1971, table 4) — 

 mainly filtration-grade material. Although transpor- 

 tation is a major factor in the cost of diatomite 

 products, high-quality filter-aid material is exported 

 regularly from the Western United States to Euro- 

 pean and other distant markets. United States im- 

 ports of diatomite are small — 47 short tons in 1969 

 (West, 1971, p. 484). 



GEOLOGIC ENVIRONMENT 



Diatoms are one of the chief agents by which 

 silica is removed from solution in marine and fresh 

 waters; their role as consumers of dissolved silica 



