348 



UNITED STATES MINERAL RESOURCES 



tact of a dike or plug with the country rock ; in this 

 type of occurrence the glass apparently devitrifies 

 most rapidly. 



Perlite very commonly surrounds cores of ob- 

 sidian, and there is a full range of occurrences from 

 obsidian containing a minor amount of perlite along 

 a network of fractures, through nearly equal 

 amounts of perlite and obsidian, to complete perlite. 

 Most obsidian cores range from about 1 inch in 

 diameter to microscopic. Ross and Smith (1955) dis- 

 covered a very marked, abrupt contrast in water 

 content between the obsidian cores (a few tenths of 

 1 percent) and the surrounding perlite (2-5 per- 

 cent). They attributed the contrast to hydration 

 of the glass after emplacement, on the basis of 

 marked differences in the indices of refraction be- 

 tween obsidian and perlite and the relative ease of 

 dehydration of perlite below 600°C as compared 

 with dehydration of obsidian between 800° and 

 1,000°C. They suggested that the water in obsidian 

 was pristine and that the water in perlite repre- 

 sented hydration from snow, rain, and ground 

 water. Investigations of the deuterium content of 

 the water in obsidian and perlite by Smith and 

 Friedman (1957) and Friedman and Smith (1958) 

 showed that most of the water in perlite was in- 

 deed of meteoric origin. These conclusions and the 

 increasing devitrification of glass with increasing 

 age are of importance to the consideration of perlite 

 resources. 



As with the other volcanic glasses, perlite is 

 widely distributed throughout the western conter- 

 minous United States (Jaster, 1956). In 1970 the 

 production of crude ore came from New Mexico (87 

 percent), Arizona, California, Nevada, Colorado, 

 Idaho, and Utah. A very few deposits have had 

 short-term production in Oregon and Texas; other 

 deposits are known in Montana, Wyoming, and 

 Washington. 



Basic data on which to build a detailed estimate 

 of the recoverable reserves of perlite in the United 

 States are lacking. Simply, from the broad geologic 

 knowledge of the very large volumes of extrusive 

 rocks available, no shortage is foreseeable. As an 

 example, some 600,000 tons of crude perlite was 

 produced in 1970, and about 420,000 tons of ex- 

 panded perlite was consumed, a ratio of about 1.5 

 tons of crude for 1 ton of expanded. The perlite 

 industry is moving toward less dependence on the 

 construction industry, and so an annual growth rate 

 (6 percent) should be assumed that can take into 

 account consumption in additional new, and perhaps 

 more rapidly growing, fields. Through the year 2000, 

 the cumulative need for crude perlite production will 



total about 48 million tons. Schilling (1960, p. 108) 

 estimated that in north-central New Mexico alone 

 "There is over 100 million tons — probably several 

 hundred million tons — of readily minable, commer- 

 cial-grade perlite in the No Agua deposit." This is 

 the district which produced nearly all the perlite 

 mined in 1970. Even if this estimate should be an 

 order of magnitude too large, a single district can 

 apparently furnish at least one-fourth of the antici- 

 pated demand. 



Numerous sizable deposits also are present in 

 Cahfornia (Chesterman, 1957, 1966), Arizona, Ne- 

 vada, and elsewhere in New Mexico, all with reserves 

 that appear to be in millions or tens of millions of 

 tons. Other large deposits, but fewer, are present in 

 Colorado (Bush, 1951, 1964), Utah, and Idaho. No 

 quantitative data are available to estimate the 

 amounts of paramarginal and submarginal resources, 

 but the paramarginal resources (determined more 

 by costs of stripping and transportation than by 

 lower expansibility or greater dilution) are likely 

 of the same order of magnitude as the reserves, 

 whereas the submarginal resources are considerably 

 smaller. Using the same approach as for pumice and 

 pumicite, the total resource can be approximated 

 very conservatively at about 650 million tons. 



The hydration of silicic volcanic glass by meteoric 

 water raises the problem of how far the glasses are 

 hydrated inward from their margins. The extent is 

 surely a function of the thickness and permeability 

 of the overlying rocks and of the structure, texture, 

 and attitude of the host rocks. In the absence of 

 data on the extent of hydration, any reserve or 

 resources estimate must be considered to be a 

 maximum. 



PUMICITE 



Laboratory investigations by the Oklahoma Geo- 

 logical Survey in 1944 revealed that pumicite was 

 capable of considerable expansion upon heating in 

 excess of 1,100°C (Burwell, 1949). Additional inves- 

 tigations by the Kansas Geological Survey in the 

 1960's provided many more data on temperatures 

 and strengths and suggested industrial uses as filter 

 aids and ultralightweight construction materials 

 (Bauleke, 1962; Hardy and others, 1965). Discus- 

 sion of geologic habit, reserves, and resources is 

 given in the section on "Pumice and Pumicite." 



VERMICULITE 



In contrast with all the other naturally occurring 

 lightweight aggregate materials, vermiculite is a 

 single mineral rather than an assemblage of min- 

 erals. The rocks that are suitable for use as light- 

 weight aggregates are abundant materials that oc- 



