LIGHTWEIGHT AGGREGATES 



347 



miculite, in part reflecting the dominance of the 

 vermiculite market by a single company, and in part 

 reflecting the movement of perlite into the filter- 

 aids field at the same time as a decrease in perlite 

 use in concretes and plasters. 



Perlite, pumicite, and vermiculite share the com- 

 mon problems of open-cast mining, but because 

 many of the deposits are located in remote or 

 sparsely populated areas, public concern with the 

 mining operations has been minimal to date (1972). 

 That is not true of the processing plants. Large 

 amounts of fines are produced in the grinding cir- 

 cuits in the processing of perlite, and additional 

 amounts are produced in the explosive expansion 

 ("popping") of perhte and pumicite. Their disposal 

 is a problem from several aspects. Because both 

 materials are glasses, the fines are pointed, sharp 

 slivers and shards. Dust control is a major prob- 

 lem, both in preventing and alleviating pollution 

 beyond the limits of the plant itself and in protect- 

 ing plant workers from respiratory injuries and dis- 

 eases. These problems are not so severe for vermicu- 

 lite, which yields thin flakes to the fines ; where wet 

 classification methods are used, the disposal of 

 vermiculite slimes presents other difficulties. 



VOLCANIC ROCKS 



PERLITE 



Obsidian, perlite, and pitchstone are volcanic 

 glasses of generally silicic composition and are the 

 equivalents of rhyolites, rhyodacites, trachytes, or 

 latites. Among themselves, they differ chemically 

 primarily in water content; obsidian has no more 

 than 2 percent H2O, perlite has 2-5 percent, and 

 pitchstone has more than 5 percent. The general 

 range of composition (table 68) is SiOa from 70 to 75 

 percent and AI2O3 from about 10 to 15 percent. The 

 water content seems to be the only chemical 

 component critical to expansion, and even then 

 obsidians with as little as 0.2 percent H2O are 

 reported to have expanded satisfactorily (King, 

 1948). Originally, perlite meant a glassy rock char- 

 acterized by concentric cracks, the "perl" (from the 

 German "Perlstein") referring either to the resem- 

 blance of broken-out fragments to pearls or to the 

 pearly luster of the surfaces. Today's commercial 

 usage includes any glassy rock (except pumicite), 

 regardless of composition or origin, that can be 

 made to expand by heating to about 2,000°F 

 ( 1,100 °C). Most commercial perlites fit the petro- 

 graphic definition that emphasizes a 2- to 5-percent 

 water content. 



Many of the traditional applications of perlite are 



in the construction field, but an increasing percent- 

 age of the annual production over the past several 

 years has gone into filter aids (1964, 15 percent; 

 1970, 23 percent). Diatomite is the principal ma- 

 terial used in filtration, but perlite seems to be 

 making significant inroads into the market. In the 

 construction field the principal competition comes 

 from exfoliated vermiculite; in 1970 some 200,000 

 tons of perlite and 180,000 tons of vermiculite were 

 used. Schroeder (1970a) gave a rather detailed dis- 

 cussion of perlite processing, marketing, uses, and 

 substitutes. 



Perlite is found in three major associations; in 

 glassy zones in welded ash-flow tuffs, in lava flows 

 from volcanic domes, and in the wall zones of fel- 

 sitic intrusive plugs and dikes. Smith (1960) has 

 described the central zone of dense welding in a 

 simple cooling unit of an ash flow as a glass with 

 virtually no porosity, composed of completely col- 

 lapsed pumice and ash, and bordered successively, 

 both above and below, by a zone of partial welding 

 and a zone of no welding. Sparse phenocrysts of 

 feldspar, quartz, biotite, and hornblende may be 

 present in all the zones. Ash flows may be deposited 

 before cooling of an underlying one has proceeded 

 very far; as a result, composite cooling units may 

 result. In these, the zones of dense welding may 

 coalesce, and the intermediate zones of partial weld- 

 ing and no welding are obliterated. In such a stack 

 of flows, very thick densely welded zones of glass 

 can develop. Characteristically, the welded zones 

 pass laterally into zones of devitrification toward 

 the central area of the cooling unit ; thus, a zone of 

 glass borders and envelops a zone of felsitic tuff. 

 However, if cooling is suflSciently rapid to prevent 

 crystallization, the glass may be preserved for some 

 millions of year. Preservation of glasses, regardless 

 of origin, is increasingly rare from the Pliocene 

 back to the beginning of the Tertiary and is very 

 rare in rocks older than that. 



The glass that develops in flows and in volcanic 

 domes represents the rapidly cooled lava liquid, 

 rather than collapsed pumice. Glass develops near 

 the surface of the flow or dome and may be rolled 

 under as the flow advances. In the process it may 

 be brecciated, remelted, and annealed. As the flow 

 comes to a stop, a zone of glass can be near its 

 surface and near its base, and if cooling is rapid 

 enough the zones may coalesce to make a flow that 

 is virtually all glass. As with the ash-flow units, 

 the glass may pass laterally into a felsitic central 

 zone. In a somewhat analogous manner, a selvage 

 of rapidly chilled glass can be formed at the con- 



