LIGHTWEIGHT AGGREGATES 



349 



cur in common geologic terranes, although they may 

 not be ubiquitously distributed. The varieties of 

 Cenozoic extrusive rocks previously described are 

 good examples; they occur in a great many locali- 

 tes throughout the western conterminous United 

 States. In searching for recoverable reserves of ver- 

 miculite, however, one must find a suitable host 

 rock, then the site where it is in the proper geo- 

 chemical environment, and then the specific areas 

 where alteration has produced the mineral in the 

 concentration needed to be useful. For vermiculite, 

 the search must start with an understandfng of the 

 mineral and its origin. 



Mineralogically, vermiculite is a micaceous, hy- 

 drated magnesium iron aluminum sheet silicate, of 

 variable composition, composed of a basic unit 

 formed of two tetrahedral silicate sheets which face 

 each other and which are joined by an octahedral 

 sheet that contains the aluminum and iron (Grim, 

 1962) . This configuration is also the basic structural 

 unit of a single mica layer; the micas are built up 

 by a repetitive stack of these layers. Each layer, 

 on its upper and lower surfaces, has regularly ar- 

 ranged "holes" that overlap corresponding sites on 

 overlying layers. In biotite, potassium atoms occupy 

 these sites, and their electronic charges help to hold 

 the layers together. In vermiculite, only some of the 

 sites are occupied, and these only by magnesium, 

 but a double layer of water molecules also is present 

 in these interlayer regions. 



Commercially, "vermiculite" includes any mica- 

 ceous mineral that is capable of expansion, regard- 

 less of its chemical composition or the regularity of 

 its crystal structure. Pure vermiculite can be ex- 

 panded (exfoliated) up to 30 times its original 

 thickness when heated to about 1,600°-2,000°F 

 (870°-l,100°C), impure varieties expand less. The 

 exfoliation results from the flash conversion of the 

 interlayer water molecules into steam; the mineral 

 develops a high degree of porosity as the layers are 

 forced apart into an accordionlike structure. The 

 resulting material is easily compressed along the 

 line of expansion, but it has moderate strength in 

 other directions. Schroeder (1970b) has listed many 

 of the uses of exfoliated vermiculite; most are con- 

 nected with the construction industry, which in 1970 

 consumed more than 80 percent of the output, about 

 equally divided between aggregates for concrete, 

 cement, and plaster, and insulating uses, mostly as 

 loose fill. 



Vermiculite is an alteration product of a variety 

 of mafic minerals; it has not been found as a pri- 

 mary rock constituent. The geologic literature has 

 many descriptions of vermiculite derived from bio- 



tite, phlogopite, diopside, tremolite, augite, horn- 

 blende, olivine, chlorite, and serpentine. The altera- 

 tion may have included an intermediate biotite or 

 chlorite stage, for X-ray diffraction studies of the 

 vermiculite reveal it to be interlayered with biotite 

 or chlorite, although macroscopic and microscopic 

 examination may show no biotite or chlorite else- 

 where in the rock. Biotite is the common parent 

 mineral, and the similarity of the biotite and ver- 

 miculite crystal structures permits an intimate inter- 

 layering on a molecular scale in a series that ideally 

 extends either regularly or irregularly from pure 

 biotite through 1:1 biotite-vermiculite to pure ver- 

 miculite. In a general way, varieties in the series 

 from perhaps 10-50 percent vermiculite are called 

 hydrobiotite. The alteration has been considered to 

 be hydrothermal (Prindle and others, 1935; Leigh- 

 ton, 1954) or supergene (Murdock and Hunter, 1946 ; 

 Roy and Romo, 1957 ; Bassett, 1959) and partly both 

 (Boettcher, 1966). The character of the alteration 

 has a major impact on the resources of vermiculite. 



Almost all vermiculite deposits are associated with 

 mafic and ultramafic rocks, both igneous and meta- 

 morphic, that have been intruded by silicic and in 

 many instances by alkalic igneous rocks, and in 

 some instances by carbonatites. They can be broadly 

 grouped in three different types, but there are strong 

 points of similarity among all the classes: (1) large 

 ultramafic intrusives, such as pyroxenite plutons, 

 sometimes zoned, that are cut by syenites or alkalic 

 granites and by carbonitic rocks and pegmatites 

 (examples are deposits at Libby, Mont., and Phala- 

 borwa. Republic of South Africa) ; (2) small to large 

 ultramafic intrusives, such as dunites and unzoned 

 pyroxenites and peridotites, that are cut by pegma- 

 tites and syenitic or granitic intrusive rocks (ex- 

 amples are deposits in the Blue Ridge region of the 

 Southeastern United States) ; and (3) ultramafic 

 metamorphic rocks (usually amphibolite schists that 

 commonly are layered) that may have originally 

 been igneous rocks, sometimes cut by or in contact 

 with pyroxenites or peridotites, and cut by pegma- 

 tites (examples are deposits in the Enoree district, 

 South Carolina, and most of the deposits in Colo- 

 rado, Wyoming, and Texas). With the major excep- 

 tion of the deposit at Libby, almost all the vermicu- 

 lite deposits in the world appear to be in rocks of 

 Precambrian age. 



The world's largest vermiculite deposit at Libby, 

 Mont., is a zoned pyroxenite pluton of middle Cre- 

 taceous age that cuts Precambrian sedimentary 

 rocks of the Belt Supergroup (Boettcher, 1967) — a 

 type 1 deposit. The pluton has a core of biotitite in 

 a mass of biotite pyroxenite. A shell (like a ring 



