90 



UNITED STATES MINERAL RESOURCES 

 Table 17. — Characteristics of berryllium minerals known to form large 



or hydrothermal. Each of these must be further sub- 

 divided : the pegmatitic into fine-grained unzoned 

 deposits and coarse-grained zoned deposits; the 

 hydrothermal deposits into several categories, in 

 general conforming to Lindgren's classification of 

 hypothermal, mesothermal, and epithermal. 



PEGMATITIC DEPOSITS 



The pegmatitic deposits that are significant as 

 sources of beryllium minerals are composed of 

 major amounts of quartz, sodic plagioclase, and 

 microcline, with or without spodumene, muscovite, 

 or lepidolite. The association of beryl with spodu- 

 mene is close, and few spodumene-bearing pegma- 

 tites lack beryl. Muscovite is also a common associ- 

 ate of beryl, especially in nonlithian pegmatites. 

 Commonly, however, the muscovite found with beryl 

 will not yield sheet mica, which limits the produc- 

 tion of beryl as a byproduct from mica mines. 



The fine-grained unzoned pegmatite dikes that 

 have attracted most attention as potential sources 

 of beryl are aggregates of albite, microcline, spodu- 

 mene, quartz, beryl, and muscovite. These are mod- 

 erately uniform in composition, differing from place 

 to place mainly in the size and orientation of the 

 mineral particles. Spodumene and microcline crys- 

 tals range in maximum dimension from a fraction 

 of an inch to a few feet, and lathlike crystals of 

 spodumene may be parallel or random in orienta- 

 tion. Albite, quartz, muscovite, and beryl form a 

 fine-grained matrix that surrounds the spodumene, 

 microcline, and lumps of quartz. Fine-grained beryl, 



generally white, constitutes about V2 percent of 

 the pegmatite. 



These pegmatites are important sources of spodu- 

 mene, scrap mica, and feldspar, but the small 

 amount and fine grain size of the beryl have pre- 

 vented its recovery, even in conjunction with flota- 

 tion of the other minerals. 



Most zoned pegmatites are composed of the same 

 minerals as the unzoned pegmatites. They differ in 

 that the proportions of minerals and the textures 

 of the rocks vary markedly from place to place in 

 a dike, generally changing from the wall to the 

 core of the pegmatite body. This internal structure 

 of pegmatite dikes is uniform throughout the world. 

 Near the walls the rock consists of plagioclase and 

 quartz, with or without muscovite or microcline, 

 and it has a very coarse granitic texture. Toward 

 the center of the dike, this composition gives way 

 to a zone of blocky microcline that may be mono- 

 mineralic or may contain plagioclase and quartz 

 interstitial to the microcline blocks. The center of 

 the dike is occupied by a core of quartz. The mar- 

 gins of the quartz core are the loci of crystals of 

 beryl; some crystals are very large, 3 feet or more 

 wide and 20 feet long. The beryl is not evenly dis- 

 tributed along the margins of the cores but is in 

 pockets separated by barren rock. 



The much less common, but more spectacular, 

 zoned lithian pegmatites usually have spodumene- 

 bearing zones between the microcline-rich zone and 

 the quartz core. Many of these zones are rather 

 similar in composition to the unzoned spodumene- 



