Abstract
Recent optical and structural investigations have shown that ; oth high- and low-temperature forms of plagioclase can exist naturally over the range An0-70, and in the more basic plagioclases (An70_90) Gay (1954) has shown that a structural distinction can be effected although no definite optical differences have yet been established. In the anorthite range slight optical and distinct structural differences have been observed between synthetic plagioelases and crystal lapilli on the one hand and all other natural plagioclases on the other (Gay, 1953).

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