Method of Distinguishing C-twin and A-twin of Plagioclase under Ordinary Polarisation-Microscoope

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The writer has divided the types of twinning of plagioclase, from a petrological point of view, into C-twin and A-twin (Jour., Geol., Soc., Japan, Vol., 56, No., 655, 1950, p., 149-156)., The present paper describes an easy method of distinguishing them under an ordinary polarisation-microscope without the aid of a universal stage., Under the microscope the twinned plagioclase is distinguished into the following four types (see figure): Type 1-----Polysynthetic twin (1, 2, 4 of the figure) and its modification (3, 5)., Type 2-----Simple twin (6, 7) and its modification (8, 9, 10)., Type 3-----Complex twin and its modification ; this is subdivided into 3a (11-15) and 3b (16-25)., Type 4-----Penetration twin (21-25)., Of these, type 1 is almost always A-twin, and those of types 3 and 4 are always C-twin., However the following three cases are distinguished for the twin of type 2 : 1) The different twin units of a crystal show different retardation when the twinning line is placed in diagonal position with reference to the cross hairs., 2) The twin units show, in that position, equal retardation but show different retardation when a gypsum plate is inserted., 3) The twin units show, in that position, equal retardation even when gypsum plate is inserted., The first two cases are C-twin, but the third case is practically indeterminate.,
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