TRIPHYLITE LITHIOPHILITE AND TRIPLITE ZWIESELITE IN GRANITIC PEGMATITES - THEIR TEXTURES AND GENETIC-RELATIONSHIPS

Associations and assemblages of triphylite-lithiophilite and triplite- zwieselite-magniotriplite solid solution members occurring in 8 granit ic pegmatites were investigated by thin sections and electron micropro be analyses. Several types of intergrowths were detected. They are tog ether with the chemical composition evidence for distinct genetic rela tionships between the two phosphates solid solutions. It is demonstrat ed that the minerals of the triplite and triphylite series are either cogenetic or that the triplite-zwieselite-magniotriplite solid solutio n members are the earlier minerals. The only exception is the occurren ce at Valmy, where magniotriplite is formed after triphylite, which is intimately intergrown with sarcopside and graftonite. In most of the occurrences not only one generation but more or less complex crystalli zation sequences of the phosphate minerals have been confirmed by the hypothesis of GINSBURG (1960). The following stages of phosphate forma tion have been observed in the pegmatites considered, forming evolutio nary sequences or nor: 1. Formation of triphylite, which is lamellar i ntergrown with sarcopside and graftonite. This association only occurs in a poorly fractionated pegmatite stage. 2. Formation of triplite-zw ieselite-magniotriplite. Replacement of lamellar triphylite. 3. Format ion of cogenetic triplite-zwieselite and triphylite-lithiophilite. 4. Formation of triphylite-lithiophilite either in an evolutionary sequen ce with triplite-zwieselite or replacing these minerals. 5. Formation of triphylite-lithiophilite without any genetic relationship to tripli te-zwieselite.