Chessboard-twinned albite is abundant in some Franciscan metaconglomerates and associated metagraywackes in the Diablo Range of California. The chessboard albite occurs in pebbles of sedimentary rocks, felsic volcanic and plutonic pebbles, and in the graywacke matrix of the metaconglomerates, where it has probably formed during blueschist-facies metamorphism. The mode of occurrence of chessboard albite in felsic igneous pebbles suggests that this albite formed by replacement of potash feldspar (albitization); potassium released from the feldspar reacted with aluminous silicates to form white mica. Similarly, chessboard-twinned albite in Franciscan metagraywackes may also result from albitization of detrital potash feldspars. Thin-section examination of many Franciscan metagraywackes has revealed that chessboard albite is selectively distributed among pumpellyite- and lawsonite-bearing rocks which now lack potash feldspars. Variation of chessboard albite content in such rocks may be related to differences in the original potash feldspar content.

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