| Text Description: |
Granite, the matrix of a "plum pudding" rock, from a beach boulder near the head of the north arm of Skan Bay. The texture is hypidiomorphic-granular; the original rock contains many inclusions but this analysis is of matrix only. Felsic minerals compose 93-97 percent of the rock: 10-20 percent quartz, 50-70 percent alkalic feldspar heavily altered to kaolin (10-20 percent), and 15-25 percent plagioclase partly altered to epidote (0.1-0.5 percent), sericite (0.1-1 percent), and kaolin (lumped with above). The plagioclase composition, by extinction angle, is A18 [mean of 5 crystals, standard deviation is 9 percent An, average amount of zoning per individual plagioclase crystal is 0 Percent An per crystal]. Mafic minerals compose 3-7 percent: 2-3 percent hornblende and 0.1-0.5 percent is biotite both partly altered to epidote (0.1-1 percent) and penninite (0.1-0.3 percent), 1-1.5 percent is magnetite, 0.1-1 percent is sphene slightly altered to lucoxene, 0.1-0.5 percent is rutile occurring as stubby yellow rods in sphene, and 0.1-0.3 percent is penninite (interstitial). Comparison of the mode and norm for this rock indicates that the alkalic feldspar contains large quantities of both orthoclase and albite. |
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