Ch. 1: Geology of Maine Pegmatites

Ch. 1: Geology of Maine Pegmatites Page of 170 Ch. 1: Geology of Maine Pegmatites Text size:minus plus Restore normal size   Mail page  Print this page
42                 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
microcline, and a member of the isomorphous series of plagioclase feldspars. It should be pointed out, moreover, that if water or other gases were present, as it is almost certain they were, they formed additional components whose amount the analyses do not reveal, but whose influence on the proportions of the other con­stituents may have been great. If graphic granites crystallized from magmas of eutectic proportions these were therefore eutectics of at least four components. The series of analyses (p. 41), though suggesting that the proportions between the constituents of graphic granites are controlled by some laws, can hardly be regarded as proving their eutectic origin. The theoretical value of such analy­ses in elucidating the laws governing rock solutions is impaired by the fact that they take no account of the gaseous components of the magmas.
Vogt a states that many graphic intergrowths, especially when developed on a microscopic scale, represent the last portions of the magma to crystallize. This fact he cites as in harmony with the conception that they represent eutectic residues. Although this ma}' be the true relation in some cases, in others the graphic granite was unquestionably not the last crystallization from" the magma. In the Fisher feldspar quarry in Topsham, for example, where large masses of graphic granite pass gradually and irregularly into large areas of pure quartz and feldspar, the tests of Wright and Larsen (see p. 30) have shown that the quartz of the graphic inter­growths crystallized above 570° C, whereas the quartz of the large pure areas crystallized below 575°. The latter was therefore the later crystallization. Almost all the gem and cavity bearing por­tions of the Maine pegmatites grade into normal pegmatite con­taining abundant graphic granite. From the presence of cavities and of the rare minerals, from the general tield relations, and from the fact that the quartz of the pockets and of the gem-bearing por­tions, wherever tested, is of the low-temperature variety, there can be no reasonable doubt that these gem and cavity bearing portions rather than the bordering graphic portions were the last parts of the pegmatite to crystallize.''
In considering the significance of the graphic intergrowths found in pegmatite, it is necessary to consider not only the intergrowths of feldspar and quartz, but also the almost equally regular inter­growths of muscovite and quartz, garnet and quartz, black tourma­line and quartz, etc. As muscovite, tourmaline, and garnet are-less abundant than feldspar in the pegmatites, their intergrowths
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