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Show Crawford and Baranek: Economic Minerals 109 quartz will be formed from feldspar or whether feldspar will develop at the expense of muscovite and quartz are not fully un ... derstood. Temperature, pressure, the amount of water present in the system, and the presence of mineralizers such as fluorine, boron, etc., no doubt play an important part. Although no fluorite nor topaz..have been recognized in the dike, the presence of fluo rine mineralizers is apparent, not only from the presence of tourmaline but also from the presence of highly etched quartz crystals in pockets in one of the beryl feeder channels. Under the microscope there is ample evidence that the equi. librium conditions, whatever they may have been, shifted back and forth during the development of the pegmatite minerals in the Grover Pegmatite. There appears to have been four genera tions of muscovite and more than one each of quartz, albite, and microcline. For some reason, not understood, the frankly beryl appears to be correlated with the albite-rich areas, rather than with the microcline. A possible explanation for this lies in the probability that the same mineralizers that brought in the beryl lium also favored the development of muscovite at the expense of the pre existing feldspars, and that the soda in these earlier feld spars, not being utilized to an appreciable extent in the muscovite molecule, was recrystallized in the form of albite near the musco vite and beryl crystals. Another fact that lends plausibility, to this view, is that beryl usually, though not always, contains soda as a part of its molecule. Since potassia does not seem to enter into the beryl molecule it seems reasonable to assume that the beryllium might stay in solution until a sufficient concentration of soda (liberated as a by-product from the early generation of muscovite from soda-bearing potash feldspars) forced the beryl lium out of solution. This would account for its habitual associa tion with muscovite as well as with albite. .... .... Furthermore, the replacement of muscovite be explained of potassia cule. by microline can by the foregoing equation, since there is an excess resulting from the formation of each muscovite mole However, it is not clear why so large amounts of this late generation of microline should form without more albite being Perhaps the relative abundance of "soda" -microline, present. micro-perthite and perthite associated with this late generation microline is thus accounted for. A still further explanation may lie in the fact that the is more soluble potassia than soda and travels farther after being liberated. Thus the potassia is furnished in continuously larger quantities out to the extremities of the dike, farther from its original source. One important source for the enormous quantities of potassia that form the great pegmatities particularly as an aftermath of the mtamorphism of a vast area of feldspathic rocks, is regional the byabout intrusives, |
