| OCR Text |
Show I' I Crawford and Baranek: Economic Minerals 111 meta-tantalate. The evidence for placing the columbite here is not absolutely conclusive, since the material definitely proved to be columbite was not found in contact with sufficient other minerals work out its relationship to all of them. It is clearly earlier than the greenish rosette type of muscovite, and columbite mould the ing edge of beryl crystals and slightly penetrating the latter, leads us to think it probably accompanied or immediately followed the beryl. More details concerning this mineral are given else where in this report. Biotite and magnetite, the two next minerals in' the sequence are also placed without conclusive evidence. The sample studied showing magnetite came from an outcrop on the west end of the Grover Pegmatite zone and consists of "knots" of magnetite in a quartz-feldspar-biotite pegmatite quite different from the pegma tite as shown in the open' cut. The biotite is excessively black and in one direction is practically opaque even in a thin section. It is distinctly different from the biotite in the wall rock and ap pears to have been formed from muscovite by the action of iron rich solutions which, on becoming still more concentrated, de to posited magnetite. The remainder of the series, numbers 20 to 37 inclusive, have been placed by fairly reliable evidence. The complete series of minerals identified and their apparent sequence of deposition as determined by careful microscopic study is shown in the fol lowing list: The Paragenetic Series 1. of Minerals in the Grover Pegmatite Apatite, present in subhedral residuals in Idaho Springs schist and gneiss. 2. Magnetite, present in subhedral residuals in Idaho schist and gneiss. Springs 3. Hornblende, inclosing magnetite but embayed and replaced. by andesine of schist. 4. Biotite, replacing hornblende and replaced by andesine of schist and gneiss. Andesine, moulding· biotite and hornblende in schist and gneiss. Quartz, replacing andesine in Idaho Springs gneiss. Epidote, (with rims of clinozoisite) replacing hornblende. 5. 6. 7. 8. 9. 10. Muscovite, "magmatic" in Idaho Springs gneiss, biotite and replaced by orthoclase. replacing Oligoclase, replacing andesine, and replaced by, gneiss. quartz, in Saussurite, (zoisite and clinozoisite) replacing andesine in gneiss. |
