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September 1933Volume 8, Number 6

Instructor in Mineralogy,
Virginia Polytechnic Institute, Blacksburg Va.

*This work was done in the laboratory of the Virginia Polytechnic Institute, Blacksburg, Va.

IN the issue of Grand Canyon Nature Notes for March, 1932, there appeared an article by Mr. B. F. Moomaw, Jr., entitled, "Identification of the Common Pink Mineral found near Phantom Ranch". In this article Mr. Moomaw stated that the mineral was orthoclase. His conclusion was based on certain physical properties, notably the specific gravity and index of refraction. Also it was stated that examination with the petrographic microscope showed no polysynthetic twinning on this mineral. The results obtained were again checked, using the same specimen, in January, 1933, and were found to be correct. One point, however, was noticed to have been overlooked. The previous examination with the petrographic microscope had been on granular fragments of the mineral. In the latter examination by the writer, an oriented thin section parallel to the basal cleavage (.001) was made. The fragments used in the first examination may not have been transparent enough or correctly oriented to show twinning, but the thin section used afterwards showed definitely the characteristic grating type of twinning so well known in microcline. With the microcline were found intergrowths of plagioclase, probably of the variety oligoclase. A microphotograph was made of the sections, and accompanies this statement. The indices of refraction of both orthoclase and microcline are so close that it would be impossible to distinguish them by the immersion method on unoriented grains. Their specific gravities are identical. Hardness is the same and similar color shades are common. Also crystallographically the minerals appear the same, as microcline, which is triclinic, differs from monoclinic orthoclase only 30 minutes in the interfacial angle between the (b) and (c) faces. Cleavage for each is the same. Composition is the same; hence it is readily understood how either mineral might be mistaken for the other. It is only with the microscope and an oriented thin section that the difference is readily distinguished. The writer himself had called the mineral orthoclase for two successive seasons.

Microcline X40 Waesche Photo

Microcline Oligoclase Photo

Examination of thin sections made from a number of the Archean rocks found in the vicinity of Phantom Ranch shows that orthoclase is present in the granites only in minor amounts, and that the major part of the pink feldspar is microcline or plagioclase. The microcline predominates in the coarser pegmatites. The plagioclase probably grades from albite to andesite with oligoclase being the most plentiful.

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