Kyanite, Sillimanite, and Andalusite Deposits of the Southeastern ...

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thick and a few hundred yards long. Several parallel layers usually occur together. Most of the deposits form low knobs or hills, and the sillimanite quartzite is exposed in ledges a few feet high. The sillimanite typically weathers bone white in contrast to the glassy luster of the quartz. Barren coarsegrained quartzite and white mica schist occur with nearly all the deposits. The sillimanite quartzite deposits in the Kings Mountain district are all less than 1 mile from a large body of quartz monzonite. GEOLOGY OF THE DEPOSITS 15 «. FIGURE 15. Photomicrograph of sillimanite quartzite from Ryan-Purcley property, Gaston County, N.C. Shows typical mats of fibrous sillimanite (f), coarser prisms of sillimanite (s), coarse white mica (m), a.nd quartz (q) groundmass. Small dark grains are rutile. Note way in which fibrous sillimanite fingers out into quartz groundmass. Plane-polarized light. FIGURE 14. Photomicrograph of kyanite-chloritoid quartzite from locality 1.2 miles S. 10° W. of Crowders Mountain village, Gaston County, N.C. Shows typical fine-grained quartz (q), radial aggregates of chloritoid (ch), and kyanite (k). Crossed nicols. Quartz and sillimanite are the principal minerals. Sillimanite typically occurs in matted aggregates and bundles of very fine fibrous crystals (fig. 15); coarse prismatic crystals are also present in places. The sillimanite content is generally between 10 and 35 percent. Andalusite is an abundant mineral in parts of the deposit on the Will Knox property (fig. 16). Kyanite also occurs in this deposit, but is less abundant than sillimanite or andalusite. All 3 of the Al2Si05 minerals are present in 7 thin sections from various sillimanite quartzite deposits. Other aluminous minerals that are found locally in amounts of a few percent or more are white mica, diaspore, topaz, and lazulite. Rutile is a widespread accessory mineral in sillimanite quartzite; other accessory minerals are listed in table 2. Chemical, spectrochemical, and calculated mineral content of sillimanite quartzite from two deposits in the Kings Mountain district are given in table 3. ANUALTJSITE-PYROPHYLLITE-QTJARTZ DEPOSITS Significant amounts of andalusite and other highalumina minerals were recently discovered in some of the North Carolina pyrophyllite deposits by Broadhurst and Councill (1953) and at Boles Mountain, S.C. by us. These deposits are lenticular bodies of various shapes and sizes in sericite schist and silicified zones in silicic volcanic rocks. The deposits all lie in the Piedmont belt of volcanic rocks and slates (pi. 1). Their shapes and structures are not well known because none of them has been mapped in detail. Stuckey (1928, p. 35-37) has concluded that these deposits were formed by the replacement FIGURE 16. Photomicrograph of andalusite-sillimanite quartzite from Will Knox property, Gaston County, N. C. Shows coarse anhedral andalusite (a) with numerous quartz inclusions, coarse prisms of sillimanite (s) penetrating andalusite, fibrous sillimanite (f), and inequigranular quartz (q) groundmass. Crossed nicols.
16 KYANITE, SILLIMANITE, AND ANDALUSITE DEPOSITS, SOUTHEASTERN UNITED STATES of tuffs and breccias of dacitic and rhyolitic composition. His principal points of evidence are the preservation of primary features (bedding- and breccia structure) in the mineralized rocks and the presence of isolated masses of unaltered or only partly altered country rock in some deposits. Andalusite generally occurs as light-gray to bluish grains in a matrix of pyrophyllite and very fine grained quartz. Both andalusite and quartz are replaced by pyrophyllite (fig. 17). Diaspore is commonly present in minor to moderate amounts. Stringers and aggregates of diaspore seem to be associated with the pyrophyllite replacement in some localities (figs. 17, 18, 20); in one deposit, diaspore and andalusite are intergrown, and both are veined by pyrophyllite (fig. 19). A clay mineral, with optical properties similar to those of kaolinite, is present in very small amounts in some deposits (figs. 17, FIGURE 18. Photomicrograph of andalusite-diaspore-pyrophyllite rock from Boles Mountain, Edgefield County, S.C. Shows andalusite (a) partly replaced by pyrophyllite (ph); fine to coarse grains of diaspore (d) in pyrophyllite; several kyanite crystals (k), rutile grains (r), and patchy areas of clay mineral, probably kaolinite (ka). Holes in section (h). Planepolarized light. fine grains is a constant accessory in these deposits; other accessory minerals are listed in table 2. DEPOSITS IN MICACEOUS SCHISTS AND GNEISSES KYANITE SCHIST AND GNEISS Kyanite is a rather abundant constituent of schist or gneiss in certain areas of the Blue Ridge of western North Carolina and northern Georgia. Less important deposits of kyanite schist are also known FIGURE 17. Photomicrograph of andalusite-pyrophyllite-quartz rock from Carolina Pyrophyllite Co. mine, Bowlings Mountain, Granville County, N.C. Shows fine-grained quartz (q) partly replaced by pyrophyllite (ph) ; some quartz grains containing numerous small pyrophyllite flakes (q-ph); andalusite (a), veined and partly replaced by clay mineral, probably kaolinite (ka), with narrow stringers of diaspore (d) in center of some clay veinlets ; rutile(r). Crossed nicols. 18). Small amounts of kyanite (fig. 18), topaz, corundum, and lazulite occur locally in some deposits. Fine-grained topaz resembling chert in the hand specimen and having a cryptocrystalline texture is found in parts of the deposit at Bowlings Mountain, N.C. Topaz of identical character was first discovered at the Brewer gold mine, Chesterfield County, S.C. (Pardee, and others, 1937), where it occurs in veins and irregular masses in dense siliceous rock; pyrophyllite, andalusite, and kyanite are also present in the Brewer deposit. Rutile in FIGURE 19. Photomicrograph of andalusite-diaspore-pyrophyllite rock from North State Pyrophyllite Co. prospect, Hillsboro, Orange County, N.C. Shows andalusite (a) and diaspore (d) in matrix of pyrophyllite (ph). Minor veining of andalusite and diaspore by pyrophyllite. Plane-polarized light.
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Kyanite, Sillimanite, and Andalusite Deposits of the Southeastern ...

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