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

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at some places in the Piedmont of Virginia, North Carolina, and Georgia. Muscovite, quartz, and kyanite are the dominant minerals in kyanite schist; biotite, garnet, and graphite are commonly present too, and staurolite and tourmaline are found in some areas. Sodic oligoclase is an abundant mineral in the garnetiferous kyanite gneiss of the Burnsville-Swannanoa area, North Carolina (fig. 20). The kyanite GEOLOGY OF THE DEPOSITS 17 FIGURE 21. Photomicrograph of sillimanite-biotite-muscovite schist from Cages Mountain, Caldwell County, N.C. Shows biotite (b) and muscovite (m) intergrown with fine fibrous Sillimanite (s) ; in some areas biotite and Sillimanite are very intimately mixed (b-s). Holes in section (h). Planepolarized light. FIGURE 20. Photomicrograph of kyanite-bearing gneiss from Yancey Cyanite Co. mine, Celo Mountain, Yancey County, N.C. Shows kyanite (k), garnet (g), biotite (b), muscovite (m), quartz (q), and plagioclase (pi). Black areas of opaque mineral (o) are magnetite and pyrrhotite. Holes in section ( h). Plane-polarized light. crystals are commonly blue. They have a wide range in size at different localities, from about an eighth of an inch to several inches in length. Layers of schist or gneiss in which kyanite is rather evenly distributed commonly alternate with layers that contain little or no kyanite. Some quartz veins or pegmatites carry kyanite. The kyanite content of zones that are 100 feet or so thick ranges from a few percent to about 10 percent; individual layers contain 15 percent or more of kyanite. SIKLIMANITE SCHIST AND GNEISS Sillimanite schist has recently been found widespread in the region, particularly in a belt in the western Piedmont that extends from North Carolina to eastern Georgia and in another belt in the Blue Ridge of western North Carolina and northern Georgia. Sillimanite typically occurs in quartz-mica schist, in which biotite is generally much more abundant than muscovite; feldspar is absent from much of the rock, though orthoclase is found in places. Other minerals commonly present are garnet, staurolite, chlorite, magnetite(?), pyrite, ilmenite, and tourmaline. A little kyanite accompanies the Sillimanite at some localities. Sillimanite occurs most commonly as clusters of fine fibrous crystals closely intergrown with mica (fig. 21); it also forms coarse prismatic crystals in some deposits (fig. 22). Sillimanite may be concentrated in certain layers distributed rather uniformly through the rock. Sil- FIGURE 22. Photomicrograph of sillimanite-biotite-quartz schist from Gideon property, about 1^ miles southwest of Cross Anchor, Spartanburg County, S.C. Shows prismatic crystals of sillimanite (s), biotite (b), quartz (q), and a little muscovite (m). Some sillimanite crystals are cut nearly normal to the c axis (showing rectangular cross sections with diagonal cleavage), and other crystals are cut nearly parallel to the c axis. Plane-polarized light.
18 KYANITE, SILLIMANITE, AND ANDALUSITE DEPOSITS, SOUTHEASTERN UNITED STATES limanite aggregates commonly occur as small platy lenses less than an inch in diameter; masses several feet across, consisting of nearly pure sillimanite, are found at several places. Small pegmatite lenses and quartz veins are commonly abundant in the rock. Large granite bodies occur in most areas of sillimanite schist. Numerous samples of sillimanite schist, taken from road cuts, trenches, and drill holes, have shown that the average sillimanite content across widths of 50 feet or so is less than 10 percent in many places, though samples from some localities have shown a content of 15 to 20 percent. Low-grade schist or granite has been found by drilling at shallow depths below some surface exposures of high-grade rock. DEPOSITS IN QUARTZ VEINS AND PEGMATITES Large bladed crystals of light-gray to blue kyanite are common in quartz veins and in pegmatite in areas of kyanite schist and gneiss, especially in the Blue Ridge. Many of these deposits have attractive showings of large kyanite crystals, and prospecting and small-scale mining have been attempted in such deposits in North Carolina and Georgia; the operations, however, have not been profitable because of the small size and irregularity of the deposits. RESIDUAL DEPOSITS AND PLACERS Kyanite, sillimanite, and other insoluble minerals have accumulated in the residual soils overlying rocks containing these minerals as the result of the deep weathering and rock decay that have affected the Southeastern region. In some localities these insoluble minerals are more abundant in the soil than in the parent country rock. This is particularly true in some areas of sillimanite schist where abundent distinctive float of rock fragments or of sillimanite nodules or crystals has become concentrated in the soil and has been spread laterally from the point of origin. Under these circumstances, the float mantle may give the impression that the underlying body of sillimanite schist is larger and contains more sillimanite than it actually does. Abundant accumulations of large kyanite crystals are found in the soil overlying parts of the kyanite schist belt of Habersham and Rabun Counties, Ga. No information is available on the occurrence of andalusite in the residual soils of the pyrophyllite deposits that carry andalusite. The Al2Si0 5 minerals become concentrated in stream alluvium along with the other heavy minerals. Placer deposits containing abundant large subrounded kyanite crystals (fig. 23) have been mined in some streams that drain the kyanite schist belt of Habersham and Rabun Counties, Ga.; kyanite also occurs in terrace gravels which are 30 feet or more above present stream levels, and which are the remnants of ancient stream deposits. Kyanite and sillimanite are among the heavy minerals in the Southeastern beach sands. Along the Atlantic coast between Charleston, S.C., and Miami, Fla., Martens (1935) found that the content of sillimanite and kyanite ranged from 3 to 13 percent of the total heavy-mineral content, and that sillimanite was several times more abundant than kyanite. However, in three samples of beach sand from the gulf coast of northwestern Florida, kyanite was far more abundant than sillimanite. Miller (1945) found the same situation in his study of the Florida beach sands: kyanite made up 30 to 45 percent of the heavy minerals in the Pensacola Bay FIGURE 23. Waterworn kyanite crystals from A. P. Green placer deposit on Raper Creek, about half a mile above junction with Soque River, Habersham County, Ga. area but only about 3 percent of the heavy minerals along the east coast of Florida. In the sands of Trail Ridge and other areas in central Florida, kyanite and sillimanite occur in about the same proportions as in the Atlantic beach sands. These inland sands usually contain only a few percent of heavy minerals, of which kyanite and sillimanite together make up from 5 to 15 percent of the total heavy minerals (Spencer, 1948; Thoenen and Warne, 1949). At the Trail Ridge titanium mine of the E. I. du Pont de Nemours & Co. near Starke, wet spiral concentrate contains 80 percent heavy min-
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