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

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KYANITE DEPOSITS 97 EXPLANATION Stratigraphic order of rocks is unknown Muscovite-quartz schist V Limit of outcrop _j6O Strike and dip of foliation Strike of vertical foliation x Prospect pit 1000 l__ 1000 2000 I i Contour interval 50 feet Datum assumed 3000 Feet _I Prospect adit Topography compiled from aerial photographs by L.A Brubaker Geology by G. H. Espenshade, 1953 FIGURE 55. Geologic map of Graves Mountain, Lincoln County, Ga. ments of closely folded and broken beds, or are replacement bodies that have been formed along two systems of fractures having these northeasterly trends. Watkins (1942) thought that the kyanitequartz rock was in a tight syncline whose axial plane dips steeply northwest, and that there was possibly a fault along the southeast side. The kyanite-quartz rock forms large bare massive outcrops and bold rough crags, 40 feet or more high; abundant talus mantles the slopes. Most of the rock is composed of very fine grained quartz with numerous thin stringers and scattered crystals and clots of small white to light-gray or green kyanite crystals. These kyanite string-erg, generally less than half an inch thick, are closely spaced and parallel in places, but also form networks of randomly trending stringers. The clots and veinlets of kyanite stand out as rough projections and ribs on the weathered rock surfaces, the fine-grained interstitial quartz having been etched away to leave numerous small pits. Limonite is common in the weathered rock, generally in rather small amounts, but in places it is abundant and accompanied by dark-red earthy hematite; fine-grained pyrite is disseminated through the fresh rock. Considerable
98 KYANITE, SILLIMANITE, AND ANDALUSITE DEPOSITS, SOUTHEASTERN UNITED STATES muscovite is present in the rock along the flanks and ends of the ridge. Grains of bluish opalescent quartz, from 2 to 10 mm across, are very abundant in micaceous kyanite-quartz rock at the eastern and western ends of the ridge (fig. 1). Examination of loose grains of bluish quartz weathered from the kyanite-quartz rock showed that some of the grains seem to have bipyramidal crystal form, and it was suspected that the blue quartz grains might have originally been quartz phenocrysts. This conclusion was also reached by R. L. Smith after examining the blue quartz; he states (written communication 1953) : Some of the blue quartz in these samples occurs in wellformed hexagonal bipyramidal crystals, some of which show poorly developed prism faces. Many of the blue quartz grains show several crystal faces while others have been altered to the point where they show no recognizable crystal form. It seems reasonable to assume a common origin for all of the blue quartz grains * * * . In my opinion the blue quartz in these samples was formed as a high temperature phenocrystic quartz in a high silica rhyolite. The blue color is probably not primary, but rather related to the later metamorphism. Possibly it is due to dispersion caused by the presence of tiny rutile needles. Quartz veins are especially abundant along the southwestern part of the ridge as individual veins and networks of irregularly intersecting veins, which range in thickness from a few inches to more than 10 feet. Johnston (Prindle and others, 1935) has pointed out that kyanite is abundant along the margins of some quartz veins and decreases in amounts away from the veins. We have also observed some quartz veins in the kyanitequartz rock that have no apparent concentration of kyanite along or near their margins. Pyrophyllite seems to be most common in the area where quartz veins are numerous. It is light gray to tan, has a pearly luster, and forms aggregates of small granular crystals and clusters of radiating slender crystals about 14 to y% inch long. It occurs alone in crosscutting stringers about half an inch thick; it is found with quartz veins and in the adjacent wall rock, and also with coarse kyanite. In thin section, pyrophyllite is seen to have replaced kyanite. Watson and Watson (1912) state that masses of radiating aggregates of pyrophyllite several feet thick are well exposed in low cliffs along a road on the northwestern slope of the mountain. Lazulite (an aluminum-rich phosphate) occurs as small blue crystals, generally less than half an inch in size, sparsely distributed through kyanite-quartz rock. It seems to be most common along the central part of the ridge, particularly where quartz veins are numerous, but it occurs in the kyanitequartz rock rather than in the quartz veins. Johnston (Prindle, and others 1935) says that lazulite appears to be confined to a zone 2 to 3 feet wide extending in a N. 30° E. direction up the south slope of the hillside toward the saddle; it is common here but is not restricted to this zone. On some weathered surfaces, lazulite is altered to a claylike material that contains small blue specks of unaltered lazulite. Shepard (1859) and Watson (1921) described the occurrence of lazulite at Graves Mountain. Watson includes a chemical analysis of the mineral whose calcium oxide determination (3.12 percent) is evidently too high; a recent analysis (Pecora and Fahey, 1950) found 0.06 percent calcium oxide in a lazulite specimen from here. Rutile occurs at Graves Mountain in exceptionally large and well-formed crystals that have been prized by mineral collectors since Shepard (1859) first described them. Multiple-twinned crystals are common; the largest crystal is said to have weighed about 4 pounds. The different forms of rutile crystals found here were described by several German crystallographers between 1860 and 1890; references to these articles may be found in Watson and Watson (1912) and Johnston (Prindle and others, 1935). The best rutile crystals were found in the saddle and on the northwestern slope in coarse kyanite-quartz rock containing considerable hematite and limonite. Several pits were dug to recover the mineral; according to Kunz (1907, p. 52) the value of the mineral specimens sold from here in the last century was about $20,000. Apparently the easily found large crystals were gathered up many years ago, because Watson and Watson (1912) state that they could find none at the time of their visits; a few rutile crystals, 1/2 to 1 inch long, can be found at present. Tiny grains of rutile .are widespread throughout the rock as inclusions in kyanite and quartz. Watson and Watson (1912) give the following partial analyses, in percent, of rocks from Graves Mountain: SiO 2 ... . .._..._...._.-._. ___ 79.18 A1203 __-._ 14.14 ____--_-_ -_-_-_ 3.17 Schistose kyanite-muscovite Massive kyanite-quartz quartz rock with limonite rock (from crest of (from northwest slope) mountain) 69.74 24.86 .53 Several lines of evidence favor the replacement origin of the kyanite-quartz rock here. The abundant blue-quartz phenocrysts are among the strongest items of evidence. These blue-quartz grains must
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Kyanite, Sillimanite, and Andalusit
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UNITED STATES DEPARTMENT OF THE INT
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IV Page Kyanite deposits Continued
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VI CONTENTS TABLES TABLE 1. Major d
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KYANITE, SILLIMANITE, AND ANDALUSIT
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Type of mica Muscovite . . . Do....
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Kyanite, Sillimanite, and Andalusite Deposits of the Southeastern ...

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