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2008 annual meeting – Spruce Pine Mining District: Little Switzerland, North Carolina
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Microprobe Analyses
Electron microprobe analyses of some feldspars were done by W. C. Luth at Sandia
National Laboratory using a ARL electron microprobe. Other microprobe analyses were
done at the Department of Geology at the University of Georgia using a JEOL-8600
Superprobe. Machine conditions were: accelerating voltage of 15kV and a sample
current of 5 nA. Beam diameter varied with mineral phase analyzed: 10 µm for mica and
feldspar and 2 µm for garnet. Synthetic and natural minerals were used for standards.
Data were reduced using standard techniques.
X-Ray Diffraction
X-ray diffraction studies of K-feldspar of the LUF1 pegmatite was done at Stanford
University using a Phillips X-ray diffractometer. Samples were powdered in a corundum
mortar. The feldspar powder was mixed with a internal standard (annealed CaF 2 ) and
placed on a glass slide. A solution of acetone and glue was mixed with the powder and
the slurry was spread on the glass slide. X-ray procedures followed the method of Wright
(1968).
MINERALOGY OF THE LUF1 PEGMATITE
Feldspars and quartz grains in the LUF1 pegmatite are variable in size, ranging from less
than a mm to 10s of cms. Muscovite forms subhedral to euhedral crystals ranging from
less than a mm to10 cm in diameter. Anhedral to euhedral garnet crystals up to 5 cm in
diameter occur in LUF1, but most garnet is less than one cm in diameter. The margins
of some of the larger feldspar and quartz grains are surrounded by a narrow zone of finegrained
recrystallized quartz, feldspar, and muscovite that forms a mortar structure. Bent
and broken feldspar twin planes and undulose extinction in quartz also support postmagmatic
deformation in the LUF1 pegmatite.
Feldspar
Plagioclase is more abundant than K-feldspar in the LUF1 pegmatite (Figure 3). Albite
twinning is well developed in the plagioclase and is often bent and broken. The
plagioclase is antiperthitic with stringers of K-feldspar included in the plagioclase. Much
of the plagioclase contains graphic quartz intergrowths (Figure 3). Plagioclase grains are
unzoned and there is no compositional variation with position within the LUF1
pegmatite. Average plagioclase composition is Ab 91 Or 1 An 8 . Exsolved K-feldspar in the
plagioclase is also uniform at Ab 5 Or 95 An 0 (Table 1).
K-feldspar forms large, slightly pink crystals in the LUF1 pegmatite (Figure 3). Graphic
intergrowths of quartz and K-feldspar are common in the pegmatite, especially in
feldspar crystals away from the quartz core (Figure 3). Most of the K-feldsapr is perthitic
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