Download Guidebook as .pdf (2.2 Mb) - Carolina Geological Society

______________________________________________________________________________________
2008 annual meeting – Spruce Pine Mining District: Little Switzerland, North Carolina
______________________________________________________________________________________
minerals”. The Pink mine was noted to be a locality for samarskite (Parker, 1952;
Lesure, 1968), but had not previously been reported to contain uraninite. The Goog Rock
mine also had no prior report of uraninite being present.
In appearance, uraninite crystals typically are subequant to equant with a dull to vitreous
luster, black color and black streak. Their hardness is 5½, roughly the same as glass.
Individual grains of uraninite range from 0.5 cm to a little more than 2 cm long; at some
localities (e.g., Goog Rock mine), these may be concentrated to form much larger masses
of uraninite. Alteration of the uraninite is widespread and locally results in yellowishcolored
flakes or orange-red to reddish brown banded masses. Insofar as possible, we
avoided these alteration products, focusing instead on what was most likely primary
uraninite.
Backscattered electron images (Figure 1) reveal a variety of textures in uraninite.
Although some grains appear homogeneous (e.g., Figure 1A), most are not. On a very
fine scale there are patchy variations in brightness related to small heterogeneities in
composition. The brighter areas in Figure 1B, for example, are slightly richer in uranium
than adjacent areas. In some grains there is also a distinct, broad banding (Figure 1C);
the darker bands usually give lower totals, possibly due to incipient oxidation and/or
hydration of the uraninite. Such areas are more susceptible to beam damage, which may
be caused by the presence of water. In other cases, the banding is clearly compositional,
as evidenced in Figure 1D, where the image brightness is related to the extent of calcium
substitution for uranium (discussed below).
Representative electron microprobe analyses of uraninite from Spruce Pine pegmatites
are given in Table 1. Many of the analyses are characterized by low totals (more than
half of the analyses sum to less than 98 weight percent). There are several possible
reasons for this. First, the uraninite may be partially oxidized. That is, some U 4+ may
have been oxidized to U 6+ . According to Janeczek and Ewing (1992), uraninite of ideal
UO 2 composition probably never occurs in nature; instead, the structural formula for
uraninite is better represented as UO 2+x (0 < x < 0.25), where x = U 6+ /(U 4+ + U 6+ ). The
relative amounts of U 4+ and U 6+ cannot be directly determined from our analyses because
the electron microprobe measures only total uranium. To the extent that uranium is
present as U 6+ , the analyses in Table 1 will be low because not enough oxygen will have
been assigned to make uranium oxide. A second possibility is hydration of the uraninite.
Alteration of primary uraninite leads to the formation of a variety of hydrous uranyl
minerals collectively referred to as gummite (Frondel, 1956). Although we deliberately
avoided samples with obvious alteration products, some of the material we analyzed may
have experienced incipient alteration and hydration. This would be consistent with the
beam damage occasionally observed. Still another possibility is that radioactive decay
may have disrupted the crystal structure of uraninite, causing it to become metamict or
partially metamict. X-ray diffraction patterns of two randomly selected uraninite samples
reveal well-defined x-ray peaks identifiable as uraninite (Figure 2), so we conclude that
______________________________________________________________________________________
Page 32
______________________________________________________________________________________