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MINERALS INDEX
Microcline |
| KAlSi3O8 |
| Triclinic |
Forms
c(001), b(010), m(110), M(110), f'(130), z(130),
x(101), y(201), e(021), n(021), p(111), and o(111)
| forms | Illustrations | |
| 1 | c, b, m, M, x, y, p, o | Figure 71 |
| 2 | e, b, m, M, f, z, y, e, n, p, o | Figure 72 |
Occurrence
Microcline is a normal constituent of the local pegmatites, in which, especially at
contacts with limestone, many crystals of microcline are well formed and some are more
than a foot long. Characteristic combinations are shown in the figures, but the habit has
a wide variety of detail.
| Figure 71 Crystal of microcline showing the forms c(001), b(010), m(110), M(110), x(101), y(201), p(111), and o(111). Trotter mine. |
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Figure 72 Crystal of microcline showing the forms c(001), b(010), m(110), M(110), f'(130), z(130), y(201), e(021), n(021), p(111), and o(111). Trotter mine. |
The color ranges from brilliant green in the amazon stone to muddy yellowish green and yellowish white. The finest green crystals were found during the. sinking of the Trotter shaft, where microcline was abundant. Large and well-formed but poorly colored crystals were abundant at Sterling Hill also.
Alteration
Feldspar pseudomorphs from Mine Hill were described by Roepper (116) as slightly altered
crystals of anorthite. They were found, with crystals of hornblende and titanite, in
limestone and were from an eighth of an inch to 3 inches across. They display typical
feldspar habits and are tabular parallel to the base. Their surfaces are deeply pitted,
and some are coated with a drusy crystalline layer appearing like prehnite, but others are
smooth and of talcose appearance. Traces of feldspar cleavage are still visible. Crystals
from this locality were seen in the Hancock collection and had all the external appearance
of microcline from the district. They are so rough that contact measurements, such as
Roepper gives to prove their derivation from anorthite, have little weight.
In thin section this material is heterogeneous, showing a mosaic made up of zoisite, epidote, talc, and possibly feldspar, and minute crystals of apatite and titanite. The crystalline coating resembling prehnite could not be identified in the section. The composition and specific gravity given by Roepper approximate zoisite more nearly than anorthite, and the crystals are probably replacement pseudomorphs after microcline, consisting largely of zoisite.
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Website
© by Herb Yeates 1997-2001.
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This
page created: January 12, 2001 6:35 PM
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