amphibole-riche roche; Ger-
Amphibol-reich gestein; Nor- stein rik på
Rus- [= amphibolite] )
AMPHIBOLE-RICH ROCKS (See also THE JADES, NEPHRITE entry.)
A. Amphibole-rich rock cabochon (greater
axis - 1.7 cm) from Converse County, Wyoming. R.V. Dietrich
collection. (© photo by Dick Dietrich)
equivalent of anthophyllite" rock is said to exhibit schiller and
exhibit an appearance that is characterized as labradorite-like.
I have not seen this rock and know of no additional descriptive
information about it
that seems noteworthy here.
-- see NEPHRITE entry.
Nuummite and a similar rock from Wyoming -- see Figure A -- are relatively coarse-grained nonfoliated amphibole-rich rocks. Colors - brown (including golden brown), green and nearly black; H. 5-6; S.G. ~ 2.8; individual grains, which are typically a few millimeters in greatest dimension, have dull to subvitreous lusters, are subtranslucent to opaque, and commonly exhibit golden brown, rose-red, silver-gray and/or greenish iridescence.
blue nephrite" is a massive
blue rock, commonly gneissic "composed of a submicroscopic mixture of
tremolite, and another (blue) amphibole, potassium
(Johnson et al., 2000,p.68) -- see Figures B & C.
Although the amphiboles
the major portion of this rock, their compositions are such that this
rock should not be considered to be nephrite or jade.
Colors - "mottled saturated 'royal' or 'lapis' blue to a mottled
desaturated grayish blue ... similar to 'denim' lapis" (op. cit.);
H. 6-7; S.G. ~2.9; subtranslucent.
OTHER NAMES: Some amphibolites and amphibole gneisses constitute geological units that have been named according to the scheme used chiefly for sedimentary stratigraphic units (see Appendix B, Glossary) -- i.e., ... Amphibolite with the ellipsis replaced by a geographic name. An example is the Ketchepedrakee Amphibolite of the Piedmont-Blue Ridge province of Alabama.
Special names, other than those already mentioned, that have been given to amphibole-rich rocks used as gemrocks follow:
USES: Each of the listed amphibole-rich rocks has been fashioned into attractive cabochons and miscellaneous shaped pieces for jewelry. Amphibolites and amphibole gneisses of some rock sequences are rather attractive and have been used, albeit rarely, as gemrocks -- e.g., in the past, for beads; more recently as paperweights and bookends.
OCCURRENCES: Most amphibolites and amphibole gneisses occur in terranes with metamorphosed sedimentary and/or volcanic rocks as the chief bedrock -- e.g., the Wyoming rock occurs in a weathered gneiss-schist unit, near its contact with a peridotite mass.
NOTEWORTHY LOCALITIES: The "blue . . . anthophyllite" rock is from near Butte, Silverrod County, Montana. "Ghost jade" comes from the vicinity of Yerington, western Nevada (Johnson et al., 2000, p.68). Nuummite occurs in the Nuuk region (including offshore islands) of southwestern Greenland (Appel and Jensen, 1987) and in the Sahara Desert of central Mauritania (Renfro, 2011). The illustrated amphibole-rich rock occurs southwest of Douglas, Converse County, Wyoming (Dietrich et al., 1988). "Siberian blue nephrite" is recovered "as a byproduct of nephrite jade mining in Sakha (formerly Yakutia) in the Murrun Mountains of central Siberia" (Johnson et al., 2000, p.67).
REMARKS: Amphibole is the group name
for several related species and varieties, many of which have been
given individual names -- e.g., tremolite. The word amphibole was
first applied to minerals of this group in 1801 by the famous French
mineralogist Abbé René-Just Haüy (1743-1822).
His choice was based on the Greek word αμφιβολος (amphibolos - equivocal or
ambiguous) in allusion to the diversity of composition and
appearance of the species then known to comprise the group. The
rock name amphibolite is, of course, based on the mineral name.
The extraordinary appearance of the Wyoming rock depends at least in part on the fact that its amphibole grains have been partially to completely dissociated to a mixture of goethite and opaline silica. This fact, which has been established by microscopic, x-ray, and chemical investigations (Dietrich et al., 1988), is not discernible macroscopically.
To date, I have seen ancient beads, but no other
gemstones or ornaments, other than two handsome paperweights I have,
fashioned from "run of the mill" amphibolites or amphibole gneisses.
The amphibolite paperweight with its smooth, here and there glistening
makes me think these relatively common rocks have a good
for use as gemrocks; the second, a banded amphibole gneiss, is
shown as Figure B in the
SIMULANTS: None that I have seen or seen described.
REFERENCES: No general reference.
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