I am always on the lookout for “strange” minerals or
at least something rather unfamiliar to my limited mineralogical knowledge. Therefore, while attending the recent Rocky
Mountain Federation show in Sandy, Utah, I nabbed a single and isolated specimen
labeled “turgite” that was in the “sale bin”.
After all, what is a frugal collector to do when something interesting
and “cheap” pops out in front of you?
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Graves Mountain Goethite var. Turgite. Note bladed and mammillary goethite covered
by hematite. Golden-colored material in
upper right is pyrophyllite. Length of specimen ~12 cm.
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After returning home I begin to inquire about
turgite in my reference books and on various web sites (esp. www.mindat.org).
It turns out that turgite is not recognized as a true mineral since it
is a mixture of both hematite and goethite.
Some rockhounds continue to use the term as a variety of hematite [Fe2O3]
and/or goethite [Fe+3O(OH)] as in “goethite var. turgite”.
Whatever the case, turgite usually appears as mammillary,
botryoidal, stalactitic or bladed goethite that has its surface covered by
hematite. Or perhaps the goethite has
even partially altered to hematite. What
makes turgite “famous” and quite collectable is the fact that the hematite often
displays a beautiful spectrum of colored iridescence and resembles a fire agate
(at least to me). Good specimens often
are quoted for hundreds of dollars. Now
my specimen cost me a couple of bucks and only has a few spots of golden iridescence—until
closely examined under a binocular microscope!
As a bonus, my turgite specimen has some nice
crystals/exposures of golden-colored pyrophyllite, an aluminum silicate [Al2(Si4O10)(OH)2]. There are also “spots” where tiny crystals of
quartz are very loosely cemented and contain microcrystals of pyrite and rutile
(both almost impossible to photograph).
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Very soft blades of pyrophyllite. Width ~3.5 cm.
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Graves Mountain, where my specimen was collected, is
perhaps Georgia’s best known collecting locality and is especially noted for
specimen minerals (besides turgite and pyrophyllite) of kyanite [Al2(SiO4)O],
pyrite [FeS2], rutile ([TiO2] world-class), lazulite
[(MgFe+2)Al2(PO4)2(OH)2],
and numerous others. In fact, the
Mountain was originally mined for the aluminum-rich, refractory kyanite used in
such items as spark plug insulators and later as insulated space shuttle tiles
(T. Hanley, 2005).
It appears, from my limited knowledge, that the
geology of Graves Mountain is quite complex!
It lies in the “foothills” of the Appalachian Mountains, and is thrown
into the Piedmont Physiographic section of the mountains. In introductory terms, the great Appalachian
Belt resulted from the collision of parts of Gondwana (Africa and South
America) with Laurentia (North America).
The tectonic events started in the Ordovician (Taconic Orogeny), hit
another high point in the Devonian (Acadian orogeny) and culminated in the Alleghenian
Orogeny (Mississippian through Permian).
These events were compressional in nature and produced the: 1) gently
folded late Paleozoic rocks of the inner region, the Appalachian Plateau; 2)
more steeply folded and faulted early to middle Paleozoic rocks of the Valley
and Ridge (sandstone producing ridges; shale producing valleys); 3) the badly
deformed metamorphic terrane of Precambrian and earliest Paleozoic rocks of the
Blue Ridge; and 4) the complicated and varied terrane of faults, metamorphic
rocks and igneous intrusions known as the Piedmont.
One section of the greater Piedmont is the Carolina Slate
Belt, essentially an old volcanic terrane (rocks ~560 Ma; right around the
Precambrian-Cambrian boundary) that has been subjected to later
metamorphism. These rocks originated in
an island arc system that was part of the proto-Atlantic Ocean. As proto-North
America banged into Gondwana the island arc was “squeezed” and accreted (stuck
onto) to North America (Allard, 1999). Perhaps this island arc appeared something like
the modern Philippine Islands in the Sea of Japan.
Graves Mountain itself has a unique geology in that
the principle rocks are pyritiferous kyanite granofels and sericite (type of
muscovite) schist. Allard (1999) believes these rocks are the result of
metamorphism of a hydrothermal alteration system---a subsurface hot water vent!
I presume the surficial goethite (stable Fe+3) formed from the oxidation of
pyrite (and perhaps lazulite) with the unstable Fe+2.
So, if you have not seen an iridescent goethite/hematite
from Graves Mountain, take a look on the web.
They can be beautiful specimens for your cabinet.
REFERENCES
CITED
Allard, G. O., 1999, Graves Mountain, Slate Belt,
Georgia Geology in Graves Mountain
and Magruder Mine, Wilkes and Lincoln Counties Georgia, M. V. Hurst and C.
Winkler III. Eds.: Southeastern Geological Society Guidebook Number 38. http://segs.org/wp/wp-content/uploads/2010/01/SEGS-Guidebook-38.pdf
Hanley, T., 2005, The New Georgia Encyclopedia: www.georgiaencyclopedia.org.