GOETHITE FROM GRAVES MTN, GEORGIA

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?

 

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.

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). 

 

Very soft blades of pyrophyllite.  Width ~3.5 cm.

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.

HUNTING FOR TOPAZ: UTAH AND COLORADO

The mineral topaz is one of the more widespread gem minerals in the western U. S. and is valued by collectors for both display specimens and faceted stones.  At a hardness of 8 (Mohs Scale), topaz, a silicate of aluminum with fluorine and hydroxyl [Al₂SiO₄(F,OH)₂], is the hardest of the silicate minerals and therefore quite durable.  However, it does have perfect basal cleavage and so may fracture or split along that plane.  This brittleness is a common problem if the stone is roughly handled, either during faceting or after completion.  Large faceted stones are perhaps best displayed in pendants rather than in rings since additional protection from “hard knocks” is provided.  Although cut topaz will take a nice polish, it has a weak dispersion factor (.14), especially when compared to diamond (.44).  Most often this comparison is noted in a round brilliant cut as topaz simply lacks the “fire” associated with diamonds and some other gemstones.  To offset these weaknesses, buyers note that the price of cut stones is usually very reasonable and one can afford the purchase of some really large gems, especially those of the colorless/clear variety. 

Pendant of clear topaz ~2.2 X 1.6 cm.

Specimen collectors value topaz since many crystals have interesting terminations, including some with double terminations.  However, these double points are somewhat rare as the crystals commonly break along the basal cleavage. Topaz comes in a variety of natural colors but the buyer should be aware of imitations!  Yellow or golden topaz is one of the more valuable of the topaz gemstones; however, some dishonest dealers pass off faceted citrine as topaz.  There is an entire range of valuable orange to sherry to red to pink topaz crystals with many specimens coming from Brazil.  Intense pinks have often been heat treated to satisfy the purchaser but most people are unaware of the treatment.  Most certainly examine any red topaz setting to see if red foil has been placed under a pale pink stone!  Perhaps blue topaz is the most popular color in the market today and good colored stones command a decent price.  But again be aware---colorless stones may be irradiated with gamma rays to produce the darker blue color.  There is nothing wrong with treated stones as long as the dealer apprises the buyer of the process.  In fact, the deep blue of London Blue Topaz (irradiated) is an absolutely beautiful stone and makes for a dazzling pendant set in sterling silver.  

Natural red topaz collected from La Ochoa, Durango, Mexico.  Width of specimen ~1.9 cm.

Red topaz set in ring, ~ 2.0 X 1.5 cm.  My guess is the stone was heat treated and/or surface enhanced in order to produce the vibrant color..

Colorado has produced thousands of topaz specimens from numerous localities with some gem crystals approaching five pounds in weight (Eckel and others, 1997)!  Local rock club members are familiar with the best known source of Colorado topaz---the pegmatites associated with the Pikes Peak Batholith (Precambrian in age, around 1.05 Ga).  Several of the Colorado mineral societies have claims that may produce topaz and certainly some society members have individual claims.  For many years a commercial topaz operation was open to the public near Spruce Grove Campground as the Topaz Mountain Gem Mine.  The intrepid rockhounders could purchase “buckets” of material and hunt for the rough crystals.  Today mines of the Glacier Peak Mining L.L.C. at Topaz Mountain “produce the finest U. S. topaz crystals and cutting rough …and perhaps the finest bi-color sherry and blue topaz ever found” (personal communication, Joseph L. Dorris).  If interested in collecting, I would encourage prospective topaz hunters to contact Pinnacle 5 Minerals L.L.C. at www.pinnacle5mineralscom  

A bi-color sherry and blue topaz crystal collected from rocks of the Pikes Peak Batholith, Tarryall Mountains, Park County Colorado.  Stone is ~3.0 X 3.0 X 4.4 cm.  Stone and photo courtesy of Pinnacle5 Minerals.

One of the more interesting aspects of the Pikes Peak topaz is that many of the crystals are found in stream sediments and weathered granite. Prospectors screen for the crystals rather than “pound rock”.  These alluvial specimens are commonly frosted on the surface but have not been transported a great distance from their granitic source. 

Other collecting localities associated with the Pikes Peak Batholith include: Cameron Cone and Bear Creek Canyon in El Paso County; Devil’s Head and Long Hollow (with giant crystals) in Douglas County; Badger Flats area and the Tarryall Mountains in Park County; Wigwam Creek area and the South Platte District in Jefferson County, the latter having produced “sky-blue” crystals weighing over 50 pounds; and Glen Cove on the peak itself in Teller County (Eckel and others, 1997).

However, mineral collectors in Utah (and other states) are well aware of the “other” Topaz Mountain, the famous locality in the West Desert of Utah.  Mineral hunters may owe its discovery to Captain James H. Simpson who served the Army Corps of Topographical Engineers in the southwest part of the U. S. during the 1850’s and was most active in Texas and New Mexico.  However, in 1858 he was assigned to the army of General Albert Sidney Johnson during the “Utah War”.  One of his assigned tasks was to survey a new wagon road from Camp Floyd (the encampment of federal troops southwest of Salt Lake City) to California that was situated south of the more popular Humboldt River Road.  Leaving Camp Floyd in October Simpson only made it as far as the West Desert of Utah but “discovered” the Thomas Range, and most likely Topaz Cove.

Topaz Mountain in Thomas Range, Utah.

The West Desert of Utah is an interesting place (and one of my favorite localities to explore) that displays a tremendous variety of topographic and geologic features.  The area is part of the Great Basin (geographic term) or Basin and Range (geologic term).  This physiographic province stretches from the Wasatch Fault at Salt Lake City (western boundary of the Wasatch Mountains) westward to Reno (eastern boundary of the Sierra Nevada Mountains) and from Idaho-Washington south into Mexico.  The Great Basin refers to the fact that very few streams breech the area and most drainage is internal.  The Basin and Range designation indicates that large normal faults have created uplifted block mountains (horsts) and down-dropped valleys (grabens).  Popular thought is that the ranges are generally composed of fossiliferous Paleozoic sedimentary rocks---for example, the House Range with its famous trilobite collecting localities.  However, the Great Basin also has experienced extensive volcanic eruptions and some ranges are composed entirely of Cenozoic volcanic rocks.  That is the case of the Thomas Range, the location of Topaz Mountain.

The volcanic history of the Thomas Range is quite complex but includes: 1) eruption of flows and breccias from a caldera with subsequent collapse ~40 Ma; 2) eruption of ash flows with filling of the caldera ~32-38 Ma; 3) flows and ashes ~21 Ma; 4) faulting and tilting of the range ~7-21 Ma; and finally 5) the eruption of the topaz-bearing rhyolite ~6-8 Ma (Lindsey, 1998).  Gases percolating through the cooling lava produced cavities called lithophysae and it was within these fissures that topaz and other minerals were deposited.  I believe the composition of these topaz rhyolites (extrusive so fast cooling) are about the same composition as topaz-bearing granitic pegmatites (intrusive so slow cooling.

Today Topaz Mountain is a named feature at the southern end of the Thomas Range that has been set aside by the BLM as a public collecting locality and is commonly known as the “Topaz Cove”.  There are two ways to collect crystals: 1) take a heavy crack hammer and “pound” on the rhyolite, especially in the “honeycombed” areas--the lithophysae; or 2) walk the gullies and slopes looking for crystals weathered loose from the host rock.  The former approach produces the sherry- to amber-colored crystals so prized by collectors.  Most have a single termination and are less than one-half inch in length.  With exposure to sunlight the crystals will lose their coloration within a few weeks!  The latter collecting approach, certainly the least strenuous, will produce numerous clear (non-colored) crystals of various sizes with at least some approaching an inch in length.  Small termination points are common.  Topaz belongs to the orthorhombic mineral system but the termination points take on a variety of forms.

Variety of termination points in topaz from Topaz Mountain.  Upper figure (length) is ~3.1 cm.  Middle is ~2.5 cm.  Lower, each about 9 mm. 

Crystallography of topaz.  Sketches courtesy of The Gem and Mineral Exploration Company.

So, if you are interested in collecting examples of Utah’s state gemstone at the “other” Topaz Mountain, travel to Delta, Utah, via I-70, U. S. 50, I-15, north on U. S. 6 for about 6 miles, then west on the “Brush-Wellman” road for 38 miles.  Follow the signs trending north and west on a gravel road for a few miles and you will arrive at the collecting site—over 600 miles from Colorado Springs. Take along the usual collecting accoutrements including a crack hammer, eye protection, a screen, and “lots” of water.  This is not really a trip for summer months (heat and sun) and the area is isolated (cell phones may not function).  So, plan accordingly.

A final note:   Rocks of the Thomas Range contain a number of other interesting minerals.  See Blog posting of 4/3/2011 describing bixybite.

  

REFERENCES CITED

Eckel, E. B. and others, 1997.  Minerals of Colorado:  Denver Museum of Nature and Science and Fulcrum Publishing.

Lindsey, D. A., 1998.  Slides of Fluorspar, Beryllium, and Uranium Deposits at Spor Mountain, Utah:  U. S. Geological Survey Open-file Report 98-524.

Wilson, J. R., 1995. A Collector’s Guide to Rock, Mineral, and Fossil Localities of Utah: Utah Geological Survey Misc. Pub. 95-4.
A bi-color sherry and blue topaz crystal collected from rocks of the Pikes Peak Batholith, Tarryall Mountains, Park County Colorado.  Stone is ~3 X 3 X 4.4 cm.  Stone and photo courtesy of Pinnacle Peak t