DURANGITE: RED ARSENATE, WEST DESERT, UTAH

I can see for miles and miles
I can see for miles and miles
I can see for miles and miles and miles and miles and miles
Oh yeah

The lyrics by The Who seemed constantly in my head as I traveled the West Desert in 1967-1970.

The Thomas Range is located southwest of Salt Lake City and northwest of Delta, Utah.  Map from RealGems.org

I first laid eyes on the Thomas Range in western Utah during the late 1960’s and was duly impressed.  Coming from Kansas and eastern South Dakota, I simply had not seen so many volcanic rocks in one place in my life.  It was overwhelming as I tried to understand the mechanism behind the explosions that dumped huge piles of rhyolite (mostly) that formed the Thomas Range.  On a field trip, we were examining the isolated mountain ranges in the Bain and Range Physiographic Province, mostly looking at lower Paleozoic sedimentary rocks and their contained fossils.  We had just visited the nearby House Range and had collected “lots of” Cambrian Trilobites from the limestones and shales.  But here was a magnificent pile of volcanics that needed exploring, but alas the rocks were without fossils so on we traveled.

Topaz-rich rhyolite exposed at Topaz Mountain.

Upon returning to Salt Lake City I soon learned that the Thomas Range contained a large supply of easily found topaz crystals, and that a major beryllium discovery had been recently located on the west side of the Range (the Spor Mountain area).  Well, since the only topaz I had previously seen was in the university mineralogy collection, the first thing that came to my mind was “road trip.”

Lord, I was born a ramblin' man,
Tryin' to make a livin' and doin' the best I can.
And when it's time for leavin',
I hope you'll understand,
That I was born a ramblin' man.

                              Allman Brothers, 1973. Road trip? Yes

Later in life I again hit (several times) the Thomas Range for topaz, mostly as side trips while collecting invertebrates in the House and Confusion ranges.  The Utah West Desert is a lonely place with little water, scarce gasoline, and few non-human inhabitants.  This was brought out in great reality one day when my graduate student and I spent a fair number of hours extracting our vehicle from a draw we had foolishly tried to cross.  We were in the process of examining a proposed power line route from western Colorado to Ely, Nevada, and had made a less-than-smart decision.  But, we were able to get the pickup back on the trail and ended up camping at Painter Spring on the west side of the House Range with a gorgeous view of the night sky and without a single offending human light.

On the first part of the journey
I was looking at all the life
There were plants and birds and rocks and things
There was sand and hills and rings
The first thing I met was a fly with a buzz
And the sky with no clouds
The heat was hot and the ground was dry
But the air was full of sound

                  Ah, my favorite “desert song.”  America 1971

The West Desert 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.     

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) rhyolitic flows and ashes ~21 Ma (especially the “beryllium tuff” at Spor Mountain); 4) faulting and tilting of the range ~7-21 Ma; and 5) the eruption of a second rhyolite explosion ~6-8 Ma (Topaz Mountain) (Lindsey, 1998).  Christiansen and others (1984) noted that both the Spor Mountain and Topaz Mountain rhyolites are enriched with fluorine, and also Be, Li, U, Rb, Mo, and Sn, and hence provided the fluorine needed for the formation of topaz.  Evidently, these topaz-rich rhyolites are common across the western United States and Mexico.

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 the topaz crystals: 1) take a heavy crack hammer and “pound” on the rhyolite, especially in the “honeycombed” areas--the lithophysae (the cavities formed by gas); 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 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.

Bixbyite cube attached to topaz.  Length of topaz ~ 1.5 cm.

A few years ago, I lead an intrepid group of CSMS members to the West Desert in a quest for topaz, trilobites and obsidian.  I was lucky enough to score a couple of specimens or a rare manganese iron oxide called bixbyite [(Mn,Fe)₂O₃]. I also returned home with several tens of pounds of “raw” rhyolite, hauled down from the “amphitheater,” so I could pound away some winter day.  There were surprises in those rocks, including pseudobrookite, an iron-titanium oxide, [Fe₂TiO₅].  I also stumbled on a small crystal of red beryl, a story for another day.  What I was missing, however, was a specimen of durangite [NaAl(AsO₄)F], a pretty rare sodium aluminum arsenate fluoride that is a nice red color and commonly displayed as crystals.  Like topaz, the fluorine-enriched rhyolite supplied elements for the durangite.

Crystals of pseudobrookite in topaz-rich rhyolite.  Longest crystals ~4 mm.

Last year I was fumbling around a box of dusty thumbnail specimens at a Tucson venue and saw the Shannon Minerals specimen labeled Durangite, Topaz Mountain, Thomas Range, Juab Country, Utah.  I was excited at the find—an arsenate, from Utah, from the Thomas Range, and rare--- and, it soon joined my collection.

Durangite usually ranges from orange to red, orange-red and occasionally green, It is translucent, especially in less massive crystals.  MinDat noted specimens appear orange-yellow in artificial light and artificial crystals are always green.  The crystals are vitreous and fairly hard at ~5.5 (Mohs).  Crystals belong to the Monoclinic Crystal System and are described as “oblique pyramidal.”  Crystals are quite brittle and impart a cream-yellow streak.  Evidently the red color is imparted by minor amounts of iron.  Durangite forms a solid solution with maxwellite (sodium iron arsenate fluoride) and tilasite (calcium magnesium arsenate fluoride).

Topaz-rich rhyolite with several crystals and "masses" of durangite.  The <---- points to crystal shown below.  Width of specimen ~9 mm.

Photomicrograph of durangite crystal face noted above.  Width ~2 mm.

Photomicrograph of durangite crystal, submillimeter in width.  Arrows ----> point to tiny shards/crystals of cassiterite? (tin oxide).

As I understand, durangite is found in a very limited locations in the Thomas Range.  In fact, there may only be one location and that is usually referenced as the “Durangite Locality.”  I presume John Holfert, the Master Collector of Thomas Range minerals, discovered and claimed the site.  I don’t have the slightest idea if the Durangite Locality is still producing, or if other localities are available.  The Type Locality is near Durango, Mexico; however, most collectors seem to believe the “finest specimens are from the Thomas Range.  

REFERENCES CITED

Christiansen, E. H., J.V. Bikun, M.F. Sheridan, and D.M. Burt, 1984, Geochemical evolution of topaz rhyolites from the Thomas Range and Spor Mountain, Utah: American Mineralogist, v. 69, no. 3-4.

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

 I've been through the desert on a horse with no name
It felt good to be out of the rain
In the desert you can remember your name
'Cause there ain't no one for to give you no pain