Friday, May 5, 2017

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

Thursday, April 27, 2017

PARISITE: CERIUM RARE EARTH MINERAL



Learning is an everyday event for a slumer like me.  For example, take the element cerium!  Before today, what did I know about cerium?  Turns out not much.  I knew that: it was named after the dwarf planet Ceres; 2) cerium oxide is used in polishing high quality glass lenses, and as a final polish for some lapidary specimens; 3) it is “probably” a Rare Earth Mineral (REE); and 4) somehow it is used in gas (the lantern gas, like Coleman) mantles.   Other than those factoids my knowledge about cerium was pretty sparse.
OK, Ceres is the largest planet, or “object” that is positioned between Mars and Jupiter in the asteroid belt (that factoid came from my early 1960s era astronomy class).  In turn, Ceres was named after the Roman Goddess of agriculture and fertility. 

Cerium oxide is both unstable Ce2O3 and stable CeO2 and the latter is often referred to as cerium(IV) oxide and is the type used in polishing compounds. 

Cerium is a lanthanide element—one of the 15 metallic elements with atomic numbers 57-71.  This group also contains elements not overly familiar to non-chemists such as neodymium and europium.
Pretty amazing to me is that cerium is much more abundant in the earth’s crust than lead or tin and despite the REE moniker is not very rare.  As for lantern mantles, I fail to understand the chemistry but mixing thorium oxide with cerium oxide and coating the silk mantles produces a very white light.  I suppose today very few “younger persons” know how to properly install a mantle in a “gas lantern!”  Of course, as a kid I knew next to nothing about burning whale blubber oil but did know how to “trim a wick” in a kerosene lamp!

I have learned that in nature cerium seems to never occur as a stand- alone element and that most cerium is produced from mining and refining the Rare Earth Minerals monazite (lanthanide + thorium + PO4) and bastnäsite (lanthanide carbonate fluoride).  Several months ago, I offered a post on bastnäsite (May 9, 2013).  Knowing a little more, but not much, about REE and REM after a later posting on a field experience to a REM mine near Colorado Springs (October 17, 2015), I was surprised to see a specimen of the REM parisite offered for two bucks at a mineral store in Tucson.  I did not have the slightest idea what parisite was when I found the specimen in an isolated drawer of thumbnails.  But due to what Phillip Caputo (2013) calls the MD (Magic Droid), I soon learned much and for that low price will pick up most anything that is not in my collection!

So, parisite is a REM (and actually rare) composed of calcium, cerium, lanthanum combined with a fluoro-carbonate, or Ca(Ce,La)2(CO3)3F2.  Well, that last statement is sort of a generality because mineralogists “in the know,” and with an XRD or some other gizmo providing analytical information, use parisite as a general term and designate the minerals parisite-(Ce), parisite-(Nd) and parisite-(La) depending on the “domination” of these REE.  And then it becomes quite easy to confuse parisite (any type) with lanthanide-dominant specimens of bastnäsite, synchysite, and röntgenite,  And, so its goes.
 
Photomicrograph of a banged-up and chipped parisite crystal.  Note partial hexagonal faces at arrow.  Height of crystal ~1.3 cm.
Therefore, the question then becomes, what is the correct name for my purchased specimen.  I am going to settle on parisite-(Ce) [CaCe2(CO3)3F2], not because I have a XRD in my back pocket, but because MinDat has stated the cerium variety is the mineral species at the Snowbird Mine in Montana, the home of my small collected crystal.
Parisite is not what one would call a spectacular mineral, nor a museum piece, but one that is mainly of interest to rockhounds.  It seems only to occur as definite crystals as not as massive or encrusting forms.  Parisite belongs to the Hexagonal Crystal System and commonly occurs as double hexagonal pyramids that at times appear to be prismatic.  However, the crystals are not prime specimens and often are striated and very “rough” looking.  Crystals are often brown or amber in color but at times range down to brownish-yellow to yellow.  Specimens are very brittle, are transparent to translucent with a yellow-white streak, and have a sub-vitreous to greasy luster.  Hardness has been measured as ~4.5 (Mohs).  Almost all crystals are small in size and I suppose 3 cm. would represent a very large specimen.  

Metz and others (1985) describe the Snowbird Mine as occurring in “a lenticular, rare earth- and fluorite-rich quartz-carbonate body with a pegmatitic texture, which intrudes Belt Supergroup metasediments [late Precambrian] at the Idaho-Montana state line west of Missoula, Montana…the U-Th-Pb parisite ages (71.1 + or - 1.0 m.y.) indicate emplacement during the Late Cretaceous, probably associated with the intrusion of the nearby Idaho batholith.”

REFERENCES CITED

Metz, M.C., D.G. Brookins, P.E. Rosenberg and R.E. Zartman, 1985, Geology and geochemistry of the Snowbird Deposit, Mineral County, Montana: Economic Geology, vol. 80 no. 2.

Caputo, P., 2013, The longest road: overland in search of America from Key West to the Arctic Ocean: Henry Holt and Company, New York.

I had only one hard-and-fast rule:  avoid interstates.  They are predictable and boring and their uniformity somehow erases changes in landscape; you can drive six hundred miles, from forests into deserts, and fell that you haven’t gone anywhere.  In a sense, you haven’t.  You have no idea about the lives of the people in the towns and cities you’ve bypassed at seventy miles an hour.      Phillip Caputo

Saturday, April 22, 2017

CHENEVIXITE: A GREEN ARSENATE



I continue to be fascinated by minerals containing the element arsenic (As).  The arsenate minerals are those minerals containing the anion AsO4- - -  and are often grouped/studied together with the phosphate minerals [PO4 - - -] and the vanadate minerals [VO4- - -].  Since these three anions are about the same size with the same charge, minus-3, they often replace and substitute for each other and a new mineral is born. I have written many posts about the arsenates and they include a metallic cation plus the AsO4 anion: annabergite (nickel), austenite (copper and zinc), clinoclase (copper), conichalcite (calcite and copper), cornubite (copper), cornwallite (copper), erythrite (cobalt), chenevixite (copper and iron), mimetite (lead), and olivenite (copper).  An example, annabergite: Ni3(AsO4)2-8H2O

The arsenite minerals are those containing arsenic in a metallic role and cation (As) and often combing with other metals which in turn combine with sulfur (the anion) to form a sulfide: arsenopyrite (iron), cobaltite (cobalt), enargite (copper), orpiment (arsenic), realgar (arsenic), proustite (silver), tennantite (copper). Example, enargite: Cu3AsS4

The arsenide minerals have arsenic (As) as its major anion: algodonite (copper), domeykite (copper), nickeline (nickel), skutterudite (cobalt, nickel), lollingite (iron).  Example, nickeline: NiAs

The arsenates are the most common minerals containing arsenic while the arsenides are relatively uncommon.  The arsenites are somewhere “in-between.”

Last summer at the CSMS Show I was rummaging around and came across a specimen of chenevixite, a mineral completely unknown to me.  However, the green color indicated the possible presence of copper so I scooped it up.  The price tag in a broken-down specimen box said $2, a good bargain. 
Chenevixite is a hydrated copper iron arsenate with copper and iron as the major cations and the arsenate ion as the anion [Cu(Fe)(AsO4) – (OH)2].  It is a rare mineral and found in the secondary oxidized zone of polymetallic ores.  Chenevixite represents the oxidation product of the primary sulfides enargite and tennantite (both copper arsenic sulfides).   
Coating of green chenevixite on matrix.  Width of specimen ~5.5 cm.
Chenevixite is tough to recognize in hand specimens without knowing something about the mining location—the crystals are much too small to see without help of a magnification device.  The mineral is some sort of a green color from yellow-green to olive green to dark green.  Chenevixite appears as a massive coating on matrix and the crystals are cryptocrystalline, much too small to be picked up with my camera equipment.  Luster is hard to distinguish, not really earthy but certainly not bright, perhaps “oily.”  Hardness is ~4.0 (Mohs) and the massive form appears opaque but that is difficult to determine; it may be semi- translucent.  It does produce a yellow-green streak.

Photomicrographs of massive chenevixite coating matrix.  Individual, submillimeter,  "globs" may be observed in some masses.  Width of specimen ~1.1 cm.
With a name like chenevixite I suspected the name came from a French locality or perhaps a French scientist. It was named for Richard Chenevix (1774-1830) an Irish chemist born in Dublin but who later lived and died in Paris.  Its Type Locality is from Wheal Gorland in Cornwall, England. Chenevixite forms a solid solution series with luetheite as aluminum replaces the iron.

My specimen came from the Chuquicamata Mine in the Atacama Desert of Chile (west Coast).  The scarcity of chenevixite in the world may be due, at least partially, to the fact that it is one of the few arsenic minerals that is stable in arid regions but often leaches in more humid region.  The minerals of the Desert are usually rare in other environments.

Now, here is a question above my pay grade: bronze is a combination of copper and another metal, usually tin, whose discovery, was a great metallurgical feat since it allowed the construction of “harder” implements and weapons. However, the first bronze was made with copper and arsenic and termed arsenical bronze.  I wonder if chenevixite ever provided both copper and arsenic, was ever smelted into bronze? Another one of one of life’s persistent questions!


Albert grunted. Do you know what happens to lads who ask too many questions?
Mort thought for a moment.
No, he said eventually, what?
There was silence.
Then Albert straightened up and said, Damned if I know. Probably they get answers, and serves 'em right.          Terry Pratchett