Sunday, October 6, 2019

LÖLLINGITE: AN ARSENIDE FROM THE BLACK HILLS, SOUTH DAKOTA



I continue to be fascinated by minerals containing the element arsenic (As).  The arsenate minerals are those 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 (and often water or hydroxide) : 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).  Examples, annabergite: Ni3(AsO4)2-8H2O.

The arsenite minerals are those containing arsenic in a metallic role and cation (As) and often combing with other metal cations, which in turn combine with sulfur (S the anion) to form a sulfide: arsenopyrite (iron), cobaltite (cobalt), enargite (copper), orpiment (arsenic), realgar (arsenic), proustite (silver), tennantite (copper). Examples, enargite: Cu3AsS4, and orpiment: As2S3.

The arsenide minerals have arsenic (As) as its major anion with a metal as the cation: algodonite (copper), domeykite (copper), nickeline (nickel), skutterudite (cobalt, nickel), löllingite (iron).  Example, nickeline: NiAs

So arsenic plus oxygen forms a negative electrical charge and is an anion.  Arsenic plus a metal has a positive electrical charge and therefore is a cation.  Arsenic as a standalone has a positive electrical charge and is a cation. The arsenates are the most common minerals containing arsenic while the arsenides are relatively uncommon.  The arsenites are somewhere “in-between.”

This fall during a trip to the Black Hills of South Dakota I was able to add an arsenide to my collection, the not-so-common löllingite—iron plus arsenic, FeAs2.  Again, the iron is the positive cation while the arsenic serves as the negative anion.  Trying to describe löllingite is sort of tough.  It is a shiny, silver-white to steel-gray mineral with a metallic luster, a medium hardness of ~5.0-5.5, has a gray-black streak, is brittle, and has an “almost” conchoidal fracture. It can have significant amounts of nickel and/or cobalt substituting for some of the iron.  It looks similar to arsenopyrite and other arsenides and is often confused with such.  I identified my specimen as löllingite since mineralogists have identified the Bob Ingersoll Mine as home to löllingite (and not arsenopyrite).  In the Black Hills löllingite is found in the various pegmatites associated with the Precambrian Harney Peak Granite while at some other localities it is associated with mesothermal igneous rocks (“medium” temperature and pressure).
Silvery, metallic, massive  löllingite in a matrix of albite (with some iron staining).  Width FOV ~3 cm.

Photomicrograph of a portion of above specimen from Bob Ingersoll Mine.  Width FOV ~ 1.6 cm.  


Photomicrograph of very "fresh looking" and silvery
löllingite. Width FOV ~1.0 cm.
 
Very gemmy crystal of blue indicolite variety.  length of crystal ~ 2 mm.
The Bob Ingersoll Mine near Keystone in the Black Hills of South Dakota is one of the better-known mines in the area.  This “fame” is not due to gold or large crystals or ghosts but is because small crystals of elbaite (Tourmaline Group) that are enclosed in muscovite—an interesting situation to say the least.  Several years ago, rockhounds could hike to the mine, or visit a local rock shop and take-home numerous specimens.  This year, during my last visit, I did not see a single specimen available for purchase and a collector told me the mine was off limits!
 
Elbaite crystal encased in muscovite.  Length of crystal ~2.8 cm. Note triangular cross section of a third crystal above.

Nicely striated elbaite crystals weathered out of muscovite matrix. Width FOV ~2.4 cm.

The mineralization at the Ingersoll is found in five zoned pegmatite dikes that were intruded into a quartz-mica schist—all are Precambrian in age. The extracted ores (information from MinDat.org) produced tantalum (tantalite), niobium-rich columbium (columbite), tin (cassiterite), beryllium (beryl), lithium phosphates, feldspar, uranium, and micas (westernmininghistory.com).
 
Cassiterite (black) in matrix of muscovite and albite from Bob Ingersoll Mine.  Width FOV ~4.5 cm.  
 
Photomicrograph of mass of cassiterite (8 mm wide) in albite matrix..
Work at the Ingersoll started in the 1880s and ended in the mid-20th Century.  According to Tom Loomis, on his web site DakotaMatrix.com, the Ingersoll was the discovery site of several large crystals: “About 1915, a large beryl crystal was exposed at the Ingersoll, a nearly perfect hexagon 46 inches across the face. In 1933, another beryl crystal was exposed. This crystal was nine feet high and over eight feet wide and produced 24 tons of ore. A picture of this crystal appeared in the May 1934 issue of Engineering and Mining Journal. Yet another larger crystal was exposed in 1942. This beryl measured 19 feet long and five feet wide on one end and tapered to 19 inches at the other end. Dr. Frank L. Hess of the Rare Minerals Division of the Bureau of Mines during a short reconnaissance trip in September of 1908 to the Black Hills visited the mine and wanted to make a national monument of the crystal (Johnson, 1989). The crystal was eventually mined. The largest crystal of amblygonite was mined at the Ingersoll measuring 28 feet long and six feet in diameter. Blake (1884) reported a 20-inch square by 24-inch long columbite crystal calculated to weigh one ton. Large masses of uraninite have been reported…”


By the way, the trip to the Black Hills was the last part of September when the trees were turning, the bison roaming in the prairies and the smell of frost was in the air.  Life is good.

Lighten up, just enjoy life, smile more, laugh more, and don’t get so worked up about things.      Kenneth Branagh




Wednesday, September 18, 2019

HENDERSON CREEDITE,ANTERO MOLY, CHONDRODITE, AND WOODSTOCK


It has been one of those weeks where the hot weather has sort of fizzled my brain and sent me into memory lane.  Of course, 94 degrees with 10% humidity (Colorado) is not really “hot” compared to my former home in Kansas where my friends were suffering in 107 degrees.  My heat-addled mind often “remembers” in decades and so it was this time, 50 years ago—the summer of 69.  This time period was easy to remember since the country was in an uproar over events in southeast Asia, a great geology scenery movie starring Paul Newman as Butch Cassidy and introducing Robert Redford as the Sundance Kid was released, the U.S. put a “man on the moon” in late July, Charles Manson and his groupies went on a killing rampage in early August, and 400,000+ mostly young people converged on a farm in upstate New York to attend a music festival—Woodstock.  Lately I have been watching “documentaries” on the tube about Woodstock and it brought back many memories.  I liked much of the music (not a fan of Jimi Hendrix tho) but as a small-town kid from conservative Kansas I simply could not really identify with the drugs and seemingly open sex for everyone--was the latter really true?.  What I could identify with was the fear (and reality) of so many young men waiting for the county draft board to ship them off on a bus for a make-believe physical exam, and then off they went to southeast Asia.  If a male college student flunked just one class, it was, as Buffalo Springfield sang:   Paranoia strikes deep / Into your life it will creep, It starts when you're always afraid / Step out of line, the men come and take you away.

I lost friends in the conflict and the mind, intellect, reasoning and judgment of an entire generation was affected and remembered; our country has never healed. 
I’m fed up to the ears with old men dreaming up wars for young men to die in.     George S. McGovern

For the “hippie” pilgrims (the term of the press) of the day, Woodstock was a break in the reality of the war and a time where one, for three days, could just forget and kick back.  Let their mind rest. After the concert was over it was back to the reality of draft boards, physical exams and long plane rides:
Well, come on all of you, big strong men,
Uncle Sam needs your help again.
He's got himself in a terrible jam
Way down yonder in Vietnam
So put down your books and pick up a gun,
We're gonna have a whole lotta fun.
And it's one, two, three,
What are we fighting for ?
Don't ask me, I don't give a damn,
Next stop is Vietnam;
And it's five, six, seven,
Open up the pearly gates,
Well there ain't no time to wonder why,
Whoopee! we're all gonna die.
                                                         Country Joe and the Fish

But again, I was just the small-town kid, married for two years and working hard on a graduate degree at the University of Utah.  I had neither the time nor the money nor the inclination to tromp off to New York.  In fact, I just looked at my field book/notes and noticed that August 15-18 I was tromping around in rural southwestern Wyoming mapping some rock units and collecting vertebrate fossils.  I had been walking and searching all summer and had been concerned that a lack of fossils would not allow me to complete a dissertation and then I was in trouble, big trouble.  However, in mid-August the stars were aligned correctly, and I stumbled across an opening in the sagebrush north of Evanston and saw bone chips and part of a turtle scute. My heart fluttered as I sank to my knees and stuck my face to within six inches of the ground and started seeing small mammal teeth and bones.  I knew at that moment “things” were going to be OK and I could handle the rest of my graduate academics.  Maybe, as a popular 5th Dimension 1969 song belted out, we were in the Age of Aquarius and Jupiter-Mars aligned with my fossil find!
When the moon is in the Seventh House
And Jupiter aligns with Mars
Then peace will guide the planets
And love will steer the stars
This is the dawning of the age of Aquarius
Age of Aquarius
Aquarius
Aquarius

So, the summer of 69 was an important time in my life.  Wow, 50 years ago.  The summer of 2019 (with some poor music) has not been as eventful but never-the-less has been interesting and I always look forward to the numerous rock and mineral shows scheduled for the summer months.  A week or so ago (I am still writing a month later) I was able to wander up to Woodland Park for the show and enjoyed visiting with many vendors and picking up a few specimens.  I mean how can one attend a mineral show and not come home with something?

I have a detailed posting on pyrrhotite (July 28, 2014) so will not repeat that information.  In fact, I really did not need another specimen of the iron sulfide (Fe7S8); however, this was a very nice piece, cheap, and available.  Pyrrhotite is sort of a strange mineral is that it is magnetic and easily attracts a magnet as a variable amount of iron vacancies in its crystal structure. The specimen was collected from the Potosi Mine, Santa Eulalia District, Mun. de Aquilies Serdan, Chiluahua, Mexico.



There are nice brassy crystals scattered on the specimen.  width FOV ~3.7 cm.

Photomicrograph of pyrrhotite crystals although light reflection does not allow for the brassy color to come through. Crystals are tabular in cross section but hexagonal when viewed down the C axis (as these are).  Width FOV ~1.2 cm.
Photomicrograph calcite crystals.  Width FOV ~1.2 cm.

Photomicrograph shiny black sphalerite crystals along with pyrrhotite and calcite .  Width FOV ~ 1.2 cm.


A second specimen is not a rare or uncommon mineral but is a new specimen (for me) from a favorite locality:  molybdenite (molybdenum sulfide: MoS2) from the California Mine near Mt. Antero in Chaffee County, Colorado. The Mt. Antero gem mines (aquamarine mostly) were among the first high altitude localities that I explored upon moving to Colorado in 2006.  The geology of Mt. Antero/Mt. White has been described in numerous publications, the most comprehensive being Mark Jacobson’s book entitled Antero Aquamarines (1993).  In general, Mt. Antero/Mt. White is underlain by a rock unit termed the Mt. Antero Granite.  McIntosh and Chapin (2004) have assigned a date of 29.6 million years to the granite, or mid-Oligocene in age. Sharp (1976) described the granite as chiefly pinkish-orange, medium grained;… youngest of the plutons (intrusive igneous rocks) in the vicinity of Mt. Antero.  Miarolitic cavities (crystal lined cavities often containing unusual minerals) are common and are often filled with…beryl (including aquamarine), phenakite, and smoky quartz.

A flat plate-like matrix of beryl measuring ~5.7 cm. maximum width, and coated on both sides by bright flaky silver molybdenite crystals.  BTW, I cannot remove the photo of this specimen from the pyrrhoite photos--another Blogger quirk!
Photomicrograph from above specimen showing flaky crystals.  Width FOV ~1.0 cm.


My early trips to Mt. Antero were accessed by walking (twice) up the “Mt. Antero Road” through Baldwin Gulch off Chalk Creek Road west of Nathrop.  This is the major road that almost all rockhounds bounce and bump up. My initial walking treks were due to the fact that I wanted to summit (fairly) Mt. Antero at 14,276. Since the California Mine is a couple of miles southwest of the mountain on the flank of Carbonate Mountain at an elevation of ~12,500 feet, I decided one day (before several joint replacements) that a nice hike was in order and trudged up the Browns Creek Trail starting off US 285 south of Nathrop. 

The location of the California Mine is sort of obscured in the large talus slope on the mountain and an adit has been blasted shut. In 1953 Adams noted that “molybdenite occurs sparsely in small veinlets in the massive quartz outcrop of the vein…and is concentrated along surfaces that presumably represent the walls of open channelways or vugs. Away from such surfaces, it is in isolated flakes that in many places project into small voids between beryl and quartz crystals.”


I really did not find much of interest on the hike (except beauty and tired legs after a 12 mile in and out) and needed to retreat to a lower elevation as a summer monsoon storm was blowing in.  I do not enjoy electrical storms and quickly left the area never to return.  As best that I can tell, the California Mine had beryllium production from the beryl; however, I could not locate production figures.  MinDat also reported the presence of molybdenum and tungsten but again no production figures fell into my hands.  In fact, I am uncertain as to which mineral contained the tungsten.
Therefore, I was happy to locate the specimen shown below.  It originally came from the collection of coloradominerals.com, the owner/collector unknown since the domain site is now for sale.

Radiating aggregates of prismatic "soft" violet crystals of creedite from the Henderson molybdenite mine.Width FOV ~6.1 cm.
  

 Photomicrograph of a section from above specimen.  Width FOV ~1.4 cm.
A third specimen I acquired at Woodland Park is again a mineral that is not uncommon but certainly rather rare in Colorado. In a Posting on July 10, 2016, I described a specimen of creedite [Ca3SO4Al2F8(OH)2-2H20], a hydroxyhalide mineral (minerals with a Halide Group anion—chlorine, fluorine, bromine, iodine--- plus the hydroxide radical) from the Cressen Pit at Cripple Creek (the famous gold mine).  Although the mineral creedite had been named (1916) from a locality in the San Juan Mountains near the community of Creed, it was not until 2001 that the second Colorado locality was recorded from the Cresson Pit.  And finally, in 2007, a third locality was reported from the Henderson molybdenum mine near Climax.  The Woodland Park show was the first time I had seen a specimen from this latter locality for sale so I “nabbed it.”


The final specimen brought back from Woodland Park is actually one that was not in my vocabulary (not a hard task to accomplish)---chondrodite (not to be confused with chondrite, stony meteorites) . As a softrocker I was totally unfamiliar with this metamorphic mineral, but it was cheap at four bucks and had some nice gemmy crystals.  Chondrodite is a somewhat rare neosilicate mineral [(Mg,Fe)5(SiO4)2(F,OH,)2] and is formed in a contact metamorphic environment, usually where hydrothermal solutions (created by heat from an intrusion) comes in contact with carbonate rocks (limestone or dolomite ormaybe even carbonatite) and where fluorine has been introduced by the fluids.  Most crystals of chondrodite are yellow, orange, red, red-orange, brown, red brown and occur as individuals or masses in remnants or the original carbonate, usually calcite or marble.  Changes in color intensity in the crystals is common.  Crystals are brittle and are easily fractured in a conchoidal manner.  Chondrodite is fairly hard at ~6.0-6.5 (Mohs), has a vitreous to greasy luster and a yellow to yellow-gray streak. It is transparent to translucent.  Some red crystals are quite gemmy and have been faceted.  In addition, a few crystals fluoresce an orange (SW) to yellow orange (LW).


 Crystals of chondrodite in a matrix of rhombohedral calcite.  Width FOV ~7.0 cm.
Photomicrograph of chondrodite crystals showing changes in color intensity.  Matrix of rhombohedral calcite.

In the United States the best specimens of chondrodite seem to be associated with intrusive rocks in New York and New Jersey, especially the Tilly Foster Mine located near Brewster, Town of Southeast, Putnam County, New York.  The mine was active from 1843 until 1897 with peak iron production in the 1870s, mainly from magnetite.  MinDat lists 120 minerals and stated the mine “is famous for its excellent brucite, chondrodite, clinochlore, titanite, and magnetite crystals and antigorite or lizardite (and other species) pseudomorphs after a wide variety of minerals.” 

REFERENCES CITED

Adams,  J.W., 1953, Beryllium deposits of the Mt. Antero Region, Chaffee County, Colorado: U.S. Geological Survey Bulletin 982-d.

Jacobson, M.L., 1993, Antero aquamarines: Minerals from the Mt. Antero-White Mountain Region, Chaffee County, Colorado: L R Ream Publishing.

McIntosh, W.C. and C.E. Chapin, 2004, Geochronology of Central Colorado Volcanic Fields: New Mexico Bureau of Mines and Mineral Resources.

Sharp, W.N., 1976, Geologic Map and Details of the Beryllium and Molybdenum Occurrences, Mount Antero, Chaffee County, Colorado. U.S. Geological Survey Misc. Field Studies Map MF-810.





If you want to see a little Woodstock live see if this will load: