Tuesday, June 14, 2016

STICHTITE: PIKES PEAK GEM AND MINERAL SHOW


A wide view of the inside of the event center.  Several members of the CSMS sold items or had display cases.
The 53rd annual Pikes Peak Gem and Mineral Show was held at the Mortgage Solutions Financial Expo Center in Colorado Springs on June 3-5, 2016.  For the first time in many years the Show was entirely “inside” and that move was a nice change of pace compared to past shows---no rain, no high winds, plenty of room for displays and vendors, and no forest fires!  In fact, the 50+ vendors represented an amazing number for our venue, and a wide variety geology-related items were offered “for sale.”  I was really impressed with the diversity of minerals, rocks, gems and jewelry offered by vendors.  Kim and Bodie, along with a host of club volunteers, are to be congratulated for producing this awesome Show.

As usual, I was on the lookout for a few less expensive and rather uncommon minerals----no Brazilian amethyst or Arkansas quartz!  And, I consider my hunt successful as my modest collection picked up several nice specimens of interesting and colorful minerals. 
Stichtite encrusting chrysotile?, a serpentine mineral.  Specimen width ~6.5 cm.
One purchased item was a mineral that was completely unfamiliar to me: violet- to lilac-colored stichtite encrusting a green serpentine mineral (probably chrysotile or perhaps clinochrysotile---precise identification above my pay grade).  Stichtite is a hydrated carbonate of magnesium and chromium: Mg6Cr2(OH)16[CO3]-4H2O and is some sort of an alteration product of serpentine.  Although many of us routinely refer to serpentine as a mineral, it actually is a “group” of minerals with the generic formula of D3[Si2O5](OH)4 where D is magnesium, iron, nickel, manganese, aluminum or zinc or a combination of cations.  Several members of the “group” are asbestiform in nature and have health warnings about breathing in the fibers. I believe my specimen is the serpentine mineral chrysotile: Mg3(SiO5)(OH)4.  In addition to the chrysotile and stichtite, the specimen contains tiny grains of magnesiochromite [Mg(Cr,Al,Fe)2O4] and a somewhat substantial amount of magnetite---it readily attracts a magnet.
Bands of magnetite crystals (<----M) are interlaced in the serpentine.  Crystals of lilac-colored stichtite are replacing tiny crystals of magnesiochromite (<---------).  Crystal size less than 1 mm.
However, I purchased the specimen due to the beautiful colored stichtite contrasting with the green serpentine. It was collected from Stichtite Hill, Dundas Mineral Field, Zeehan District, Tasmania, Australia.  Most people know the island of Tasmania as home to the Tasmanian Devil, an endangered carnivorous marsupial.  This little critter is much better known that the mineral stichtite!
The Tasmanian Devil.  This cute little fellow is carnivorous and most likely would nip your finger.  Public Domain photo.
Taz, the Tasmanian Devil ©, the hero/villian in Loony Tunes cartoons.   Created by Robert McKimson and owned by Warner Bros.
Most stichtite specimens on the market were collected from a few localities (about five) with the best known being Stichtite Hill.  According to Bottrill and Brown (2000), specimen mining at the Hill produced material for carving—lilac colored stichtite on green or green-black serpentine.  Bottrill (MinDat.com, 2009) described the occurrence of stichtite deposits at Dundas as “hosted by generally massive serpentinite bodies, probably derived from altered chromite-rich dunites [olivine-rich rocks] within mid-Cambrian ultramafic complexes [rocks with low silica content and high magnesium/iron content], particularly the Dundas ultramafic complex (Brown, 1986; Burrett and Martin, 1989). These complexes are a series of thrust-emplaced [faulted], dismembered ophiolitic bodies along the Dundas Trough, which bisects western Tasmania. They were deformed and intruded by mineralised granites during the Devonian era (Brown 1986). The occurrences are erratic…”  Ashwal and Cairncross (1997) noted stichtite “occurs exclusively in Cr-rich serpentinites of ophiolites or greenstone belts…with stichtite formation invariably [post-dating] serpentinization.”  

In reference to the above paragraph, one that is tough to understand by a non-student of geology, consider the following abbreviated explanation:

The earth is composed of a number of moving plates and the plates include oceanic crust (rocks deposited in a marine environment) and continental crust (rocks that make up the continents).  When a continental plate collides with an oceanic plate, rocks of the oceanic plates are forced under (subducted) the continental plate since the later has rocks of a higher density.  Sometimes upper rocks of the oceanic plate are scraped off by the continental plate during subduction.  These rocks are termed ophiolites.  The collision of plates are sites of earthquakes, volcanism and mountain building.  During earlier studies geologists referred to the moving and collision of plates as “Continental Drift” but today we know the process as “Plate Tectonics.”  The entire process is quite complex and not quite as simple as noted here.

Many ophiolites have a high percentage of olivine and pyroxene group minerals that started out as mantle or near mantle rocks (deep seated rocks below the earth’s crust).  As these mantle rocks are brought toward the surface by tectonic forces during mountain building events along subduction zones, the minerals in the rocks begin to destabilize (oxidation, metamorphism, input of water) and the rocks change into serpentine group minerals.  Ashwal and Cairncross (1997) then believe the stichtite formed by a “reaction between serpentine and altered chromite during addition of substantial fluid.”  That fluid is either water (H2O), some phase of CO2 or carbonic acid (H2CO3)

So, the rocks at Dundas were subjected to the process of serpentinization and then invaded by fluids (post-serpentine formation) that interacted with the serpentine and altered chromite resulting in the lilac mineral stichtite along with the original green chrysotile (or some serpentine mineral).

That is the best that I can do with the process. Geologists first learned about seafloor spreading (think Mid-Atlantic Ridge) ca. 1960 although the idea of Continental Drift was thrown around in the earlier part of the 20th Century by Alfred Wegener.   After the validation of sea floor spreading the theoretical model of plate tectonics was put forth amid much debate.  My university “schooling” was during the 1960s and needless to say the plate tectonics theory was widely discussed in almost every geology class.

But back to stichtite.  The lilac- to violet-colored mineral is very soft (1.5-2.0 Mohs) and may be scratched by a fingernail.  It appears translucent to transparent in very thin “slices” I could peel away.  These thin slices almost appear micaceous or fibrous under the high power of a microscope but under eye observation the mineral appears as encrusting.  It is subvitreous to waxy in luster. It is non-magnetic but this can be deceiving.  Intermixed with the mineral and seemingly in layers beneath the encrustation are magnetite crystals and fibers.  Stick a magnet to the lilac mineral and it will adhere due to the underlying magnetite therefore appearing to indicate that stichtite is magnetic!  I could not locate identifiable stichtite crystals; however, MinDat notes it belongs to the Trigonal System.  Stichtite has a polymorph, stichtite-2H, that belongs to the Hexagonal System but has the same chemical formula (again above my pay grade for identification).
A very thin crust of stichtite.  Width of photo ~5.1 cm.

Reverse side of photo above.  The color is brighter and the encrustation appears as micaceous or fibrous plates.
Photomicrograph of a section of above photo.  Width ~ 1.4 cm.  Note bands of magnetite.
Photomicrograph of same specimen showing the fibrous nature of both the stichtite and chrysotile. Width ~ 1.2 cm.
Minerals with chromium acting as the chromophore are fun to collect since the element produces a variety of colors: the red or pink in rubies, the green in emeralds and purple or red in alexandrite.  In pink or red spinel some chromium ions replace aluminum and voila. So, is chromium the pink/lilac coloring agent in stichtite?  Maybe, but that is a question for my mineralogist friends, maybe Tom up at www.dakotamatrix.com or Pete up at USGS.




But, it was nice to add stichtite to my collection.  

REFERENCES CITED

Ashwal, L.D. and B. Cairncross, 1997, Mineralogy and origin of stichtite in chromite-bearing serpentinites: Contributions to Mineralogy and Petrology, v. 127.

Bottrill, R. S., and G. Brown, 2000, Rare Australian gemstones: stichtite: Australian Gemologist, v. 20.

Brown, A.V., 1986, Geology of the Dundas–Mt Lindsay–Mt Youngbuck region: Bulletin Geological Survey Tasmania, no. 62.

Burrett, C.F., and E.L. Martin, (ed.), 1989, Geology and Mineral Resources of Tasmania: Special Publication Geological Society of Australia, no.15.

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