IT'S THAT DANG CUROSITY AGAIN: THE SULFARSENIDE COBALTITE

The important thing is not to stop questioning. Curiosity has its own reason for existing.    Einstein

In my last posting on rouxelite I noted the mineral was a member of the Sulfide Class, minerals containing either the sulfide anion S^2-or the disulfide anion S₂^2-, that is either one or two sulfur ions, each with an oxidation state of 2 minus (2^--). The formula is often written as A_mS_n where A is a metal, often iron, copper nickel, lead, silver or zinc, S is sulfur, and m and n are numbers indicating cations /anions present. Most rockhounds will recognize the iron sulfide mineral, pyrite—FeS_2. (one iron cation and two sulfur (a disulfide) anions. Or perhaps galena, the lead sulfide PbS—one lead cation and one sulfur anion.

One of the more interesting and complex group of sulfides are the sulfosalts. These minerals contain: 1) a metal (mostly lead, copper, iron or silver although a few others, mercury, zinc, vanadium may be present); 2) a semi-metal like arsenic, germanium, antimony, or the post-transitional metal bismuth, or the metals tin or vanadium’ and 3) sulfur but perhaps selenium or tellurium (Richards, 1999). In case you are wondering, semi-metals are elements with properties both of a metal and of a non-metal and are the following: boron, silicon, germanium, polonium, arsenic, antimony, tellurium, and tennessine (radioactive, artificially produced element).  All are interesting elements: each semi-metal takes several different forms (allotropes), but all have at least one form that is shiny and metallic looking.  All are solid at room temperature and pressure, and act as nonmetals in chemical reactions. They are poor conductors of electricity (unlike metals) but make excellent semiconductors. Most are malleable and some are ductile, Semi-metals can form alloys with metals and a lead-antimony combination is an important industrial component of batteries and cable sheaths. Most semi-metals are rarely found in the natural state but are common in combination with other elements. For example, silicon is the second most abundant element on our earth (after oxygen) but does not occur uncombined in nature. So, these are the semi-metals and sulfosalts. Interesting? Yes! 

The sulfarsenide minerals are also a group of sulfides with metal(s) plus semi-metal(s) plus sulfur and therefore must seem related to the sulfosalts. However, there is a big difference in the two groups in that the semi-metal arsenic has moved from a positively charged cation to replacing some of the sulfur as a negatively charged anion as in the minerals arsenopyrite [FeAsS] and cobaltite [CoAsS]. These minerals confuse me (not hard) since I did not realize that both sulfur and arsenic may have oxidation states of 1 minus so cobaltite is Co²⁺, S^1- and As^1- . Because the −1 oxidation states for S and As are not stable in solution, the mineral reactions always involve oxidation of the S and As to some higher oxidation state in order to be released into solution (https://chem.libretexts.org/). 

I recently acquired a couple of micromounts of cobaltite that exhibit really nice crystals. One specimen is from Håkansboda, Bergslagen Mining District, Sweden, and was, at one time in the collection of Al Kidwell (he of kidwellite fame—see Posting July 7, 2015). As best I can determine the stratigraphy of Bergslagen [part of a Precambrian Shield] is quite complex with original volcanic and sedimentary rocks of Precambrian age [1.8-1.9 Ga] intruded by granite, folded and faulted, and then invaded by hydrothermally emplaced sulfides (pyrite, chalcopyrite, sphalerite, and galena) when the hot fluid interacted with carbonates and mixed with cool seawater in a seafloor environment (Kampmann and others, 2017).

Håkansboda is a very old mining district (at least to U.S. standards) and started some time in the 1400s but was never a large producer (Falun was the 800 pound gorilla in the region—see Posting July 1, 2020. Tegengren (1914 in MinDat) noted that total copper production from 1613 until 1905 was about 2100 tons with 7 tons of cobalt during 1836-1841. Currently the Håkansboda ore field is under intensive investigations and has shown good potential for copper, cobalt, iron, zinc, lead, silver, nickel, and potentially REE’s (Månbro, 2021). Although most cobalt production was as a byproduct of copper mining, some cobaltite ore is present and has produced many of the finest crystals in the world.

Cobaltite crystal. Width of crystal ~1.5 mm.

The second micro was collected by a J. Seguin in Ontario, Canada, and mounted in 1973 by Art Smith. I could not locate good information on Seguin and I presume the crystal was collected from the most famous locality in the Cobalt/Sudbury, Ontario, Mining District, the “Brazil Lake occurrence.” According to Sabina (1991) cobaltite euhedral crystals measuring up to 3 cm in diameter occur with actinolite amphibolite along the walls of a quartz-dolomite calcite vein containing pyrrhotite and chalcopyrite.

Cobaltite crystals on a micromount stand. Width FOV ~6 mm.

 The Cobalt, Ontario, portion of the District is located northeast of the town of Sudbury and regardless of its name, silver was the major metallic commodity. At one time in the early 1900s the Cobalt area mines were the world’s largest producer of silver and total production over the years totaled nearly a thousand tons.  The silver was associated with nickel and arsenic minerals like skutterudite.

I was unable to locate exact production figures of cobalt and nickel for the Cobalt area; however, Young and Perrone (2013) noted that “small high-grade deposits of nickel-cobalt arsenides furnish significant quantities of cobalt. Arsenide ores from Cobalt, Ontario, gave Canada world leadership in production for the period 1905-25. Cobalt output from this area stopped in 1971 but was reactivated in 1995 as a primary production center and now seems a major producer.

 Pink cobalt bloom (erythrite) on specimen from Cobalt, Ontario, Canada.  Width FOV ~7 mm.

Cobalt is a critical mineral in today’s high-tech and electronic world, especially for use in rechargeable batteries, to strengthen steel and other metals, and to construct rare-earth magnets. In the latter use cobalt is combined with samarium (SmCo) to create magnets with  good temperature stability and a strong resistance to corrosion. In regards to batteries, the public always hears about the need for lithium; however, cobalt, nickel, and graphite are also critical components.

Although cobalt is present as a component in many minerals, most cobalt-containing ores are mined for other minerals such as gold, zinc, silver, lead, copper, and nickel. The cobalt is then harvested as a by-product.

Most cobalt on the market comes from the mining of: 1) nickel/cobalt ores of chalcopyrite, pyrrhotite, pentlandite (nickel, iron sulfide), cobaltite and others. Pentlandite is the major nickel sulfide although other minerals on the list often contain smaller amounts. The major mines of these magmatic sulfide deposits include Noril’sk-Talnakh (Russia), Sudbury (Canada), and Kambalda (Australia); 2) copper/cobalt ores from the Central African Copper belt in the Democratic Republic of the Congo and neighboring Zambia. Here cobalt deposits usually consist of two layers: a) weathered “oxide” surface containing mainly heterogenite CoO(OH);  and b) unweathered Cu,Co “sulfide” deposits below these “oxide” caps containing mainly carrollite Co₂CuS_4.

The photomicrograph does not do justice to the mirror faces of this altered cube, maybe a cubo-octahedron., of carrollite (C). A gemmy poker chip of calcite (Ca) is attached at lower right.  The calcite rhombs (CA) form the matrix.  I don't have the slightest idea what the small green crystals (?) scattered on the matrix represent. Width of carrollite crystal ~3.2 mm.

A third major source of cobalt is the Moroccan cobalt–arsenic ores in the Bou-Azzer deposit, about the only mines in the world where cobalt is produced as a primary product.  Here the major cobalt producing minerals are skutterudite (CoAs₃ or (Co,Fe,Ni)As_2-3) and erythrite (Co₃(AsO₄)₂ · 8H₂O).

Specimen of Specimen of skutterudite from Morocco with nice octahedral crystals on fresh surface. The majority of the surface, bottom and sides of specimen, is massive and granular.  White mineral is calcite.  Specimen is difficult to photograph as bright metallic luster reflects light.  Length of specimen ~4 cm.

 

Photomicrograph clusters of erythrite sheaths from Bou Azzer.

Minor sources of cobalt come from nickel-cobalt laterite deposits where enrichments of Ni-Co from from intense chemical and mechanical weathering of ultramafic parent rocks such as dunite and “serpentinite.” The major lateritic mines operating today are in New Caledonia, a French Territory in the South Pacific. Future cobalt resources might come from manganese nodules and cobalt-rich crust on ocean seafloors.

OK, so cobalt is a valuable component needed for batteries in electric vehicles, now what? In reading about cobalt resources, I noted there are serious concerns about child labor and environmental contamination in the Central African Co/Ni Belt. Sudbury and the Canadian Cobalt Belt have faced serious environmental disasters in the past and problems still exist. The U.S Congress would like to source critical minerals from countries other than China and Russia; however, obtaining mining permits in the U.S is a multi-year task, and perhaps with good reason!

The only major cobalt mine in the lower 48 was the Blackbird Mine near Challis, and Cobalt, Idaho. The Blackbird closed in the early 1980s after more than 30 years of intermittent operations. By then, the surrounding creeks were lifeless; heavy-metal pollution had killed off most of their fish and aquatic insects.  Today, the 10,830-acre Mine is now a toxic waste site. The Superfund Site includes a 12-acre open pit, 4.8 million tons of waste rock, 2 million tons of tailings, and enough tunnels that, if they were strung together, you could run a half marathon in them and still have nearly a mile to spare. Over the past 26 years, the Blackbird Mine Site Group has restored creeks, sealed off mine portals, and constructed an intricate system of ponds and ditches designed to separate clean water from contaminated water. The end date for restoration is unknown; however, in the early 2000s, Chinook salmon returned to nearby Panther Creek. (Holtz, 2022).

The Blackbird sits in the Idaho Cobalt Belt, a 34-mile-long area in the Salmon-Challis National Forest that contains some of the largest cobalt deposits in the country. As the global market for lithium-ion batteries has grown—and the price of cobalt along with it—so has commercial interest in the belt. At least six mining companies have applied for permits from the U.S. Forest Service to operate in the region. Most of these companies are in the early stages of exploration; one has started to build a mine (Holtz, 2022).

The US Geological Survey has spent decades studying the cobalt belt and noted the belt is an alignment of deposits composed of cobaltite, cobaltiferous pyrite, pyrrhotite, arsenopyrite, chalcopyrite, and gold with anomalous rare-earth elements in a quartz-biotite-tourmaline gangue hosted in Mesoproterozoic metasedimentary rocks of the Lemhi Group.

I have a small specimen (toenail) purchased from a now defunct rock/mineral store over two decades ago. It has an older label stating “cobaltite, Cobalt, Idaho.” Good photos of Idaho cobalt seem tough to locate. The photo below seems to indicate cobaltite, arsenopyrite, and chalcopyrite.

 

Specimen of cobalt-bearing minerals from Cobalt, Idaho. Width FOV ~6 mm.
 

Several years ago, maybe in the early 1980s/late 1970s, I spent a few days up in the Salmon-Challis National Forest near Challis trying to clear a site (look for fossils) for a possible fluorite mine. I don’t remember much except it was in the Bayhorse Mining District, a fantastic “Ghost Town” was well preserved, and the only fossils I could locate were Ordovician? graptolites. I never found out if the mine was permitted.

In news from Canada, I read in Mining.com about New road paves the way for Canada’s first primary cobalt mine. “Evidently a new paved road has been constructed to the NICO deposit in Canada’s Northwest Territories that was discovered by Fortune Minerals in 1996. NICO is a fully vertically integrated project that will include mining and concentrating ores in the Northwest Territories, and transportation of the metal concentrate to the proposed refinery in Alberta for further processing to high value metal and chemical products. NICO is positioned to stand out as a North American asset dedicated to the production of cobalt chemicals needed to manufacture rechargeable batteries used in electric vehicles, stationary power storage applications, and portable electronics such as smart phones, tablets and laptops. The unique metal assemblage of the deposit also includes more than 10% of global bismuth reserves along with significant gold as a counter cyclical hedge to reduce exposure to cobalt and bismuth price volatility.”

So, if you are investor (probably younger) who likes to take chances with your money, consider cobalt or lithium or one of the REEs. At my age any “extra” resources (as if there is any) goes into U.S. Treasury Bills or Notes. The boys and girls in Washington better not default on the budget!!!

REFERENCES CITED

Holtz, M., 2022, Idaho Is Sitting on One of the Most Important Elements on Earth: The Atlantic https://www.theatlantic.com/science/archive/2022/01/cobalt-clean-energy-climate-change-idaho/621321/

Månbro, C., 2021, The Geology and Geochemistry of the Håkansboda Cu-Co deposit, Bergslagen, Sweden: M.S. Thesis, Stockholm University.

Kampmann, T., Jansson, N., Stephens, M., Majka, J., Lasskogen, J., 2017, Systematics of Hydrothermal Alteration at the Falun Base Metal Sulfide Deposit and Implications for Ore Genesis and Exploration, Bergslagen Ore District, Fennoscandian Shield, Sweden: Economic Geology v. 112, no. 5.

Sabina, A.P.,1991, Rocks and Minerals for the Collector: Sudbury to Winnipeg. Geological Survey of Canada Miscellaneous Report 49: https://geoscan.nrcan.gc.ca/starweb/geoscan/servlet.starweb?path=geoscan/fulle.web&search1=R=132238

Tegengren, F.,1924, (in MinDat): Sveriges ädlare malmer och bergverk (in Swedish). SGU ser Ca17, Stockholm.

Young, R.S. and L. Perrone, 2013, Cobalt:  www.thecanadianencyclopedia.ca

My non-investing advice: The most important thing is to live an interesting life. Keep your eyes, ears and heart open. Talk to people and visit interesting places, and don't forget to ask questions. Drink in the world around you so it's always there in your head.      Michael Morpurgo

CURIOSITY GOT TO ME: THE THALLIUM SULFOSALT ROUXELITE

Curiosity is the wick in the candle of learning.      W.A. Ward

The sulfosalt minerals, members of the sulfide group (sulfur is the major anion), are quite complex minerals, at least to an ole plugger like me. In a couple of previous posts, I “described” cylindrite (FePb₃Sn₄Sb₂S₁₄) on 1/23/17 and dufrènoysite [Pb₂As₂S₅] on 6/19/19. Sulfosalts contain a metal (mostly lead, copper, iron or silver although a few others, mercury, zinc, vanadium may be present), a semi-metal like arsenic, germanium, bismuth, antimony, or the metals tin or vanadium, and then sulfur but perhaps selenium or tellurium (Richards, 1999).

At 2022 Tucson I had several good conversations with a European dealer on some slow days and learned much. He also pulled out of his collection a specimen of rouxelite and noted that this mineral was quite rare and was only described in 2005. I looked at the micro through a loupe and it was a gorgeous mineral and so I brought it home.

Rouxelite crystals from the Sant’Olga tunnel, Monte Arsiccio Mine. The longer whiskers are ~ 2 mm in length.

Crystals of rouxelite [Cu₂HgPb₂₃Sb₂₇S_65.5 ] are acicular, elongated and striated, black in color but at times with a bluish violet iridescence, and have a metallic luster. They belong to the Monoclinic Crystal System, Prismatic Class. MinDat noted that the hardness of rouxelite could not be measured, and no cleavage was observed.

The Type Specimen of rouxelite is from the Buca della Vena mine, Apuan Alps, Tuscany Italy, where it actually is quite rare. The mine is a small Fe–Ba deposit in the Apuan Alps  and represents a complex mineralogy (no mining past 1988). Many lead–antimony sulfosalts have been collected from the mine (79 valid minerals including 14 Types). Like the other lead sulfosalts, rouxelite was formed in the latest stage of hydrothermal activity, within small veinlets cross-cutting dolomitic lenses interstratified in the Ba–Fe ore (Orlandi and others, 2005). Distribution of rouxelite in Tuscany is limited to an area from the Buca della Vena Fe-Ba deposit, near the village of Ponte Stazzemese, then extending about 11 km southwest to the Sant’Olga tunnel, Monte Arsiccio mine, near Sant'Anna di Stazzema. The only other know specimens are from the Magurka antimony deposit, Slovakia. Indeed, it is a rare mineral (www.handbookofmineralogy.org/pdfs/rouxelite.pdf).

My specimen came from the St. Olga Tunnel which is, to the best of my knowledge, an opening into the Monte Arsiccio Mine. The mine was the economically the most important pyrite ± baryte ± iron oxide deposit in the Apuan Alps. The orebodies are hosted within a Paleozoic metavolcanic–metasedimentary sequence, locally tourmalinized, close to the contact with the overlying Triassic metadolostone (“Grezzoni” Formation), belonging to the Apuane Unit (D’Orazio and others, 2021).

The rouxelite from the Monte Arsiccio Mine (72 valid minerals including 13 types) was the most recent locality described (Biagioni and others, 2014). Perhaps the most interesting aspect of this locality is that electronic gizmos (Inductively Coupled Plasma Mass Spectrometry) show there is a widespread substitution of Hg (mercury) by Ag (silver) and some incorporation of Tl (thallium) with the silver and therefore is a new example of a thallium sulfosalt (D'Orazio and others, 2021). This seemingly obscure notation may be important since thallium is a highly toxic metal associated with low-temperature hydrothermal mineralization. Weathering and oxidation of sulfides (including sulfosalts) may generate acid drainage with a high concentration of thallium and may pose a threat to surrounding environments.

At the end of this little exercise on a rare and complex mineral the question is—why learn about rouxelite? Well, to quote B.B. King, The beautiful thing about learning is that no one can take it away from you.

.  

REFERENCES CITED

Biagioni, C., Y. Moelo, and P. Orlandi, Paolo, 2014. Lead-antimony sulfosalts from Tuscany (Italy). XV. (Tl-Ag)-bearing rouxelite from Monte Arsiccio mine: Occurrence and crystal chemistry. Mineralogical Magazine. Vol. 78.

D’Orazio, M.; Mauro, D.; Valerio, M.; Biagioni, C., 2021, Secondary Sulfates from the Monte Arsiccio Mine (Apuan Alps, Tuscany, Italy): Trace-Element Budget and Role in the Formation of Acid Mine Drainage: Minerals, vol. 11, no. 2.

Orlandi P, Meerschaut A, Moelo Y, Palvadeau P, Leone P, 2005, Lead-antimony sulfosalts from Tuscany (Italy). VIII. Rouxelite, Cu₂HgPb₂₂Sb₂₈S₆₄(O,S)₂, A new sulfosalt from Buca Della Vena mine, Apuan Alps: definition and crystal structure The Canadian Mineralogist 43 919-933.

Richards, J.P., 1999, Encyclopedia of Geochemistry in Encyclopedia of Earth Sciences Series, C.P Marshall and R.W. Fairbridge, eds.: Springer Netherlands.

Zhao, F. and S. Gu, 2021, Secondary sulfate minerals from thallium mineralized areas: Their formation and environmental significance: Minerals, vol. 11, no. 8.

BLACK HILLS SOUTH DAKOTA: A FEW METAMORPHIC MINERALS

 

 The Black Hills of South Dakota represent an elliptical uplifted dome elongated in a north-south direction (as seen in the diagram by A.N. Strahler). Structurally, the Hills represent the easternmost uplift of the Laramide Rocky Mountains, perhaps 120 miles east of the Rocky Mountains in Wyoming. However, physiographically they are part of the Great Plains Province. The core of the Hills is composed of Proterozoic igneous and metamorphic rocks that were uncovered during Laramide and later Tertiary uplifting events, and then subjected to erosion. Surrounding the core are a variety of Paleozoic, Mesozoic, and Cenozoic rocks and sediments.

 When visitors flock to the Black Hills they most often arrive to take in the scenery at and near Mt, Rushmore National Monument in the central and southern Hills. The presidential carvings are in the ~1.715 Ga Harney Peak Granite, essentially the youngest Precambrian rock in the Hills. Surrounding this igneous intrusive granite is a large band of associated metamorphic rocks and pegmatites with similar radiometric dates. The pegmatites, perhaps as many as 24,000 units, occur within an area of about 275 square miles around Harney Peak and range from a few inches to more than a mile in length and up to 500 feet wide. Rocks within the pegmatites are primarily plagioclase feldspars (oligoclase and albite), potash feldspars (perthite and microcline), and quartz. Spodumene, lepidolite, muscovite, and tourmaline are major constituents in a few pegmatites (Redden and DeWitt, 2008).

 The northern Hills seem to have a more complex and varied hard rock geology with a few Precambrian rocks older than 2.5 Ga and Archean in age, and mid-Tertiary intrusions. The northern metamorphic rocks also contain a wider variety of sedimentary and igneous protoliths than the southern Hills. 

 I have looked at these metamorphic and igneous rocks, off and on, for over 55 years and don’t pretend to understand much at all. There is a reason that I settled on studying soft rocks and fossils! Redden and DeWitt (2008), in their amazing geologic map of the Black Hills, summarized the area: “…the Harney Peak Granite and associated pegmatites represent a complex system involving repeated magmatic emplacement…and metamorphism. Likely these processes extended over a considerable period of time.”

 Although most rockhounds exploring the Black Hills concentrate on pounding the pegmatites, there are several minerals in Hills that are unique to metamorphic rocks.  These minerals and rocks formed when the original protoliths underwent changes in chemistry and/or texture and/or composition. These changes are usually due to heat, pressure, and hot metamorphic fluids. Petrologists, then, are able to identify different metamorphic rocks by their distinctive mineral composition and texture.

 Another interesting aspect of metamorphic minerals is that since many are stable only within certain limits, the presence of specific minerals in metamorphic rocks indicates the approximate temperatures and pressures at which the rock underwent metamorphism. The diagram below came from the Open Mineralogy textbook and illustrates that chlorite, muscovite, biotite, andalusite, garnet (some types of each), cordierite, staurolite, kyanite, and sillimanite are metamorphic index minerals that indicate the approximate temperatures at which these minerals are stable. In the metamorphic rocks west and northwest of Custer collectable minerals (at least that I can identify) occur in schists and gneisses of medium to high grade temperatures. Redden and Dewitt (2008 and references therein) in some of their mapping indicated that many of the zones mineral overlap, especially staurolite-sillimanite, something that can be noted in the diagram. Essentially, the closer one gets to the Harney Peak Granite the higher the metamorphic grade becomes.

 Minerals as indicators of metamorphic grade. Chart accessed 2023 from https://opengeology.org/Mineralogy/8-metamorphic-minerals-and-metamorphic-rocks/

 

Metamorphic minerals help identify metamorphic facies, zones where specific minerals indicate the temperature and pressure of metamorphism. Chart accessed 2023 from: https://opengeology.org/Mineralogy/8-metamorphic-minerals-and-metamorphic-rocks/

 

 THE BLACK HILLS MINERALS—a few

Andalusite is an aluminum silicate [Al₂SiO₅] that forms under low metamorphic pressure and and low to high temperatures although it is most common in medium temperatures (see chart). Andalustite is a polymorph (same chemical composition but different crystal structure) of kyanite and sillimanite. Each of these three minerals form under different temperature and pressure conditions and therefore help to identify T/P in their host rocks. The phase diagram below is reprinted in virtually every mineralogy and petrology textbook and comes from the Open Source Mineralogy.

 

 

Andalusite belongs to the Orthorhombic Crystal System and often appears as columnar aggragates with individuals having a square cross-section. I would call it opaque but thin, cut sections may be translucent. Most field specimens are close to pink to red-brown in color, but more gemmy specimens may occur in a variety of “brighter” colors. Although MinDat lists a vitreous to greasy luster, most specimens I have seen have a more subdued luster. My major means of identifying adalusite is the square cross-section, and its mineral associations; however, andalusite is often pseudomorphosed to other minerals, due to changes in presssure and/or temperture, after after the original crystals form. Therefore, one might have “square crystals of sillimanite” that originally was andalusite.  

Andalusite seems a common mineral in/near the band of metamorphic “amphibolite” trending from 3-5 miles southwest of Custer northeasterly to Berne and on to Oreville. In fact, Roberts and Rapp (1965) stated “several excellent flawless gemstones showing superb dichrosim… have been cut from transparent andalusite found” near the Custer-Pennington County Line north of the Crazy Horse Monument. The specimen in my collection came from near Berne.

 

A “square-shaped” adalusite crystals with one end polished to show the gemmy feature. Length of crystal ~3 cm.

 Also collected west of Berne near/in the amphibolite is a specimen of (~4 x 5 cm) of metamorphic rock that appears to be part gneiss and part schist with layers of glassy blue or blue-violet cordierite var. iolite collected, as my label states, “west of Custer.” Additional descriptions of this index mineral may be found in my previous posting (Jan.5, 2023).

Cordierite [(Mg,Fe)₂Al₃(AlSi₅O₁₈)] is most characteristic of metamorphic rocks formed by high temperature and low pressure, that is in the same zone as garnet and andalusite (see chart). The proliths are often argrillaceous, alumnium-rich, silica-poor sedimentary rocks.

Cordierite. Width FOV ~7 mm.

 

Traditional garnet species

Mineral Formula Almandine Fe2+3Al2(SiO4)3 Pyrope Mg3Al2(SiO4)3 Spessartine Mn2+3Al2(SiO4)3 Andradite Ca3Fe3+2(SiO4)3 Grossular Ca3Al2(SiO4)3 Uvarovite Ca3Cr2(SiO4)3

Different varieties of garnet. Figure accessed from from MinDat.

Garnets are a group of silicate minerals that have similar physical properties but different chemical compositions. All varities have a hardness of ~6.5-7.7 (Mohs), commonly form as dodecahedrons or cubic crystals, have a vitreous or subvitreous luster, a white streak, but occur in a variety of colors with red, red brown, or green as dominat forms. Garnets have been popular gemstones for centuries, and today are among the most reasonablly priced faceted stones and are often set in silver rings, ear rings, and pendants.

Garnets can form in a variety of environments-igneous granites and pegmatites, metamorphic (both contact and regional) rocks of almost all facies, and as detridal grains in sedimentary rocks or unconsolidated sediments. However, garnets are commonly found in schists and gneisses that formed in regionally metamorphic environments—as one observes in the metamorphic rocks in Custer County. Most of these garnets in the county are almandine (Fe²⁺₃Al₂Si₃O₁₂). These garnets are the iron-rich end member of a solid-solution series with pyrope garnets having magnesium substituting for the iron.  

 

 West of Custer about 6-7 miles are exposures of a micaceous schist containing tiny “gemmy, transparent, ruby-red modified dodecahedral crystals of almandite [almandine garnets]” Fe₃Al₂(SiO₄)₃ (Roberts and Rapp, 1965).  

Staurolite is a higher-grade metamorphic mineral that usually requires more heat than garnets. Redden, in his mapping of the Berne Quadrangle (1968) was able to map the staurolite isograde as the rocks moved closer to the heat source, the Harney Peak Granite.

Staurolite crystals in a micaceous schist west of Custer in the Amphibolite Zone. Width FOV ~9 cm.

A large badly fractured crystal of staurolite in a micaceous garnet-biotite schist, Amphibolite Grade, collected near the Custer-Pennington County line on US 16. Width FOV ~7 cm.

Staurolite [Fe²⁺₂Al₉Si₄O₂₃(OH)] is another of those iron aluminum silicates that results from relatively high-grade metamorphism of aluminum-rich argillaceous sedimentary rocks. Euhedral crystals are dark brown to brownish black to reddish-brown in color with a white streak and somewhat hard at 7.0+ (Mohs). Staurolite is best known for its 60 degrees twins. The mineral is common in schists found in the Black Hills. I really don’t remember where the micaceous schist specimen of staurolite came from except in the Berne Quadrangle west of Custer.

 Sillimanite [Al₂SiO₅] is a mineral that I had never observed in the field until I reached the Black Hills. In several localities in Custer and Pennington Counties sillimanite is associated with variable pressure and high temperature in rocks close to the Harney Peak Batholith (see Chart). It is a rather strange mineral, at least in specimens I have observed. It occurs in pods or ellipsoids or knots that are most notable when they weather out of micaceous schists. These pods are usually white or dirty white, sort of fibrous and splintery, brittle, and thin layered, and come in a variety of sizes.  Hardness comes in at ~6.5-7.5 (Mohs) and the streak is white and the luster silky. It is one of the polymorphs of kyanite and andalusite (see Phase Diagram above).

 

 Sillimanite pod, obverse and reverse views. Length ~9 cm.

A somewhat forgotten, and rather rare, mineral in the Black Hills is scapolite. It is a silicate but is not really an individual mineral but a solid solution series between end members marialite (sodium chloride rich) and meionite (calcium carbonate rich): Na₄Al₃Si₉O₂₄Cl to Ca₄A_l6Si₆O₂₄CO₃.  The sodium and calcium are interchangeable with each other as are the chlorine and the carbonate radical, therefore leaving an infinite number of chemical compositions. In addition, calcium may include some strontium while the sodium may include potassium. And SO₄ may substitute for some CO₃ (Evans and others, 1969).  It appears that “pure” end members never occur in nature so intermediate compositions are the norm; however, these intermediate members vary considerably in chemical composition and remain unnamed.  Members of the solid solution series are essentially indistinguishable (visual) from each other and therefore scapolite is simply used for all. 

Scapolite comes in a variety of spectral colors ranging from colorless to white and yellow, purple, blue, red, green, pink, brown, gray, orange and various mixed compositions.  However, all varieties have a white streak. The transparency ranges from completely opaque to translucent to completely transparent while the luster ranges from vitreous to dull and pearly.  As scapolite weathers to “mica” the luster becomes dull, and the diaphaneity becomes opaque. The hardness of ~5.5-6.0 (Mohs) makes gemmy varieties more suitable for pendants rather than rings.  Scapolite crystals are Tetragonal and generally come in two distinct forms: short and fat, or long and prismatic. 

Scapolite is a rare mineral in the Hills with almost all larger specimens coming from the Susan Lode located about five miles east- northeast of Custer although Roberts and Rapps (1965) noted small grains are found in the amphibolite facies west of Custer.

 The Susan Lode is an enigma, at least to me.  I simply cannot locate information about any mining or minerals, virtually nothing except limited material on MinDat and Roberts and Rapp (1965).  The most detailed description of the Susan Lode seems to be in an unpublished BS thesis at SDSMT, a copy seems unavailable, at least to me (hundreds of miles away from Rapid City).

 

 Partial crystals of striated scapolite. Width FOV ~5 cm.

Roberts and Rapp (1965) state: “scapolite,,,occurs in great abundance along the contact between a Precambrian metasedimentary xenolith and [the Harney Peak] granite at the Susan Lode.” The color of the mineral varies and crystals are usually long prismatic, coarse, and vertically striated. So, these minerals seem to be the result of contact metamorphism due to heat from the granite affecting the metamorphic rocks. Best guess that I can make.

The metamorphic rocks of the Black Hills contain many more minerals than those listed here. My simple goal was to briefly describe a few minerals that are indicators of metamorphic temperature and pressure in the rocks west of Custer. Interested readers should consult the detailed maps completed by Redden and Dewitt, especially the Berne, Custer, and Fourmile Quadrangles as well as the Redden and Dewitt large scale Black Hills map.

 REFERENCES CITED

Evans, B.W., D.M. Shaw, and D.R. Haughton, 1969, Scapolite stoichiometry: Contributions to Mineralogy and Petrology, v. 24, issue 4.

Redden, J.A., 1968, Geology of the Berne Quadrangle Black Hills South Dakota: U. S. Geological Survey Professional Paper 297-F.

Redden, J.A., and DeWitt, Ed, 2008, Maps showing geology, structure, and geophysics of the central Black Hills, South Dakota: U.S. Geological Survey Scientific Investigations Map 2777, 44-p. pamphlet, 2 sheets.

Roberts, W.L. and G. Rapp Jr., Mineralogy of the Black Hills: South Dakota School of Mines and Technology Bulletin 18, Rapid City.