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.

TIEMANNITE AND LUANHEITE: TWO RARE MERCURY MINERALS

 

Selenium, Number 34 on the Periodic Chart of Elements, has properties that are intermediate between the elements above (sulfur) and below (tellurium) in the periodic table. It is often described as a metalloid with properties intermediate between a metal and nonmetals.  For comparison, other metalloids include silicon, boron, antimony, arsenic, tellurium, and several others. Selenium is a rare element and its abundance in the earth’s crust ranks the element 67th (0.05 ppm) while #1 oxygen has 461,000 ppm. Native selenium is rare as a mineral but does appear in some uranium-vanadium sandstone deposits. If selenium is available in hydrothermal or magmatic solutions it often substitutes for some sulfur in the formation’s sulfide minerals.

I do not have a specimen of native selenium but have acquired and described, in past Blog Postings, thumbnails of clausthalite [PbSe], klockmannite [CuSe] and berzelianite [Cu₂Se]; all are fairly rare minerals and in the selenide group.  Selenium can exist in the oxidation states of 2-, 2+, 4+, and 6+ and form selenates, selenides, and selenites (not the gypsum variety).  In selenide compounds the selenium has an oxidation charge of 2- and this group includes all ~125 naturally occurring selenium minerals (I think).

My most recent collection addition to the selenides is a specimen of tiemannite acquired at the 2022 Tucson Show. Tiemannite is a rare mercury selenide [HgSe] where the selenium anion has an oxidation state of 2-. Mercury, in nature, has three possible valence states. Elemental mercury has no valence state (Hg⁰), mercurous mercury has a 1+ state (Hg⁺), and mercuric mercury comes in at 2+ (Hg⁺⁺). So, tiemannite has a nice balance of a 2+ cation and a 2- anion. It is related to coloradoite, a mercury telluride (HgTe).

Tiemannite has a steel gray to black color, a metallic dull luster, and a black streak.  Like most metals it is opaque and soft (~2.5 Mohs); however, it does exhibit a brittleness. Although some collecting localities produce small tetrahedral crystals most specimens of tiemannite are massive to granular and compact. It commonly occurs with other tough-to-identify selenides in hydrothermal veins.

Tiemannite crystals (sub-millimeter in size) forming dendrites and other features. A large cluster of crystals may be observed in the lower right quadrant. Width FOV ~7.0 mm.

Tiemannite crystals (sub-millimeter in size). Notice the scattered individual crystals in center of photograph. Width FOV ~4.0 mm. Best my camera could produce of these microscopic crystals.   

My specimen came from the Lucky Boy Mine, Mount Baldy Mining District on the east flank of the Tushar Mountains of Piute County near Marysvale, Utah. The district is a large gold-silver producer having significant zinc-lead deposits and covers part of the Marysvale volcanic field in the transition zone from the Basin and Range Province to the west and the Colorado Plateau to the east. Upper Paleozoic and Mesozoic sedimentary strata occur along the eastern base of the range and are unconformably overlain by rocks of the Marysvale volcanic field (Chenoweth, 2007). The Lucky Boy mine was not a gold-silver mine but was producing mercury (213 flasks) by retort during 1886 to 1887 and, as far as is known, is the only U. S. deposit of the selenides of mercury to be operated commercially (Callaghan, 1972). 

The second mineral that joined my collection in 2022 (Tucson) is also a mercury mineral that, at first, confused me to no end! With the tiemannite described above I was able to observe mercury as a “normal” cation with an oxidation charge of 2⁺ and balance with the selenium anion of 2^-. My new specimen was luanheite with a formula of Ag₃Hg and I nabbed it due to mercury appearing as an anion—and so it came home with me. Last week while sorting and looking at minerals (a constant joy) I pulled out the two perky boxes and suddenly my mind hit a brick wall. Something was wrong, or so I thought. As noted above, mercury has oxidation states of 0, 1⁺, and 2⁺ so how could it be an anion? How could it match with the positive oxidation states of silver, 1⁺, 2⁺, 3⁺?  Confused was I!

As noted before in my little writings, I am trying to remain a lifelong learner and therefore relearning “basic chemistry as I advance in age. My three semesters of chemistry as an undergraduate in Hays, Kansas, were completed over 60 years ago and much of the “learning” in these classes did not stick in my brain for the following decades. That is one reason I commonly mention oxidation states in discussions—pushing, pushing my mind to try and understand. So now perhaps I have an answer.

Luanheite, according to MinDat, belongs to the silver amalgam group (yes, the same as your tooth fillings) and therefore is an amalgam mineral. So far, so good. An amalgam is an alloy and a combination of mercury with another metal, in this case silver. Most minerals, other than the native elements, are chemical compounds and held together by chemical bonding (several types of bonding) and may be transformed by chemical reactions. Amalgams, and most metals, are held together by metallic bonding where electrostatic forces are in play. This bonding is quite strong and therefore “holds together” the silver and the mercury.

With some continued interaction examining chemistry books, I found the answer. All elements in an amalgam are in an elemental state and have oxidation charges of zero. So, in the mineral luanheite, an amalgam, both mercury and silver are in oxidation states of zero. I suppose any student enrolled in CHEM 100 would know that; however, if I learned such, it “slipped my mind.”

Robert Cook wrote a great article in Rocks and Minerals (2002) describing the discovery and naming of luanheite. The crux of the story was your work is not finished till the paperwork is done. Cook posed the question, “if one discovered a pocket of this material [luanheite in Chile], a mineral unknown until the 1980s, its peculiarity and obvious rarity would suggest that timely formalization as a new species was not an urgent matter. Why then rush to publication?” As you might guess, a group of Chinese scientists had identified and published a description of the new mineral luanheite located in a gold-bearing alluvial gravel, a completely different environment from the Chilean volcanic tuff-hosted silver and mercury mine. Although the Chilean luanheite locality produces the finest specimens in the world, the Type Locality is an obscure river in China and the Type Specimen is a rounded small pebble; no photograph is included on the MinDat description.

Dark metallic gray-black sheets of luanheite on a matrix of volcanic tuff. The specimen was collected in 1985, the year that information about the rare find was published. Width FOV ~9.0 mm.

As Cook described, luanheite closely resembles native silver, ranging in color from gray, white to black, has a metallic luster, a hardness of ~2.5 (Mohs), and is soft and malleable. It usually is massive, granular, or  sheetlike; however, at the Chilean Elisa de Bordos mine it may occur in arborescent growths. The chemistry remains constant across mineral grains indicating it is a mineral and not just a jumbled mixture of silver and mercury.  

This has been a tough assignment but perhaps something sort of perked me up---Richard Faynmann: Study hard what interests you the most in the most undisciplined, irreverent, and original manner possible. I can, at times, be quite irreverent and undisciplined!!

REFERENCES CITED

Callaghan, E. (1973) Mineral Resource Potential of Piute County, Utah, And Adjoining Area: Utah Geological & Mineralogical Survey Bulletin 102.

Chenoweth, W.L., 2007, History of uranium production, Marysvale district, Piute County, Utah, in Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors, Central Utah―Diverse geology of a dynamic landscape: Utah Geological Association Publication 36.

Cook, R.B., 2002, Connoisseur's Choice: Launheite: Elisa de Bordos Mine, Northern Chile, Rocks & Minerals, vol. 77 no. 2.

TRIVIA. Although my mind might have lost some knowledge once learned in chemistry, one thing that has not disappeared in those 60+ years is a small tavern on 11^th St. in Hays, the Brass Rail. Just a laid-back place with cold drafts of Coors for $.15 (in the early 1960s). 

 

The Brass Rail on 11th St. in Hays, Kansas. Parked in front is a great 1955 Chevy. Photo and car courtesy of Jeff Thisted.