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.
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Traditional garnet species |
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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 Ca4Al6Si6O24CO3.
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.
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