REALGAR--PARAREALGAR BLACK HILLS, SD

QUARTZ WITH AN INCRUSTATION OF PARAREALGAR (ALTERED REALGAR) FROM THE BLACK HILLS OF SOUTH DAKOTA.  LENGTH OF SPECIMEN ~6.5 CM.

I recently returned home from a fall leaf peeping and collecting trip to the Colorado high country (near Leadville) and continuing to the Black Hills of western South Dakota---hence my absence from the Blog!  I have often written about the latter area as the Hills are one of my favorite places to visit-especially in the fall.  The children are in school, the campgrounds are un-crowded, and the many old mines are waiting for my return.  In addition, the Hills have numerous rock/mineral shops that require stops to visit new additions as well as the dusty old bins that time seems to forget.  It is these latter cardboard boxes that require close examination.  The bottom line is that I came home with many decent collected specimens and some of those weird and strange and rare minerals that I love to acquire for my collection.

Now realgar is not a weird or strange mineral and has been mined for centuries at various world-wide locations; however, it is a rare mineral in rocks of the Black Hills.  Realgar is a sulfide mineral, an arsenic sulfide: As₄S₄ (sometimes written as AsS).  In crystal form the mineral is usually bright ruby-red or orange-red in color and is sometimes called “ruby sulfur”.  Often transparent with a resinous luster, realgar is quit soft (1.5-2.0 Mohs) and crystals are easily damaged.  Most realgar forms in low temperature hydrothermal settings but does occur as a product in hot springs such as the Norris Geyser Basin in Yellowstone National Park, or in sublimation products via volcanic vapors.  In hydrothermal veins realgar is often found with other sulfides such as gold, silver, stibnite, lead, and especially orpiment.  The latter mineral, As₂S₃, is often confused with realgar since it usually forms in the same environments and is similar in appearance (more on this in a later blog).

Realgar is one of those minerals that is fairly unstable and over time alters to a new mineral, pararealgar (As₄S₄ or AsS)—same chemical formula and chemical composition as realgar but a different crystal form.  But since I am not a mineralogist confusion often enters my mind—both pararealgar and realgar are Monoclinic and 2/m Prismatic—so at this time I fail to understand the difference.  Regardless of my confusion, with exposure to light and over time realgar alters to this yellow to yellow-orange crust called pararealgar---ah, the wonders of modern instrumentation to determine such actions!  But hold on, if you leave pararealgar in the light for a “long time” it turns into a yellow-orange powder and sort of disintegrates into what?  Dust?  I have also noticed that not every mineralogy reference seems to accept pararealgar and simply states that realgar occurs either as the red crystals or the yellow-orange crust or powder, sometimes all on the same specimen. 

In reading an article on medieval painting techniques and natural pigments I remember the author writing about red pigment made with beautiful realgar that would fade from the canvas and turn into a yellow or yellow-orange hue.  I can assume that is true although I don’t have the slightest idea for that source---probably some magazine that I browsed through at Barnes and Noble!

So, in this small rock/mineral shop in the Hills I stumbled across a couple of dusty specimens marked “realgar, mine near Sheridan Lake?, Black Hills”.  The price was right so I picked it up knowing that, at least from my limited knowledge, that realgar was not common in the Black Hills.  I only wish that I had also picked up the second specimen.  I immediately consulted the Black Hills “mineral bible” Mineralogy of the Black Hills (Roberts and Rapp, 1965) but could not find a reference for realgar, or for orpiment, occurring in the Hills.  I continued to search my other books (and that knower of all knowledge, the Internet) and the only reference that I located was in Encyclopedia of Minerals (Roberts, Rapp and Weber, 1974):  “Realgar is found in the United States…sparingly in the Homestake gold mine, Lead, South Dakota.”

So, the problem is that Sheridan Lake is not really close to the Homestake Mine!  Maybe there is a mine near the lake that produced the specimen, or perhaps the locality information is wrong.  I may never know as I cannot locate photos of Homestake specimens for comparison with my small specimen of massive quartz (with vugs of tiny quartz crystals) with a crust of pararealgar. 

As stated previously, there are many aspects of crystallography that confuse me; however, I have hope for the future since to be conscious that you are ignorant is a great step to knowledge:)  (Benjamin Disraeli)

FORT HAYS LIMESTONE: THE NATIVE

I like to roam around the countryside looking for interesting rock outcrops, blooming flowers, nicely shaped trees, and singing birds.  I have always been interested in nature and in these later years of my life thoroughly enjoy just "being alive".  As I am fond of telling my friends, life is good.

So, in areas that I am quite familiar with (geologically speaking), it takes much to really surprise me.  Like, "holy cow, what in the world is that" sort of surprise?  A real serendipitous moment!  Well, it happened the other day during my little trip to western Kansas.  I was on a country road that I had traveled tens of times before, always looking out the window at the rocks when it popped up, there it was---"what was it"?  Am I seeing things (and no I did not have any of that funny weed that Colorado has legalized)?  It looks like a "statue" of some sort out in the pasture. I parked my pickup, crossed the fence (without snagging my pants on the barbed wire [known in the west as bob wire]), and trudged up to the "statue".  Much to my amazement there was a large hunk of Fort Hays Limestone (Cretaceous, member of the Niobrara Formation) that a stone mason had skillfully worked into a Native American "head" complete with an eagle feather bonnet.  It seemed to have been in place at least a few decades since the characteristic black patina of the Fort Hays had formed.  I didn't have the slightest idea what this old gentleman was doing out in the pasture.  It was on a slope of Blue Hill Shale where it had tumbled down from the outcrop above (before the carving), and I did not notice any nearby structures where an early rancher may have lived.  So, I decided to ask my brother, who lives reasonably close to the pasture, about the gentleman.   Since he is the smarter one of the family I thought for certain that he would know the answer, or at least make one up.  Although brother had seen the structure, explanations were not forthcoming---he didn't have the slightest idea about the origins of the gentlemen.  Well sort of did not have any ideas---he did spout something about Vikings and Rune Stones (we are of Danish hertitage so Vikings have always been our heroes).

I guess the old gentlemen in the pasture will remain a mystery to me, at least for now.  And that is OK for life always needs a few mysteries for without mysteries life would be very dull indeed.  What would be left to strive for if everything were known? (Charles de Lint)

   

KANSAS CONCRETIONS (CRETACEOUS)

During my somewhat lengthy stay in western Kansas, about a third of my life, I became sort of fascinated by some of the large concretions located in the Cretaceous rocks.  Perhaps the best known are the giant (25+ feet in diameter) sandstone concretions weathered from the Dakota Formation in Ottawa and Ellsworth counties in central Kansas (saving these for another story).  The Blue Hill Shale Member of the Carlile Formation has fairly large rounded to warty concretions with calcite crystals.  Some members of the Greenhorn Limestone contain abundant limestone concretions, often fossiliferous and commonly flattened or oval in shape.  In the far western part of the state, Wallace County, some concretions in the Pierre Shale contain crystals of barite and occasionally fossil cephalopods.

The Cretaceous rocks in central and western Kansas are often thought of as four or five different informal groups (although it is more complicated than this simple classification): 1) the Cheyenne sandstone (non-marine) and Kiowa Formation (mostly marine); 2) the Dakota Sandstone (both marine and non-marine); 3) the marine Graneros Shale, Greenhorn Limestone and Carlile Shale; 4) the marine “chalk” (Niobrara Formation); and 5) the marine Pierre Shale.  These stratigraphic units are the “heart” of the great Cretaceous Western Interior Seaway. 

The Carlile Shale, and its stratigraphic equivalents (for example the Benton Group or Formation) is a widespread unit extending from Utah to eastern Iowa/Minnesota and Texas north into the plains of Canada.  In Kansas, the Carlile is divided into three members: 1) the lower Fairport Chalk lying conformably above, and often indistinguishable from, the Pfeifer Member of the Greenhorn Limestone; 2) the Blue Hill Shale, a rather fissile, dark gray shale, often with small pyrite concretions and limonitic staining (probably derived from weathering of the pyrite); gypsum in the form of selenite is common and often litters exposures.  In most areas the Blue Hill is a slope former and poorly exposed--being covered with vegetation.  Where exposures do occur the unit is easy to identify and “spot”; and 3) the seemingly gradational (in most places) Codell Sandstone.  The Codell is a “strange one” as in some places it is a true sandstone, but a silty one, quite distinguishable from the Blue Hill while at other localities the unit is a sandy shale seemingly gradational with the upper Blue Hill. Many geologists are interested in the Codell due to the presence of numerous abraded teeth, dermal denticles, fecal pellets, and bones (fish and sharks).

In almost all localities the Codell is unconformably overlain, and has a sharp contact with, the Fort Hays Limestone Member of the Niobrara Formation.  In fact, a common profile one observes in central and western Kansas in the cliff-forming Fort Hays Limestone protecting a steep-sloped Codell overlying a slope-forming Blue Hill Shale.

BREAKS ALONG SMOKY HILL RIVER, GOVE COUNTY.  THE FORT HAYS CAPS THE UPLANDS, THE CODELL IS A SANDY/SILTY SHALE AND NOT WELL EXPOSED, THE BLUE HILL IS  SLOPE FORMING AND OFTEN COVERED WITH VEGETATION.  CONCRETIONS WEATHER FROM THE SHALE.

Perhaps the most interesting “thing” about the Blue Hill is the presence of thousands of concretions. Some are quite large (commonly on the order of four feet or so, some are at least 10-12 feet in diameter).  Hattin (1962) noted three different types of Blue Hill concretions: 1) calcareous septarians, the most abundant and interesting; 2) noncalcareous clay-ironstone; and 3) the rare sandstone.  The common septarians are what Kansans locals call “thundereggs”.  They seem everywhere the Blue Hill is exposed and farmers and ranchers commonly collect them as yard ornaments or to line driveways.  I suspect “city folk” would do likewise; however, front end loaders are uncommon implements in city garages.

When I was teaching in Kansas, people often came up to my office lugging along their “fossil turtles”.  It was my duty to bust their balloon and inform them their rock was a septarian concretion.  I tried to encourage them to closely examine the concretion and notice that the internal cracks in the concretions are secondarily filled with calcite. However, I was only successful about 50% of the time and on more than one occasion the visitor huffed out of my office with his/her fossil turtle.

Concretions seem to from when masses or globs of mineral matter precipitate out of the water, often concentrating around some sort of a nucleus.  In this case the concretions would form contemporaneously with the surrounding sediment/rock.  In other cases the concretions seem to form after deposition of the sediment and are more intruded into the sediment.  Hattin (1962) noted this occurrence in the Blue Hill concretions. 

The septarians, at sometime in their life, begin to shrink and fractures form where secondary calcite then precipitates, usually as larger, dark-colored crystals.  The characteristic septarian ridges form when the softer limestone between the veins weathers more rapidly than the secondary crystals.

It is interesting, as least to me, that another secondary calcite, in the form of tan-colored tiny crystals, commonly fills vugs and voids in the concretions. 

There are many "things" about septarian concretions that I do not fully understand.  However, I do not believe that I am alone in my confusion!  At least the professional literature would indicate such. 

If readers are interested in collecting septarians I suggest consulting a geologic map of Kansas to pinpoint outcrops of the Blue Hill, especially around Cedar Bluff Reservoir and the Smoky Hill River south of I-70 (Exit 135, Gove County).  Other localities include the Blue Hills physiographic region, especially in Mitchell County, and the breaks along the Saline River in northern Ellis and Russell Counties. 

But be aware, most exposures of the Blue Hill Shale are on private land. However, I have found western Kansas farmers and ranchers quite friendly and open to collecting concretions if visitors ask permission and close gates (or even walk to outcrops from the road).

On a recent road trip to western Kansas I visited the Gatorosa Ranch along the Smoky Hill River (see blog posting 6-4-2011) where the Blue Hill is widely exposed and concretions are numerous.  So, I nabbed my camera and snapped the photos below.

To acquire additional information about the Blue Hill Shale consult the reference by Hattin.

REFERENCES CITED

Hattin, D. E., 1962, Stratigraphy of the Carlile Shale (Upper Cretaceous) in Kansas: State Geological Survey of Kansas, Bulletin 156.

THEY ARE EVERYWHERE.

A FORM THAT IS NON-DESCRIPTIVE.

RIDGED SEPTARIAN

CONCRETION WITH A VUG FILLED WITH ORBICULAR CLUMPS OF CALCITE CRYSTALS---CLOSE-UP BELOW.  NOTE BAND OF CALCITE CRYSTALS.

MR. LUMPY

MRS. LUMPY

NOTE SEPTARIAN RIDGES

BROKEN SEPTARIAN EXPOSING RIDGES

MASSIVE SEPTARIAN RIDGES

CLOSE-UP SEPTARIAN RIDGES

RECRYSTALLIZED BLACK CALCITE VEINS

JUST AN ORDINARY CONCRETION

CONCRETION WITH CIRCULAR CORE

MR WARTY

CONCENTRIC SEPTARIAN RIDGES

BROKEN WITH MASSIVE, BUT SMALL, CALCITE CRYSTALS

CLOSE-UP OF MASSIVE CALCITE CRYSTALS

PHONOLITE AT SPEARFISH PEAK, BLACK HILLS

I travel to the Black Hills of South Dakota and Wyoming on a semi-regular basis to visit friends and relatives and to try and lasso an elusive trout or two.  The Hills are also a wonderful place to collect mineral specimens from some of the numerous pegmatites.  Campgrounds are abundant and the geology is well-exposed.

Generally speaking the Hills are cored by Precambrian rocks, both of Archean and Proterozoic ages, and their exposures create some of the best known attractions, i.e. the Needles Highway and Mt. Rushmore.  It is interesting to note that these same Precambrian rocks are perhaps 15,000 feet below the surface in the Williston Basin to the immediate north of the Hills.  A very long (time-wise) unconformity separates the Precambrian rocks from the overlying Paleozoic rocks---usually the Deadwood Formation.  At some meeting I attended (my mind fails at attempting to recall which meeting) the speaker threw out a time gap of about one billion years plus.  The Deadwood represents the transgressive phase, shoreline and shallow marine, of the early Paleozoic seaway.  As this seaway moved from west to east (today’s direction) the shoreline sandstone ranges in age from Early Cambrian (southern California), late Early Cambrian [Pioche Formation, western Utah], early Middle Cambrian [Tintic Quartzite, central Utah] (Hintze and Kowallis, 2009), early Late Cambrian [Sawatch Formation, Colorado] (Myrow and others, 2003) to latest Cambrian-Early Ordovician Deadwood Formation (Sokoloski, 2005).  All of these sandstones are termed Cratonic Quartz Arenites and are sheet-like in nature.  These rocks represent a classic example of marine waters slowly encroaching onto the Precambrian rocks of the Craton (the central stable part of the continent). 

Diagram of the Black Hills uplift by A. N. Strahler but taken from Trimble, 1980.

The remainder of the Paleozoic in the Black Hills is represented by sandstones, shales and carbonates related to numerous transgressions and regressions of marine waters; unconformities are numerous.  Perhaps the best known Paleozoic rocks are the Mississippian Pahasapa Limestone (Madison Formation), home of Wind Cave, and the Pennsylvanian Minnelusa Formation (home of Teepee Canyon Agates and ultimately Fairburn Agates)---see blog posting8/18/12).

The Triassic-Jurassic starts out with the redbeds of the Spearfish Formation (forming the racetrack around the Hills), transforms to marine carbonates and shales and ends with deposition of the non-marine, dinosaur-bearing Morrison Formation.

Marine waters again invaded in the Cretaceous period; the rocks are represented by sandstones, shales and carbonates deposited during four marine transgressive-regressive phases. 

Cenozoic non-marine rocks may have, at one time, been present in the Black Hills.  However, physical outcrops do not remain.  There seems not to be evidence of Pleistocene mountain glaciation in the Hills.

The doubly plunging anticline representing the structural feature of the Hills is Laramide in age (the Rocky Mountains uplift orogeny), probably beginning about 62 Ma (Cretaceous) and continuing into the Eocene (Redden and Lisenbee, 1996).

  

A domed Spearfish Peak, elevation 5796 feet, rising from Northern Black Hills south of Spearfish, South Dakota.

Tertiary phonolitic rocks moving down slope (gravity) on the southeast flank of Spearfish Peak.  The Junior Geologist for scale.

During my last few trips exploring for minerals I have been fascinated with tracking down a few of the Tertiary intrusives (Eocene) that dot the landscape both within the Hills (i. e. Crow Peak---see blog posting 8/14/12) and on the Plains (i.e. Bear Butte---see blog posting 6/10/12).  The best known of these intrusive structures is probably Devil’s Tower situated in Wyoming west of the Hills about 35 miles.  Most of the intrusives are laccoliths although a few are sills.

Geologic Map of the Black Hills near Spearfish, South Dakota.  Spearfish Peak, mapped in red as Tp (Tertiary Phonolitic Intrusive Rocks), dominates the skyline south of Spearfish “city”.  Map from DeWitt and others 1989.

Saw-cut surface of phonolite with massive nephaline and black sprays of aegirine-augite.  Note a feldspar lathe towards upper right corner.  The "light-colored" specks are probably nosean.

During my last trip in summer 2013 I had the Junior Geologist haul me up some back country roads to a locality termed Spearfish Peak.  This mountain dominants the landscape south of the town of Spearfish (northern Hills) yet is somewhat difficult to locate on the USFS roads.  However, it was worth the drive as the exposed rocks of the Mountain are sort of “different”, at least to me!  Roberts and Rapp (1965) described the intrusive rocks at Spearfish  Peak as a phonolite with aegirine-augite, nosean, sodalite, and nephaline.  Pretty interesting!  DeWitt and others (1989) mapped Spearfish Peak as “phonolitic intrusive rocks of Eocene and Paleocene age” and part of a group of sills and laccoliths occurring from west of Lead to near Galena.  The mineralogy seems complicated with numerous feldspathoid minerals, aegirine-augite, biotite and sphene along with feldspars and others.  They also noted that the phonolites were characterized by anomalously high concentrations of barium and strontium.  Unfortunately I have neither a polarizing microscope nor an XRD nor an XRF to confirm this analysis.

Phonolite collected on Spearfish Peak.  The dark mineral sprays are aegirine-augite.  Width of rock ~11 cm.

 However, there are quite interesting macro-crystals in the phonolite that are visible to the eye and especially impressive as seen with a binocular microscope.  Very large crystals of aegirine [NaFe³⁺SiO2O6] or perhaps aegirine-augite are scattered on the surface almost as radiating sheathes. I really never understood much about aegirine until my friend Pete at the USGS gave me a crash course on some of the pyroxenes.  That information is present in a blog posting on 3/17/13.

 

Photomicrograph of Spearfish Peak phonolite showing Aegirine (A), Nephaline (N), and possibly Nosean (?). Width ~1 cm.

Photomicrograph showing possible Nosean (?).  Width of photo ~1 cm.

Using a binocular scope it appears that I can also identify nephaline [(NaK)AlSiO4] and nosean [sodalite group Na8(Al6Si6O24)(SO4)-H2O].  I could not locate the described sodalite.

Whatever the case, this field trip to Spearfish Peak was exciting and I was able to collect a somewhat uncommon rock—phonolite--- with some uncommon minerals exposed in an interesting laccolith.     

REFERENCES CITED

DeWitt, E., J.A. Redden, D. Buscher, and A.B. Wilson, 1989, Geologic map of the Black Hills area, South Dakota and Wyoming: United States Geological Survey Miscellaneous Investigations Series Map I-1910.

Hintz, L.F. and B.J. Kowallis, 2009, Geologic history of Utah: Brigham Young University geology Studies Special Publication 9

 

Myrow, P.M., J.F. Taylor, J.F. Miller, R.L. Ethington, R.L. Ripperdan, and J. Allen, 2003, Fallen arches: Dispelling Myths concerning Cambrian and Ordovician paleogeography of the Rocky Mountain Region: Geological Society of America Bulletin. 

Redden, J.A., and A.L. Lisenbee, 1996, Geologic Setting, Black Hills, South Dakota, in C.J. Patterson and J.G. Kirchner, eds., Guidebook to the Geology of the Black Hills: South Dakota School of Mines and Technology Bulletin 19.

 
Roberts, W.L. and G. Rapp Jr., 1965, Mineralogy of the Black Hills: South Dakota Schools of Mines and Technology Bulletin 18.

Sokoloski, W. P., 2005, Sedimentology and ichnology of Late Cambrian to Early Ordovician sandstone in the Deadwood Formation, Northern Black hills, South Dakota, and Southeastern Bear Lodge Mountains, Wyoming: MS Thesis, University of Toledo.

Trimble, D.E., 1980, The geologic story of the Great Plains: United States Geological Survey Bulletin 1493.