POLYMORPHS: ANATASE AND BROOKITE

Last Saturday I took the opportunity to attend a local estate auction where the handbill advertised thousands of rock, mineral and fossil specimens.  I thought---sure, “thousands”, no way and they are probably junk.  What a pleasant surprise was in store as there were “thousands” of specimens and while some were junque, many more were very nice and collectable.  Most of the fossils were local Cretaceous baculites and really not collectable.  There were also flats of pretty unspectacular microcrystalline quartz available and some went for quite a few bucks!  Why?  But then there were nice collectable individual specimens thrown into flats (beer cases) with about two thirds carrying correct labels.  Some of the flats contained specimens from a single locality such as unlabeled topaz-bearing rhyolite from western Utah.  Most other flats were mixed with perhaps the most interesting being opal (looked Australian) with barite (almost certainly Hartsel, CO).  I guess the packer saw that both were “shiny”. 

I had a great time visiting with, and bidding against (although very hard), about 9-10 other members of CSMS.  I would guess that the ten of us purchased the great majority of the collectable specimens.  The auctioneer would start out with a flat and request bids for any single specimen or two and then sell the remainder of the flat at a single price.  I thought that some of my buys were “quite good”!

One partial flat that I purchased had a small crystal of anatase, something that was not in my collection and a mineral not all that familiar to me ---but I wanted it.  So for $5 I was able to “get it” along with a beautiful, water-clear, double terminated, scepter quartz crystal, a piece of rhodochrosite (with crystals), a nice terminated apatite crystal, some gemmy-green, titanite (sphene) crystals, and several other specimens.  A great buy.

Driving home I kept probing the back recesses of my mind—what do I know about anatase?  Where had I seen specimen(s)?  Then something popped out, or turned on, and I remembered by blog about the mineral brookite (March 7, 2012).  There must be some relationship!  So, in examining the posting I noted my words, and there it was—anatase:  Brookite is a titanium dioxide, TiO_2, crystallizing in the orthorhombic system.  MinDat notes that brookite is one of five titanium dioxide minerals (rutile, anatase, akaogiite, unnamed) that occur in nature---all belong to different crystal systems!  So, anatase is a polymorph of brookite!

Anatase is in the Tetragonal System while brookite is orthorhombic; both are usually found in primary sources as single crystals.  I say primary since all of the polymorphs are not rare in concentrations of heavy minerals from sedimentary rocks.  Anatase has a hardness of about 5.5-6, an adamantine to metallic luster, and a dark steel blue to black color.  The source for the larger crystals seems to be secondary and derived, via hydrothermal solutions, from titanium-bearing minerals in igneous and metamorphic rocks (www.MinDat.org)

 

The specimen now in my collection has a label indicating that Minas Gerais, Brazil was the place of origin.  All-in-all, it was an exciting day.  Now, off to finding shelf space!

 

Anatase crystal from Brazil partially covered with a “clay mineral”.  Width at junction of crystal and clay is ~5 mm.

Brookite crystal from Arkansas.  Length is ~1.75 cm.

ADDENDUM, 5 June, 2103.  Partial large crystal of anatase from Cuiaba District, Minas Gerais, Brazil.  Length ~2.0 cm.

 

CRACKING ROCKS AT TEEPEE CANYON:AGATES

BISMARCK LAKE NEAR CUSTER, SOUTH DAKOTA-VIEW FROM THE CAMP SITE.

I have visited the Teepee Canyon agate area a couple of times in the last few years, but without much success.  On my initial visit I did not have the proper equipment and my undersized rock hammer was of little use.  I found a few agate fragments but certainly nothing to write home to momma about.  On my second visit I ran into a rain storm and the roads did not look very passable and I am somewhat terrified of lightening.  My momma did not raise no fools!

TEEPEE CANYON AGATES AT THE AMERICAN FEDERATION 2012 SHOW.

 But, I am persistent and in summer 2012 I was bound and determined to go back to the Canyon and pound on the rocks.  Teepee Canyon is located approximately18 miles west of Custer, South Dakota, about 2 miles west of Jewel Cave National Monument off U. S. 16.  As soon as travelers leave the Monument they should look to the west, up slope, and begin to spot piles of broken rocks, big pieces.  Sawmill Spring Road, (FS 456) leads off to the west and about a mile further West Teepee Canyon road takes off.  My best advice is to follow one of these roads/tracks and look for quarries where past prospectors have tried their luck.  The land is managed by the U.S. Forest Service and all previous claims have expired (as I understand it).  The gentleman in the local USFS office in Custer told me that only hand tools were allowed and to stay in the area of previous mining (I left the big fircrackers in the vehicle).  It is my understanding that after the fire of 2000 other agate producing areas external of the main Teepee Canyon site had been discovered.  I don’t know if these are claimed.  What I do know is that there are enough “big rocks” at Teepee Canyon to last me a lifetime!

THE DIGGINGS AT TEEPEE CANYON. 

 The agates are located in chert nodules housed within the lower Minnelusa Formation.  I suppose these nodules are the result of silica-rich meteoric waters circulating through the unit with resulting diagenesis producing the chert.  Why some nodules are agatized—I don’t have the slightest idea.  Just as I am uncertain how/why agates really form!  The formation of agates in several types of rocks is extremely complicated, even for the “experts”.

The Minnelusa sits on top of a widespread carbonate unit termed the Mississippian-age Madison Limestone, or the Pahasapa Limestone as it is generally known in South Dakota.  In the early part of the Pennsylvanian (younger than the Mississippian) the climate was warm and quite wet in the future South Dakota and a red soil developed on the Madison.  Later in the period marine waters returned and rocks of the Minnelusa were formed. The Minnelusa is an interesting rock unit composed of beds of sandstone, dolomite, and thick beds of anhydrite.  Braddock (1963) noted that dissolution of some of the anhydrite had caused numerous collapse structures and collapse breccias in the Black Hills.  Originally designated as Pennsylvanian in age, fossils in the Minnelusa indicate both a Pennsylvanian and Permian age.  I presume that since the agates seem to be in the lower part of the formation, they belong to the Pennsylvanian.  I was able to locate brachiopods in some of the beds at Teepee Canyon but am uncertain as to their exact age.

CHERT NODULES IN MINNELUSA FORMATION.

BRACHIOPODS IN MINNELUSA FORMATION.

The nodules at Teepee Canyon are composed of chert, often red to tan in color, and of various sizes.  Very few of these nodules are agatized so locating a good agate is “hard work”.  First of all, the enclosing carbonate matrix is mostly a fine-grained dolomite that is extremely hard and quite difficult to “break”; hence, the need for a large crack hammer, eye protection, heavy clothes, and preferably steel-toed boots.  Many prospectors/miners at Teepee Canyon have cut into the outcrop and removed “really large” pieces of dolomite to crack open.  I decided early on that sort of mining was not on my agenda and simply cracked open smaller pieces mined by others.  The nodules in the carbonate were quite numerous and it appeared, at least to me, that some of these would polish quite nicely.  But, I was after agates!  I found numerous smaller pieces, all fragments less than one-half inch long, but then—there it was, a real Teepee Canyon agate.  Now, it is nothing spectacular, perhaps 1.25 x 1.0 inches in size, but never-the-less it was mine, and it satisfied my yearning!  So, after about two and one-half hours of cracking rocks in a hot sun I decided to take my find and return to camp on beautiful Bismarck Lake by Custer State Park.

AGATE ROUGH COLLECTED SUMMER 2012.

RED SOIL DEVELOPED ON TOP OF MADISON FORMATION NEAR JEWEL CAVE NATIONAL MONUMENT.  SOIL FORMED IN A WET AND WARM CLIMATE.

 mike

REFERENCES CITED

Braddock, W.A., 1963, Geology of the Jewel Cave SW Quadrangle, Custer County, South Dakota: U.S. Geological Survey, Bulletin 1063 G.  

CROW PEAK: A POCKET LACCOLITH

A SMALL LACCOLITH NEAR SUNDANCE, WYOMING.

The Laramide-age Black Hills, the dominant landform in western South Dakota extending into Wyoming, is surrounded along the northern boundary by igneous intrusions.  The June 10, 2012,  blog posting described one of these intrusions near Sturgis, South Dakota, known as Bear Butte.  Near Sundance, Wyoming, there are a series of related, small Eocene laccoliths (an igneous mass intruding between layers of sedimentary rock and doming the overlying layers; at times the domed sedimentary rock is eroded away and igneous rock is exposed)  intruding through the Mesozoic rocks; all are easily visible from the highway--Sundance Mountain (5829 feet), Black Buttes, Inyan Kara Mountain (6368 feet), Missouri Buttes (5055 feet) and the Bear Lodge Mountains (~6600 feet).  A little further to the northwest of Sundance is the best known of the intrusions, Devils Tower (may not be part of a laccolith but some sort of an igneous intrusion; 5112 feet).

SECTION FROM MISSOURI BUTTES EASTWARD TO BELLE FOURCHE RIVER THROUGH DEVILS TOWER.  FROM USGS PROFESSIONAL PAPER 65, 1909.

INYAN KARA MOUNTAIN SOUTH OF SUNDANCE, WYOMING, A SACRED PLACE TO NATIVE AMERICANS.  THE PEAK WAS VISITED BY MEMBERS OF THE 1874 BLACKS HILLS EXPEDITION LEAD BY GEORGE ARMSTRONG CUSTER.  A SCIENTIST IN THE EXPEDITION (DONALDSON)  STATED THERE IS NO GRANITE OR OTHER PRIMARY ROCK IN THE MOUNTAIN, NEITHER BASALT OR TRAP. THE WHOLE IS AN IMMENSE UPHEAVAL OF NON-FOSSILIFEROUS, SEDIMENTARY, METAMORPHIC ROCK.  PHOTO AND QUOTE FROM WYOMING STATE HISTORIC PRESERVATION OFFICE.

OFFICE.

NORTHWEST-SOUTHEAST SECTION THROUGH INYAN KARA MOUNTAIN.  FROM USGS PROFESSIONAL PAPER 65, 1909.

I-90 then proceeds into South Dakota toward the city of Spearfish.  I have always enjoyed camping around, and visiting, this community for a number of reasons.  Spearfish Canyon leaves town and follows Spearfish Creek southwest toward Savoy and a summit.  A wonderful section of Paleozoic rocks are exposed along the route and the Creek offers some great fly fishing opportunities.  One can also locate the “final scene” from the movie, Dances with Wolves (http://dances-with-wolves.purzuit.com/video/LR4j8iWu-OI.html).  At this locality, up Forest Road 222 (west of Savoy), Wind in His Hair shouts from the cliff (Pahasapa Limestone of Mississippian age) as John Dunbar is leaving the tribe Shumanitutonka Ob Wachi!  Although I took exception to movie's rendition of Fort Hays (I lived there for 24 years), this final scene in the Black Hills was pretty spectacular!

CLIFF OF PAHASAPA LIMESTONE AT FINAL SCENE FROM DANCES WITH WOLVES.

 I also am fascinated by the area’s small laccoliths that just intrude and push up through the Paleozoic rocks.  I am far from a petrologist or structural geologist and really do not understand all of the emplacement mechanisms except that somehow they seem related to the Laramide Orogeny; however, they are enjoyable to observe and climb—Crow Peak (5760 feet), Citadel Rock (5410 feet), Ragged Top (6220 feet) and Spearfish Peak (5764 feet).  Most have a quartz latite porphyry (plagioclase phenocrysts in a fine groundmass of orthoclase feldspar; Lisenbee and others, 2007) in the center surrounded by the Cambrian-Ordovician Deadwood Formation (sandstone), some lower Paleozoic thin carbonates and the massive, cliff-forming Mississippian Pahasapa Limestone.  Most structures are rather circular in nature and are generally “off the beaten path”.  They evidently are Eocene in age and related to the other small intrusions.  Elkhorn Mountain further east down I-70 is an intrusion where the igneous core has not been exposed and additional small laccoliths are located south of Sturgis.

In the Spearfish area I am especially enamored with Crow Peak.  There is a beautiful “back road” leading out to the summit trail (1560 feet elevation gain) and it appears a fall drive would display some fantastic colors.  I thank my guide Junior Geologist for introducing me to both the peak and the aptly named Crow Peak Brewing Company.
mike

CROW PEAK, AN EOCENE LACCOLITH NEAR SPEARFISH.

 

SECTION THROUGH CROW PEAK.  FROM USGS PROFESSIONAL PAPER 65, 1909.

GEOLOGIC MAP OF CROW PEAK SHOWING CIRCULAR NATURE OF LACCOLITH AND IGNEOUS ROCKS IN THE CORE.  FROM LISENBEE AND OTHERS, 2007.

REFERENCES CITED

Lisenbee, A. L., J. A. Redden, and M. D. Fahrenbach, 2007, Geologic Map of the Maurice Quadrangle, South Dakota: South Dakota Geological Survey, Vermillion.