AGATES AT THE AFMS

THE GYM AT HOPKINS HIGH SCHOOL WITH EXHIBITS AND DEALERS

Agates, agates and agates; more agates than I have seen in my entire life---all spread out on tables and cases at the annual American Federation of Mineralogical Societies annual show and meeting.  This gala event was hosted by the Minnesota Mineral Club and held in Minnetonka, Minnesota (Twin Cities metro area).  The club sponsored a number of formal seminars on agates (paid admission) plus several free presentations concerning the honored gemstone.  Because of previous scheduling conflicts, I was only able to attend the Saturday events but was overwhelmed by the expertise of the presenters, as well as by the sheer numbers of agates displayed in formal exhibits, and “for sale” by the dealers.

DOES THIS SEEM AN OXYMORON?  I THOUGHT ALL GEODES WERE "OLD"!

It was easy to observe that the major emphasis of the show (exhibits and dealers) was on the famous Lake Superior Agates (the Lakers).  That fact is understandable since Minnesota is the major home of most of the discoveries in shoreline gravels of Lake Superior, although later glacial action has moved specimens into adjacent Wisconsin, Michigan and even South Dakota (and perhaps others).  The original source of the agates is the basalt located in the Midcontinent Rift System (MRS).  This geological rift (think about the great East African Rift Zone) begin to form in the Precambrian (Proterozoic Era) perhaps 1.1 Ga splitting the stable part of the North American “continent” or plate (referred to by geologists as the craton).  For some reason the rift “stopped splitting” (a failed rift in geological jargon) and the continent healed.  Most of the rocks in the rift, generally igneous and sedimentary, are buried below the surface of the earth and are only know from geophysical studies and drill holes.  However, the rocks become exposed around Lake Superior and the agates erode from the basalts.  Since the rift rocks include substantial amounts of iron, the agates have some sort of a red or orange color---oxidized iron.  Most likely the agates formed post-deposition of the basalt and are the result of percolating silica-rich groundwater filling the many vugs or vesicles in the basalt.

A MIGHTY LARGE AGATE.

The second most popular agates at the show were the Fairburn Agates from neighboring South Dakota.  I have written about these gems in previous articles but can only restate the obvious---they are spectacular in all aspects.  Generally Fairburns are found on the plains in states surrounding the Black Hills, with most discoveries in South Dakota.  The original source was from the Paleozoic Minnelusa Formation in the Black Hills—generally termed Teepee Canyon Agates at their source.

OFTEN FAIRBURN AGATES ARE NOT "CHEAP".

A BUNCH OF FAIRBURN AGATES.

The meeting and the show were a great success and the Minnesota Mineral Club is to be congratulated for their hard work and dynamic membership.
mike          

WHOA, NICE CEPHALOPODS!

Coiled cephalopod, Hoploscaphites sp., in concretion.  This same concretion contains Baculites, gastropods, and bivalves.

 South Dakota has an amazing variety of geological features present on the landscape and an interesting array of formations cropping out along the roads.  I have a particular love for the state since in 1965 I left my home in central Kansas and traveled north for additional education.  It was a long trip, 325 miles, for a small town kid sort of striking out on his own (my undergraduate education was less than 100 miles away from home, and in the same state).  In fact, it was downright scary!  

The first thing that I noticed upon my arrival in eastern South Dakota was the fact that most of the rock exposures were covered by what we lovingly called “glacial crap”.  The second was that in some of the stream valleys, where a few exposures existed, there were old friends from western Kansas—the Niobrara Formation, Carlile Formation, Dakota Formation, etc.  These rocks made me feel more “at home”!

But the real excitement came from our field trips to “West River” and the badlands and the Black Hills.  I had visited these areas the previous summer on a road trip with a couple of college buddies but at age 20 our minds were mostly on other “things”.  We poked around at the rocks, collected a few, did some hiking but always kept our eyes out for the other “things”.  I knew that I wanted to return to South Dakota with a knowledgeable guide.  And so I did.

In the summer of 1966 I started a thesis project West River and to break up the driving from Chamberlain (my work station) I often stopped at the little community of Wasta.  Today Wasta is located at Exit 98 on high speed (75 mph legal but often faster) I-90.  In the “old days” Wasta was just a little store on U.S. 16 where one could purchase a cold soda, and explore the Pierre Shale.

I had previously examined a few outcrops of the Pierre in extreme western Kansas (Wallace County), mostly near McAllaster Buttes, and found the formation to be rather unfossiliferous (a few nondescript baculites) and kind of “ugly”.  The Pierre represents deposition in the deepest waters of the Western Interior Seaway during the latest Cretaceous, and many tens/hundreds of miles from the nearest shoreline.  I sort of put the formation in the back recesses of my mind---until I explored South Dakota!

The Missouri River Trench, as South Dakota geologists call the feature, exposes the Pierre from near the South Dakota—Nebraska border (Fort Randle in the south central part of the state) all the way north into North Dakota.  In fact, there are even exposures along the Missouri River near Yankton in the south eastern section.  Geologists from South Dakota Tech have been studying the unit for years and have described a number of new vertebrate critters.  In addition, invertebrate fossils are numerous.

From the Missouri River Trench west to the Black Hills (and beyond) the Pierre crops out at numerous localities but especially along the major streams. I found that special exposures were located along the Cheyenne River near Wasta.  Suddenly the Pierre lost its ugliness (my earlier opinion) and became exciting!

Exposures of the Pierre along the Cheyenne River yield concretions and these concretions are often fossiliferous.  Most fossils are mollusks including the pelecypods Inoceramus sp. and Pteria sp., the cephalopod ammonites Baculites sp. and Hoploscaphites nodus, the gastropods Margaritella flexistriata, Amauropsis sp., Drepanochilus sp., Anisomyon sexculcatus, the scaphopod Dentalium gracile, as well as the coral Micrabacia americana (Sharman, 2008).  But most collectors are not interested in the snails but in the beautiful straight-shelled cephalopods (Baculites sp) and the coiled cephalopod Hoploscaphites.  These ammonites often have mother-of-pearl present and the suture patterns on the shells are quite distinct.  They are really spectacular fossils and collectors will notice South Dakota specimens in museums around the world.

Baculites sp. in concretion.

 Today, as I understand the situation, the private land is very difficult to access and collect.  Therefore, if you plan on collecting I would suggest contacting the various rock and mineral clubs or rock stores located in or near the Black Hills.  I have found them to be a wealth of information.

It is not always fun experiencing the aging process but---the alternative is not very appealing either!  What I do have are my memories and I often think of those days where collectors could wander around South Dakota, visit with the ranch owners in a friendly fashion, and bash open concretions looking for those ammonites.

REFERENCES CITED

Sharman, G. R., 2008, Petrology, Geochemistry and Paleontology

of Fossiliferous Concretions from the Cretaceous Pierre Shale, South Dakota [abs.]: Geological Society of America Abstracts with Programs, v. 40, no. 5, p. 85.

mike

COLORADO FIRE II

The good news is that I am back home and my house and neighborhood were undamaged.  The bad news is that 346 homes in the city of Colorado Springs were vaporized and reduced to ashes.  In addition, countless others have been damaged.  The good news is that there were no serious injuries (I believe) among the hundreds of emergency workers.  The bad news is that at least two bodies have been located in a burned residence.  The fire is the most destructive in the history of Colorado and thus far nearly 18,000 acres have burned; it is 55% contained.  In addition, the USFS noted that the growth potential for the fire is "extreme".  Somewhere near 32,000 people evacuated the fire area in Colorado Springs and perhaps 3000 of our friends in Teller County (Woodland Park) left their homes.  Today most of us are back home with the major exception of Mountain Shadows, the area experiencing the greatest home loss.

The fire captured my attention like no other natural event.  It was a sad situation to see homes burning, but on the other hand watching the forces of nature at work was a shot of adrenaline.  Many of us here in the mountain west live at or near the urban-wildland junction and so I suppose we should expect natural disasters.  As far as I known this is the first major burn along the range in recorded history.  The Ponderosa Pine forest was "full" and seemed waiting for the right moment to ignite.  The USFS and other agencies are attempting to ascertain the cause of the fire.

It is good to be home.  But, do rocks burn?

mike   

COLORADO FIRE

I am taking a small break from this blog since all of my notes, etc were left behind during a very rapid evacuation from my home in Colorado Springs! The wild fire has been and is amazing, massive and very scary. Check out denverpost.com for some great photos of the Waldo Canyon fire.  Many forest fires burn in the countryside.  The west side of Colorado springs was on fire and over 300 homes are gone.  It was very difficult to decide what part of your entire life should be saved in 15 minutes of throwing things in the pickup in a choking smoke and ash fall.  Thus far our home has been spared and the fire line is holding.  I vacated my office leaving behind hundreds of specimens and books with a tear in my eye wondering--do rocks burn?  It was a terrible feeling but at least my spouse, dog and I were able to leave, along with 32,499 other people.  The roads were in gridlock and the city in a panic mode.  The temps that day in Colorado Springs were the highest In recorded history and the humidity was 2%!  Now that is dry. Will return as soon as I can:) Mike

FAIRBURN AGATES: GEM OF SOUTH DAKOTA

MAYBE AGATE HUNTERS COULD GET A ROOM AT THE FAIRBURN HOTEL?

 If I were to quiz 10 rockhounds of the CSMS and ask them to name the two most “famous” agate types in the United States I suspect nine of them would list Lake Superior Agates and Fairburn Agates; I would agree.  I first started looking for Fairburn Agates in South Dakota back when I was chasing fossils in 1966 for some of my research.  In those long ago days the agates were not common, but were still available to the intrepid explorer willing to tramp around the exposures.  I have a nice specimen collected in 1966, could not locate one during a hunt in 2009, but was again successful last summer.

A COUPLE OF NICE FAIRBURNS.

 Fairburns are valued for their colorful fortification patterns with an abundance of reds (iron oxide), oranges (iron oxide) and blacks (manganese oxides).  The derivation of their name comes from the small community of Fairburn, located south along I-90, ~25 miles, of Rapid City near SD 79.  This area is also perhaps the easiest for collectors to locate.  Agate hunters should travel east from Fairburn along French Creek Road (good gravel road) for about 12 miles to a sign locating the original collecting area managed by the Buffalo Gap National Grasslands.  Although known to collectors for decades, these Fairburn beds still yield an occasional agate, and as many colorful specimens of jasper, quartz and chalcedony as can be carried out in your collecting bag.

ENOUGH MICROCRYSTALLINE QUARTZ TO FILL YOUR COLLECTING BAG!

 The geology at the collecting site is as interesting as the specimens.  French Creek has eroded the area and exposed the Cretaceous Pierre Shale, the Chamberlain Pass Formation (Eocene) and Eocene/Oligocene White River Group (mostly the Chadron Formation), and Tertiary terrace gravels.  Rocks of the White River Group are famous for producing mammals in Badlands National Park to the northeast of the beds.  However, rockhounds also will notice very distinct red-orange-yellow “beds” between the Pierre and the Chadron.  Originally named the Interior beds (for exposures near the village of Interior, SD), this “bed” is now known to be a series of ancient soils developed on the Pierre (mostly yellowish in color) and the overlying Chamberlain Pass (at least the upper part of the Chamberlain Pass has a red ancient soil imprinted).  These soils most likely developed in  tropical-like climates (warm and wet) but perhaps alternated with dryer periods.

THE PALEOSOLS DEVELOPED ON THE PIERRE SHALE ARE MOSTLY YELLOW IN COLOR WHILE THOSE DEVELOPED ON THE CHAMBERLAIN PASS ARE MORE RED. NOTE THE LAG GRAVEL BEDS COVERING THE SURFACE.

 The really interesting story about the Fairburns involves their relationship with the Teepee Canyon Agates in the Black Hills.  This latter agate site, in limy beds of the Paleozoic Minnelusa Formation, is located west of Custer, SD, along US 16 about two miles west of Jewel Cave National Monument, or perhaps 14 miles from Custer.  Most geologists now believe that the Minnelusa Formation in the Black Hills is the source of the Fairburn Agates and the siliceous pebbles were transported out to the plains by Tertiary streams draining the Hills.  Pabian (Agates Database: www.snr.unl.edu) noted that conglomerate beds in the lower part of the Chamberlain Pass Formation are the source of the Fairburn agates at their "type locality"---the original Fairburn beds.  However, most people associate Fairburn Agates with the Chadron Formation stratigraphically above the Chamberlain Pass Formation.  Hunters locate agates in situ in Chadron conglomerates, and in the lag gravels covering many outcrops where the finer sediments have eroded away leaving behind a veneer of pebbles (including agates). I remain uncertain if I can distinguish between the Chamberlain Pass and Chadron formations so perhaps agates come from both units.  In addition, I have seen Fairburn-type agates collected from the late Cenozoic “high level” terrace gravels, and beds attributed to the Ogallala Group and/or "Western Derived Gravels" (as distinguished on the State Geological Map of South Dakota).  Collecting localities for Fairburn agates seem to surround the Hills wherever the Chadron is exposed out on the Plains.The agates noted in the other gravels probably represent reworked sediments of the Chadron.

YOU CAN'T GET LOST.

 If you plan on collecting at Teepee Canyon the agate “diggings” are very evident since prospectors must quarry out large pieces of limestone and then break out the cherty nodules, some large and some small.  Large crack hammers, pry bars, leather gloves, and eye protection are required.  The agate localities, both in Teepee Canyon and next door in Hell Canyon, are located on USFS land and are open to collectors, at least were open during my last visit.  

ROGER CLARK'S FAIRBURN AGATE BOOK.

 I would suggest that if you are interested in Fairburn Agates---pick up a book or three written by Roger Clark, the premier expert on the agates.  Book one is South Dakota's Fairburn Agate is only available on the used book circuit.  Book number two, Fairburn Agate: Gem of South Dakota, and number three,  Fairburn Agate: South Dakota State Gemstone are in print and published by Silverwind Agates.  The photos are just amazing.

CHASING AGATES AND FOSSILS 1966

ADDENDUM:  see the following blogs
Stumbling upon a Fairburn Agate:    8/19/12
Cracking rocks at Teepee Canyon:    8/18/12
A Fairburn Agate from Wyoming:    10/6/13  

ASTROPHYLLITE: A MINERAL FROM THE STARS? NO, ST. PETERS DOME, COLORADO.

ASTROPHYLLITE CRYSTALS FROM SAINT PETERS DOME AREA, PIKES PEAK MASSIF.  WIDTH OF SPECIMEN IS ~2.2 CM.

Astrophyllite is another one of those minerals that would have never popped into my mind if not for a move to Colorado Springs.  Somehow I don’t remember anything about this mineral from my basic mineralogy class; however, that was decades ago and much of that class information seems lost in the deep recesses of my mind!  I certainly never saw the mineral in Kansas, or Missouri, or Wisconsin (my other homes).  But, in exploring around the Pikes Peak Massif near Colorado Springs I heard about this brass- to golden-colored bladed mineral that was found in some of the pegmatites and “granites”.  I guess the name, “star leaf”, comes from the fact that at some localities the mineral occurs as “star bursts or rosettes”; however, all of the Colorado specimens that I have seen are rather bladed or tabular, soft (~3 on Mohs scale), some perfect basal cleavage, and sort of a greasy luster.  It really doesn’t look like much of any mineral that I observed previously except perhaps phlogopite.  However, this mica has cleavage plates that are transparent and flexible.  Astrophillite plates are brittle and opaque.

Astrophyllite is a rather complex mineral, at least to me, a hydrous potassium sodium iron titanium silicate: (K,Na)₃(Fe,Mn)₇Ti₂Si₈O₂₄(O,OH)₇.  I’m just glad that chemical formula was never on a test!

Astrophyllite acquired at Tucson, 2014.  Collected from the Khibiny Massif, Kola peninsula, Russia.  Note the golden-orange chatoyancy of some crystals.  i presume the matrix is something like albite.  Width of specimen ~5cm. 

OK, since astrophyllite did not arrive here from the stars, why is it here near Pikes Peak, one of the few known localities in the world (others seem to be Brevig, Norway; Mont-Saint-Hilaire, Quebec, Canada; Greenland; and the Kola Peninsula, Russia)?  It seems to occur with quartz, feldspar, thorite, galena, riebeckite, and zircon in both pegmatites and fractures of the Mount Rosa Granite (Eckel and others, 1997).  So, I really don’t know the answer to why.  What I do know is that the igneous rocks of the Pikes Batholith around Saint Peters Dome (Precambrian,~1.08 Ga) seem to produce a suite of minerals where many, if not rare, are certainly uncommon. 

GOLDEN BLADES OF ASTROPHYLLITE FROM SAINT PETERS DOME.  WIDTH OF SPECIMEN IS ~2.5 CM.

REFERENCES CITED

Eckel, E. B. et al, 1997, Minerals of Colorado: Denver Museum of nature and Science and Fulcrum Publishing, Denver.