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.

BEAR BUTTE; A LACCOLITH ON THE PRAIRIE

BEAR BUTTE, AN EOCENE LACCOLITH NEAR STURGIS, SD

Sturgis, South Dakota, is an interesting small town (~6600 permanent residents) on the northeast flank of the Black Hills.  Most people know the town as the host of an extremely large and rather raucous motorcycle rally held annually in early August.  At this time of the year the population soars to well over a half million temporary residents and non-cyclists avoid the area, and actually the entire Black Hills, at all costs.  At other times of the year stop in for a visit; Exit 30 on I-90.

To a geologist the most interesting aspect of the Sturgis area is Bear Butte sticking up off the prairie a few miles north of town (off SD 79 and in Bear Butte State Park).  Also known as Mato Paha (Lakota), the butte is a sacred to a variety of Native Americans and visitors often see prayer bundles attached to trees, especially along the summit trail.  The Butte is believed by many to be the spot where a creator communicates with her people through vision and prayer.  Each year Native Americans make pilgrimages to the Butte for prayer and spiritual renewal (National Trust for Historic preservation, 2011).  Since it is sacred, during my several visits I chose not to hike the summit trail--just a personal choice.

Bear Butte is an igneous feature termed a laccolith and has domed up the overlying sedimentary rocks. The intrusion is Eocene in age, ~50 Ma, and seems related to other Tertiary intrusions along the flanks of the Black Hills, i.e. Devils Tower and Missouri Buttes in Wyoming, and Crow Peak in the northern Hills.  The rock type comprising Bear Butte is a rather monotonous porphyritic phonolite, a “strange” sort of fine-grained rock (with some larger grains, hence the porphyritic texture) composed mostly of alkali feldspar (orthoclase) and nepheline.  The summit is at 4426 feet and there is a vertical relief of ~1250 feet.

TILTED BEDS OF MADISON LIMESTONE ALONG FLANK OF BEAR BUTTE.

 Most of the Butte is composed of the igneous rock; however, on the east side there is a vertical bed of Minnesula Sandstone and tilted beds of Madison Limestone (Mississippian).  These are remnants of older sedimentary rocks pushed up by the intrusion.

Another interesting aspect of the Park is Bear Butte Lake.  The lake did not exist until 1921 when a wildcat oil well penetrated artesian water, lots of water, in the Madison Limestone.  The water was diverted into the “new” lake.  Today the park’s small campground is located at the lake.

BEAR BUTTE LAKE.

 

 If you are interested in western history, there are two other sites worth a visit; both are named for Major General George Gordon Meade.  Meade was the commanding officer of the U.S. Army of the Potomac, receiving his appointment on June 28, 1863, after President Lincoln accepted the resignation of General Joe Hooker.  A few days later the Battle of Gettysburg erupted (July 1-3) and since the U.S. declared a union victory, Meade was given the honor of being the "winning" general.  He seemed never again to be so successful and Lincoln criticized him for not aggressively pursuing Lee.  Meade later served under Lt. General Ulysses Grant for the remainder of the conflict, and in fact, Grant made his headquarters with Meade.  After the War he served the military in various other posts.  Meade's portrait is on the 1890-91 one-thousand dollar Treasury Note, and several geographic localities bear his name. 

1890 UNITED STATES TREASURY NOTE, $1,000, WITH SKETCH OF GENERAL GEORGE MEADE.  SCAN COURTESY OF TOM CHAO'S PAPER MONEY GALLERY. 

George Armstrong Custer suffered his inglorious defeat at the Battle of the Little Bighorn in June 1976.  By the late summer of that year settlers in and around the northern Black Hills had convinced General Phillip Sheridan for the need of U.S. Army troops to protect against the Dakota and Cheyenne Native Americans.  Camp Sturgis was established near Bear Butte in August and was replaced in 1878 by Fort Meade.  The Fort has been continuously garrisoned since that time and today is a Department of Veterans Affairs Hospital and a training center for the South Dakota National Guard. 

FORT MEADE ca. 1888.  BEAR BUTTE IN BACKGROUND.  PHOTO COURTESY OF LIBRARY OF CONGRESS.

The second area to visit is the very tiny Fort Meade National Cemetery, opened in 1878 and closed in 1958 with only 188 burials.  I find the cemetery interesting because of its diversity:  two Medal of Honor recipients and "Lucy, Child, Sioux Indian".  An operating nearby cemetery is Black Hills National Cemetery.

REFERENCES CITED

National Trust for Historic preservation, 2011, 11 Most Endangered Places, Bear Butte: www.preservationnation.org

mike

LARGE ZIRCON CRYSTALS FROM ST. PETERS DOME, COLORADO

LARGE DIPYRAMIDAL ZIRCON GRAINS ASSOCIATED WITH FELDSPAR AND QUARTZ.  SPECIMEN FROM NEAR EUREKA TUNNEL.  WIDTH OF SPECIMEN ~2.4 CM.

Zircon, a zirconium orthosilicate (ZrSiO₄), is one of those minerals that is quite common in rocks and sediments of the earth’s crust; however, most people are unaware of its presence.  Zircon is a common accessory mineral in most igneous and metamorphic rocks, and a clastic residue in some sedimentary rocks such as sandstone.  However, the grains are usually quite small and generally unnoticeable (except to the experienced mineralogist).  My experience with zircon has been in examining: 1) metamorphic and igneous rocks in “thin section” with a polarizing microscope; 2) the “heavy mineral” element of some sandstones extracted via heavy liquids (such as bromoform) and a centrifuge; and 3) gem-grade faceted stones.  

When looking at thin sections of igneous and metamorphic rocks zircon grains are quite distinctive due to their high relief and crystal shape.  At one time I taught a course in sedimentology and lab students worked recovering heavy mineral grains from clastic sedimentary rocks.  Zircon grains were almost always present as the mineral is quite hard, ~7.5, and rather inert to chemical weathering.  As such they can survive many generations of weathering and redeposition.  I had never really seen larger (observable with the naked eye) zircon grains in a field setting until I moved to Colorado Springs!

Zircon is usually radioactive with trace amounts of thorium and uranium atoms replacing some of the zirconium atoms.  As such, zircon grains are commonly used in radiometric dating of rocks.  As I understand the process, geologists use both U-235--->Pb-207 and U-238--->Pb-206 dating techniques on zircon grains.  The former has a half-life ~700 million years while the latter ~4.5 billion years.  Fission track dating is also used on some specimens.

One of the most interesting aspects of zircon dating is with detridal grains found in sedimentary rocks.  For example, geologists have dated very old Archean (early Precambrian) rocks on a number of continents, ~3.5 to 4.0 Ga.  However, they have also dated individual zircon grains from younger metasedimentary rocks (Narryer Gneiss Terrain, Western Australia) as ~4.4 Ga (Wilder and others, 2001)!  It appears that the original rocks containing the crystals were destroyed (just normal activities associated with plate tectonics) but the resistant zircon grains were preserved and redeposited.  This dating indicates older rocks were around before the appearance of these ~3.5-4.0 Ga rocks!

Gem-grade zircon is valued as a “diamond replacement” stone as it has a high refractive index, high dispersion values, and a brilliant adamantine luster.  Most zircon used in jewelry is mined from  placer deposits in Southeast Asia, and most stones are heat treated to improve upon their color (i.e. blue zircon heated in the presence of oxygen will produce a yellow stone).  Yellow and green faceted stones seem more valuable than light blue or clear stones—if sales sites on the internet are an indication.

But, back to Colorado.  Pegmatites and granites associated with the 1.08 Ga Pikes Peak Batholith may be one of the better places in the U.S. to collect large zircon crystals.  Eckel and others (1997) described collecting as follows: The St. Peters Dome area, El Paso County, is a noted source of good crystals of zircon…Particularly fine-quality specimens are found in the famous cryolite locality of the old Eureka tunnel…about ¾ mile northeast of St. Peters Dome…Most of the crystals are brown to nearly black and are dipyramids with small, if any, basal faces…   They [individual crystals] can range up to 3 to 4 cm or more. 

LARGE ZIRCON CRYSTAL.  WIDTH ~7 MM.  SPECIMEN FROM EUREKA TUNNEL.

   

So, zircon grains and crystals are common in Colorado, occurring as an accessory mineral in igneous and metamorphic rocks, and as detridal grains, including some paleo placers, in sedimentary rocks and sediments.  The rocks of the Pikes Peak Batholith offer some fantastic collecting opportunities.

PHOTOMICROGRAPH OF INDIVIDUAL ZIRCON DIPYRAMID.  HEIGHT OF PYRAMID ~3 MM.

LARGE ZIRCON CRYSTAL (CENTER) WHERE RADIATION SEEMS TO HAVE DISTORTED THE SHAPE--OFTEN TERMED CYRTOLITE VARIETY.  SPECIMEN FROM NEAR HELEN HUNT FALLS.

REFERENCES CITED

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

Wilde, S. A., J. W. Valley, W. H. Peck, and C. M. Graham, 2001, Evidence from Crustal Zircons for the Existence of Continental Crust and Oceans on the Earth 4.4 Gry Ago: Nature, v. 409. 

mike

RIEBECKITE; AN INTERESTING MINERAL

MASS OF RIEBECKITE FROM MOUNT ROSA GRANITE.  NOTE INDIVIDUAL COLUMNAR CRYSTALS.  WIDTH OF SPECIMEN IS ~4 CM.

For decades I explained to students and laypersons that the common rock termed “granite” had an easy mineral identification:  about 20% or more quartz, 60% to 80% feldspar (both alkali types such as orthoclase, and plagioclase), and a dark mineral such as biotite and/or hornblende.  There are numerous varieties of granite but all contain either hornblende or biotite.  Wrong!  I just needed to move to Colorado and discover the Precambrian Pikes Peak batholith.  In addition to the biotite-hornblende myth, I thought that the Pikes Peak Granite was just a “big ole pluton” with a fairly homogeneous mineral composition and generally pink in color; the pegmatites present contained larger crystals.  Wrong again!

Smith and others (1999) pointed out that the Pikes Peak Batholith is a composite system (~1.08 Ga) composed of at least two different granite types.  The first is a “potassic” series (~64%-78% by weight of SiO₂ with biotite and or hornblende), mainly the “real” Pikes Peak Granite, and a “sodic” series (44%-78% SiO₂).  Rocks of the latter series were emplaced via at least seven different smaller plutons, among them the Mount Rosa Granite.  Both of the “series” were emplaced close together in time and space although the Mount Rosa Granite has been intruded into the Pikes Peak Granite (Gross and Heinrich, 1965).  

Near St. Peter’s Dome on the flanks of the Pikes Peak Massif, the Mount Rosa Granite crops out, and its major minerals include microcline feldspar, quartz and riebeckite.  So here, the major dark-colored silicate mineral in the granite is the somewhat rare amphibole, riebeckite (Gross and Heinrich, 1965).  Although most of the Mount Rosa Granite is fine grained, some pegmatites are present (see specimen photo).

Riebeckite is an iron-sodium silicate [Na2][Fe²⁺₃Fe³⁺₂]Si₈O₂(OH)₂ that is unique in that both ferrous and ferric iron are present.  The mineral, with a hardness of 5-6, is usually dark blue to black in color and the crystals are columnar aggregates.  The ends of the individual columns are usually broken and rarely terminated.  My specimen is actually pegmatitic in nature. 

One interesting aspect of the riebeckite-bearing Mount Rosa Granite is the level of radioactivity present, mostly due to the presence of thorium.  Exploration pits have been constructed and 500 tons of ore were processed for their radioactive content; however, commercial processing seems unfeasible (I think) (Gross and Heinrich, 1966).

And finally, a variety of riebeckite termed crocidolite is an “asbestos” mineral with a fibrous habit and dangerous if ingested.  

REFERENCES CITED

Gross, E. B. and E. W. Heinrich, 1965, Petrology and Mineralogy of the Mount Rosa Area, El Paso and Teller Counties, Colorado: I The Granites:  The American Mineralogist, v. 50.

Gross, E. B. and E. W. Heinrich, 1966, Petrology and Mineralogy of the Mount Rosa Area, El Paso and Teller Counties, Colorado: III Lamprophyres and Mineral Deposits:  The American Mineralogist, v. 51.

Smith, D. R. and J. Noblett, R. A. Wobus, D. Unruh, K. R. Chamberlain, 1999, A review of the Pikes peak Batholith, Front Range, Central Colorado: A “Type Example” of A-type Granitic Magmatism: Rocky Mountain geology, v. 34, no. 2.