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.

FOX HILLS FORMATION

Discoscaphites collected from Fox Hills Formation, South Dakota.

 

There are a number of quite interesting highways traversing east-west across northern South Dakota; however, my favorite is the somewhat lonely SD 20 heading east from Reva Gap to Mobridge on the Missouri River.  Actually the road starts at the Wyoming-South Dakota state line at Camp Crook.  This route crosses over a variety of rocks that are somewhat unfamiliar to readers in Colorado Springs.   Above the ubiquitous Pierre Shale are the, in ascending order, Fox Hills Formation, Hell Creek Formation, Ludlow Formation, Cannonball Formation, and Tongue River Formation.

The Fox Hills, a unit that is also present around Colorado Springs, grades upward from the marine shales of the Pierre into marine and then into marine and brackish water sandstones.  The unit represents the end of the great Western Interior Seaway and rocks are essentially shoreline deposits of the receding waters.  Although the Fox Hills near Colorado Springs contains a few fossils, mostly small clams, there are parts of the formation in north-central South Dakota that are extremely fossiliferous.  Some concretion layers have produced literally thousands, perhaps tens of thousands, of beautiful ammonites (and a plethora of other invertebrates and vertebrates), especially of the genera Discoscaphites , Sphenodiscus, and Hoploscaphites.  They represent some of the youngest ammonites in the U. S. (remember the ammonites became extinct at the end of the Cretaceous). 

Somewhere around the town of Timber Lake, a “long time ago”, I was able to collect some of these concretions with most specimens going to a museum.  However, I was able to hold on to a single specimen. Perhaps individuals may still collect on private lands.

mike

COLORADO SILVER, THE SHERMAN BROTHERS, AND MORGANITE

MINE DUMPS WEST OF CARBONATE HILL, LEADVILLE, CO.  LEADVILLE WAS ONE OF THE LARGEST SILVER CAMPS IN THE WORLD UNTIL THE 1893 CRASH.  PHOTO FROM USGS LIBRARY.

 Most persons with at least a passing interest in U.S. history would recognize the name of William Tecumseh Sherman, and if you lived in the southeastern part of the country the mention of his name in public might bring on trouble.  Likewise, Sherman is not popular among Native Americans.  During the U.S. Civil War General Sherman led his men on the infamous, scorched earth, “total war”, march to the sea through Georgia.  After the conflict he became Commanding General of the United States Army and as such was responsible for troops engaged in hostile actions with Native Americans in the western U. S.

General Sherman had a brother (actually he had three) by the name of John Sherman who was a successful U.S. Congressman, Senator, Secretary of the Treasury, and Secretary of State.  Today people hear his name associated with a law passed in 1890, The Sherman Antitrust Act.  This piece of legislation is intended to protect the American public from cartels and monopolies.

In the history of Colorado (and some other western states) Sen. John Sherman, Chair of the Senate Finance Committee, is known as the author of the Sherman Silver Purchase Act enacted in 1890.  This legislation stipulated that the U. S. government purchase about 4.5 million ounces of raw silver every month (in addition to silver already being purchased as mandated by the Bland-Allison Act).  The Sherman Act was a great boon to Colorado mining companies that had a vast supply of silver at their disposal, both in storage and in the ground, and raised the price of silver to over $1 ounce.  But since Sherman was a senator from an agrarian state (Ohio) the act was supposed to help farmers who had accumulated large debts (mostly to due to bad luck and droughts).  Supporters thought the law would stimulate the economy, combat deflation, create inflation, and then drop the dollar in value (so the debts could be paid in cheaper dollars---and I thought “voodoo economics” was invented in 1980).  Well, the entire scheme backfired and people begin to buy gold with their redeemable notes, rather than holding silver, and the country’s gold reserves became depleted.  In 1893 President Grover Cleveland repealed the Act and silver was on its way out and most mines in Colorado suffered.  Large scale unemployment was rampant in Colorado and mines all over the state closed down.  The “lucky” ones were able to produce gold or copper or lead-zinc.

Today, J. P. Morgan Chase & Co. seems always to be in the news---mostly as the “bad guy”.   In the late 1800’s and early 1900’s J. P. Morgan, the financier, always seemed to be in the news---often as the “bad guy”.  However, in 1893 the U.S. Treasury had been nearly depleted of gold as banks thought gold a better investment than silver, something the Sherman Act did not anticipate.  At any rate, J. P. Morgan, the financier, sold/loaned the U.S. about 3.5 million ounces of gold.  Always the shrew banker, he received a long term bond in exchange for the gold and certainly made money.  But, his immediate action did perhaps save the nation?  Could any individual, today, come up with those sorts of resources?

It is interesting to note that although the Act was repealed in 1893, the debate on the national scene went on for many years.  History books in “my era” (a long time ago) spent several pages talking about the “Goldbugs” of the Republicans and Wm. McKinley (a gold standard) vs. the “Silverites” of the Populists and Democrats and Wm. Jennings Brian (free silver) in the presidential election of 1896.  The free silver issue really was not settled until the Federal Reserve System was established in 1913.  But, hold on.  Many of us can remember how two brothers from Texas tried to corner the silver market in the 1970’s and early 1980’s.  In the early 1970’s the price of silver on the commodities market was less than $2 per oz.  By about 1980 the brothers owned 50% of the disposable silver and prices artificially soared to over $50 an oz.  Silver mines all over the west were retooling and share prices of silver mining companies were high (at least out of my reach as a small time investor)!  But then, the bubble burst as the Federal Reserve “came to the rescue” and the price of silver collapsed (as did the stock market; remember the prime rate of 22% in 1980?).

Perhaps the political intrigue of that time in history was just as dark and dirty as today?  I once read a bibliography of J. P. Morgan and it was simply fascinating.  He was an astute businessman involved in a wide range of activities and “older” (read my generation) history books will tell you that he organized over 40 corporations and owned numerous railroads, a true venture capitalist.  He was disliked by many but loved by a few.

Morgan collected gemstones, many/most of which later landed in collections of the American Museum of Natural History. George Kunz, the chief jeweler for Tiffany & Co, suggested in 1910/1911 that the pink variety of beryl [Be₃Al₂(SiO₃)₆] be named morganite after J. P.  Of course, Kunz was the person who curated Morgan’s gemstones.  Kunz may be best known for  his namesake, the pink variety of spodumene, kunzite [LiAl(SiO₃)₂].  The pink color in both morganite and kunzite is likely due to manganese ions.

mike