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