Thursday, May 29, 2014

WYOMING YOUNGITE AND OTHER ODDITIES



I have often been intrigued by minerals from Wyoming; however, it seems as though I was always busy collecting fossils for research and did not have time to chase after minerals.  So, my modest collection seems rather void of many good specimens. 
Specimen of Youngite purchased at AFMS/RMFMS 2014 Show in Tulsa.  Width ~12 cm.
 

Specimen labeled Youngite near Guernsey, WY.   Width ~8 cm.

Perhaps the most interesting specimen is this rather weird thing called youngite or youngite agate or youngite jasper.  Youngite is a brecciated reddish-brown to peach jasper that was then re-healed by deposition of cream-colored chalcedony.  Often the entire rock is then covered by druzy quartz crystals.  At one time youngite was fairly common on the market as collectors were able to pull numerous specimens “off the walls” in youngite cave(s) and voids near Guernsey.  As I understand, the caves are in the Guernsey Limestone and the breccia clasts are actually derived from the limestone.  Sutherland (1985) believed the silica in the cementing chalcedony and the druzy covering was derived from the overlying White River Formation of Tertiary age. The Guernsey is exposed around the perimeter of the Hartville Uplift (see Blog posting Dec. 22, 2012) and is Devonian-Mississippian in age).  I remain uncertain why the main cave was closed to collectors but it is located on land owned by the state.   Today any new specimens of youngite seem to be found alone shorelines of Glendo and Guernsey Reservoirs.



Druzy calcite “stalactites” covering massive calcite and crystalline limestone.  Labeled youngite but is calcite rather than quartz.  Width of specimen ~9 cm. Note fluorescing calcite in photo below.

I also have a specimen labeled “Youngite Wyoming” that is “something else!”  It is a specimen that is stalactitic in nature and at first I thought it was youngite since is fluoresces that seemingly diagnostic bright green color.  However, the druze is actually tiny calcite crystals sitting on a “massive” white limestone overlying a crystalline gray limestone.  It evidently was pulled from a creek or lake since the crevices are filled with unconsolidated sand grains. 
 

Stalactitic druzy quartz covering massive calcite.  Labeled near Glendo. Maximum height ~6 cm.
A second specimen labeled “near Glendo” represents some fairly large stalactites—coarsely crystalline calcite covered by snow white druzy quartz.  The druze fluoresces a pale green but not nearly as bright as some youngite specimens.  I presume the specimen originated in some sort of a void or “cave” in a Paleozoic limestone.
 

Labeled Seam Quartz, Sweetwater County, Wyoming.  Width ~6 cm.
In Tucson this year I acquired a specimen labeled “Seam Quartz, Sweetwater County, Wyoming.”  It is druzy quartz surrounding and filling voids in a reddish-brown fortification agate.  However, the agate banding extends into the white quartz.  I am unable to find additional information about the specimen.   



Sweetwater agate collected near Jeffrey City.  Inclusions not visible. Width ~3.7 cm.


Sweetwater agate under UV light with characteristic green glow.
Wyoming is one of those states with a large number of locations that produce chalcedony specimens.  Virtually any outcrop of Cenozoic rocks seems to have pebbles of microcrystalline quartz in the form of petrified wood, “plain ole” chalcedony, and then a variety of “agates.”  However, I use the term agate rather loosely since my preference is to have an agate banded or included.  In today’s world agate often refers to pebbles of chalcedony, and perhaps even quartz.  One of the better known of the Wyoming agates are the Sweetwater Agates.  These chalcedony nuggets are usually included (manganese) and occur as small pebbles in lag gravel and seem derived from the Miocene Split Rock Formation or perhaps the Oligocene White River Formation.  They are found at many localities across the center of the state but are seemingly concentrated north of Jeffrey City.  Perhaps these rather plain looking agates are best known for “glowing” bright green under a UV light.  

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