As noted in previous postings Rebecca Nohe Estate Sales recently dispersed the rock and mineral collection of longtime Colorado Springs Mineralogical Society Member Laurann Briding. The sale attracted a large number of buyers wearing masks who were admitted via reservation (due to Covid-19 pandemic). I was in the second group on Friday and came home with a few interesting specimens. The narrative that follows is a continuation from Part I and starts with fluorite and calcite from the Illinois-Kentucky Fluorspar District.
The flat held a nice fluorite with calcite that has a small old-looking label stating it came from Cave-in-Rock, Illinois. This, of course refers to the famous Illinois-Kentucky Fluorspar Area with the most famous mine being (probably) the Minerva #1. Most specimens that simply say Hardin County, Illinois, or even just Illinois, are routinely assigned to the Minerva #1 (Ozark-Mahoning No. 1 Mine). There certainly are rockhounds and mineralogists who can assign the specimens to specific mines; however, that is above my pay grade. I have several specimens from the Area picked up on a field trip back in the 1960s, but this specimen was included in the flat and actually is pretty attractive.
The fluorite (CaF2) in the specimen is composed of interlocking washed-out, honey-yellow cubes (with one small purple cube) along with a second generation of much smaller clear cubes. Scattered around on the cubes are what appears to be three or more generations of calcite crystals. The largest are clouded, poorly formed, colorless crystals with a matte luster that I first thought were witherite. However, they effervesced rapidly in dilute HCL and so I called them calcite. A second generation consists of long, prismatic, sharply terminated, colorless to white crystals some of which are encased, partially or completely, within the fluorite. There appears to be a later generation of a snow-white, slender long crystals covering these original prismatic crystals. Finally, there are clusters of very tiny, prismatic, colorless, transparent calcite crystals scattered around. Quite a selection.
The colors of fluorite in the District vary considerably but perhaps it best known for purples and blues with color caused by various elements substituting for some of the calcium in the chemical composition. The cubic fluorite crystals have perfect cleavage and will produce a nice octahedron. Tourist stores across the nation sell these cleaved specimens by the thousands.
Although we have fluorite here in Colorado associated with pegmatites, the Illinois-Kentucky District fluorite was deposited in fractures and faults associated with fairly flat lying Mississippian Age limestones (~330 Ma). Low temperature hydrothermal brines, of later age (~150-250 Ma), then migrated into the voids while also partially dissolving some of the wall rock before depositing the fluorite. The District seems related to the Mississippi Valley Type mineral deposits that produced the lead-zinc districts of Missouri, Wisconsin, and other states in the Midwest. The question of the day is did the brines originate locally around hidden igneous intrusions, or did they migrate from the southern U.S. (today’s geography) that was tectonically active due to plate collisions?
The Mexican State of Zacatecas is in the north central part of the country and is known for the tremendous abundance of minerals and especially silver. MineraliA (2011) noted that “today the state produces 60% of the national product of silver, placing it as the second largest producer in the world. The soil is riddled with veins of silver, gold, mercury, iron, zinc, lead, bismuth, antimony, salt, copper, quartz, kaolin, onyx, calcite, cadmium, and wollastonite.
One of the great mining towns in Zacatecas is Concepcion Del Oro. Iron, lead, copper, zinc, silver, and gold have been mined since at least the mid-1500s and the production of silver, gold and copper continue today. The mines around the town are not well known for pyrite; however, a pyrite specimen was in the mixed flat and so it came home with me. I wanted to check it out since I am not into collecting pyrite due to possible “pyrite disease” and the release of corrosive sulfuric acid and harmful sulfur dioxide gas.
In examining the specimen under a scope, I decided “something was kooky” with the way the numerous pyrite crystals were displayed. The 5 x 7 cm. specimen is covered with small, gemmy, terminated quartz crystals with (up to ~6 x 11 mm) projections of interlocking pyrite crystals (actually the crystals look glued together). It was confusing. In searching the photo gallery on MinDat (Concepción del Oro Municipality, Zacatecas, Mexico) I did notice an interesting specimen posted by Dan Winder: “Nice pseudomorph showing granular pyrite that has replaced elongated hexagonal crystals (or stacks) of pyrrhotite. The tallest of these is 5 cm in length and looks like a calcite.” Now these pseudomorphs certainly looked like the original mineral was calcite but what about pyrrhotite? So off I go to a browser and type in “pyrite after pyrrhotite.” Bingo, serendipity again. Dan Weinrich has a nice specimen of “Sparkling pyrite replacing previous pseudo-hexagonal crystals of pyrrhotite” collected from Romania, and others from Russia. Keep looking. Luis Burillo Minerales has a number of specimens of the same; however, they were collected in Kosovo. When all else fails, try EBAY! For $160 one may purchase a single clump of “Pyrite after Pyrrhotite, Noche Buena Mine, Zacatecas, Mexico.” That got me closer although the mine is in a different Zacatecas municipality than Concepcion Del Oro; however, it is close. The Mine also happens to be one of the largest silver mines in the world with reserves of about 1 million oz of gold and 32.4 million oz of silver.
So, an interesting way to solve a small problem of probable interest only to an ole rockhound like me. The replacement of pyrrhotite (Fe1-xS where x=0-0.125) to pyrite (FeS2) seems to involve dissolution and then replacement with perhaps an intermediate formation of marcasite thrown in (see Qian and others, 2011) for a more complete explanation.
And finally, at least for this round, the flat contained a sparkly group of brown to brownish-red quartz crystals. Closer examination shows the crystals are gemmy, terminated or double terminated, with the color imparted by the iron minerals goethite and/or hematite. The specimen is not anything that I would pick up on an individual basis but, it was in the flat. The specimen was collected from Indian Mountain, Alabama, a location well known for producing phosphate minerals (see Posting Dec. 8, 2017). However, close examination did not produce visible phosphates. MinDat had single photo of included quartz from the locality; otherwise I am shy of information except that an old looking, handwritten label stated it was collected by one Preston Watts.
REFERENCES CITED
Gujie Fang Xia, Joël Brugger, William M. Skinner, Jiafang Bei, Guorong Chen, and Allan Pring, 2011, Replacement of pyrrhotite by pyrite and marcasite under hydrothermal conditions up to 220 °C: An experimental study of reaction textures and mechanisms: American Mineralogist, vol. 96, no. 11-12.
MineraliA, 2011, Minerals of Mexico: Oaxaca, Mexico.
Morgan, Helen, Greg Arehart, Naomi Oreskes and Half Zantop, 2014, Origin of epithermal Ag–Au–Cu–Pb–Zn mineralization in Guanajuato, Mexico: Mineralium Deposita, vol. 49.