Beautiful Rhodochrosite
[Note: This an updated version of a recent post that was badly edited by the scurvy dog who writes this blog.]
Not a single reader has asked about the photograph in the header of this blog, so I’ll save the many peoples of the world from having to ask. Rock collecting has been a lifelong weakness of mine so there was no surprise when I bought the half-pink half-grey mineral in a rock shop in Leadville, Colorado. The pinkish mineral in the sample is rhodochrosite, the state mineral of Colorado. Like most samples, it comes from the now-closed Sweet Home Mine, a failed silver mine in Buckskin Gulch north of Alma, C, between Breckenridge and Fairplay. If you are ever in Denver with spare time on your hands, the mineral collection at the Denver Museum of Nature & Science has a stunning collection on display of rhodochrosite from the Sweet Home Mine.
Colorado location of the Sweet Home Mine. Image from Google maps.
To get to the site take gravel road 8 from Alma north up Buckskin Gulch which eventually terminates at a trailhead near the base of several fourteeners in the Mosquito Range. If you get to the parking area, you’ve gone too far. We tried to find the mine by driving up the gulch above Alma, but there were no signs identifying the mine. I guess if I owned the mine I wouldn’t want a bunch of jabbering tourists stomping around either.
Buckskin Gulch left center, facing north. Image from Google Earth.
While we did not positively identify the mine on our trip, a photograph (below) was found later of a building associated with the mine. We did see it but sailed right on by. The mine is located on private property so wandering around the site is not permitted.
The mining district was discovered in the usual way- the search for placer metals like gold led miners up Buckskin Creek into the gulch looking for the source of the lode deposit. Originally a silver mining claim was made in 1873. The sporadic silver mining operation was abandoned in 1966. In 1991 the mine was bought out by Collector’s Edge Minerals, a consortium, and modernized. After a period of activity, the Sweet Home Mine was closed in 2004. However, another mine called the Detroit City Portal was begun by Collectors Edge on nearby Mt. Bross in 2016. This new operation, yielding many fine specimens was finally closed in September of 2024.
Image of Sweet Home Mine site from Mindat.org. Mt Democrat is on the horizon.
Minedat.org describes the geology as follows-
“Mineralization is generally in base metal-silver-rhodochrosite-fluorite veins predominately hosted by meta-igneous and metamorphic rocks, with minor mineralization in porphyritic dikes and pegmatites. There are five main veins in descending order of production: the Main, Tetrahedrite, Watercourse, Blaine and Blue Mud veins. The Blue Mud Vein is a barren post-mineralization fault-vein, and production from the Blaine Vein was minor. Overall, the planned extent of the mine is small (1000 feet x 400 feet) with about 5,000 feet of workings, and the overall hydrothermal alteration zone small, despite evidence of on-strike continuation of the veins in the collapsed Tanner Boy workings directly across Buckskin Gulch. And even within a vein, rhodochrosite finds were limited.”
“Three conditions were responsible for the formation of vugs: (1) changes in strike and dip of veins, (2) vein intersections, and (3) openings formed by fault bends controlled by host rock foliation. In general, the 2nd condition was responsible for major pockets, and the 3rd for most smaller pockets. Exploration focused on fault/vein intersections. Fluid inclusion studies suggest that the hottest fluid flow produced the gemmiest ruby-red rhodochrosites.” Minedat.org
Deposits found in the mine result from mineral-saturated hydrothermal fluids moving from the mineral source-rock into faults and fractures in the formation that were cooler, leading to precipitation of the minerals. The large size of the rhodochrosite crystals in the museum collection suggests that the precipitation was very gradual.
According to the Handbook of Mineralogy, Mineralogical Society of America, 2005, rhodochrosite occurs in areas of low to moderate temperature hydrothermal veins in metamorphic deposits.
(2005) Rhodochrosite. Handbook of Mineralogy. Mineralogical Society of America
According to Minedat.org, after the buyout of the Sweet Home Mine by Collector’s Edge Minerals and subsequent modernization, ground penetrating radar was used to survey for vugs. According to the AI overview by Google in a search for “vugs”-
Vugs are- “small to medium-sized hollow spaces or cavities within rocks, often lined with beautiful, well-formed crystals like quartz or calcite, formed by mineral-rich fluids filling natural voids left by dissolution, tectonic shifts, or gas bubbles in volcanic rocks, prized by collectors for their exposed crystal formations.”
Only makes sense, right? Liquids within voids in the rock have the opportunity for crystals to grow into the liquid spaces. Vugs are associated with faults and fractures which can be filled with hydrothermal fluids within a formation. Lode gold, silver, lead, etc., as well as quartz may line or even fill the vug. This is why some of the best mineral crystals are only found in mines and this certainly applies to rhodochrosite. Rhodochrosite contains manganese (II) which is oxidizable to a higher, more positive oxidation state, so protection from atmospheric oxygen deep within a rock formation prevents decomposition of the mineral.
The crystallographic structure of rhodochrosite is shown below-
Rhodochrosite is manganese (II) carbonate, MnCO3, and is insoluble in water but as a metal carbonate it is acid sensitive and therefore subject hydrolysis or chemical or microbial oxidation to Mn(III) or Mn(IV). Like a great many common ionic substances, it is not regarded as suitable for jewelry applications because it is not comprised of silicate or aluminum silicate subunits common in semiprecious and more robust minerals like sapphire, beryl or garnet. The structure is composed of MnO6 octahedra connected by trigonal carbonate units. The large buff-colored balls are manganese atoms and the smaller, bluish-colored balls connected directly to the manganese atoms are oxygen atoms. The middle-sized darker balls not connected directly to the manganese atoms are the carbon atoms of carbonate.
Manganese is not uncommon in the Colorado Rockies or the rest of the world for that matter. A mining geologist once complained to me that there was so much manganese in their gold mine tailings and overburden that it was a regulatory problem for them. For a time pyrolusite, or manganese dioxide (MnO2), was mined in Colorado near Salida. Never a large operation, pyrolusite could be used in the extraction of gold from its ore. Pyrolusite is manganese dioxide which is a well-known oxidizing substance containing Mn(4+). Manganese has multiple oxidation states: 0, +1, +2, +3, +4, +5, +6, +7, as well as several negative oxidation states. While interesting, removing manganese from ore containing a different target metal can be challenging. The various oxidation states produce different colored liquors with different extraction challenges.
Crushed pyrolusite was placed below a wooden container along with sodium chloride. To this mixture was added concentrated sulfuric acid. This generated gaseous hydrochloric acid which was then oxidized by the manganese dioxide in the pyrolusite into chlorine gas which flowed up through the slatted container of gold ore and combined with the ore to generate gold chloride. The water-soluble gold chloride was removed with water, then isolated and into this pregnant solution was dumped scrap iron. The iron reduced the gold chloride and finely divided gold precipitated out. This was a pretty danged clever method for use in the field as it required only NaCl, H2SO4 and pyrolusite mineral possibly mined in Colorado.
Oh, BTW. You might know that a way to generate a stream of fairly dry HCl gas (in a lab fume hood!!!) is to place granular NaCl into a vented flask and slowly drip conc H2SO4 from an addition funnel on it. A stream of nitrogen is used to force a flow of HCl out of the flask and through a sparge tube into your reaction flask. This HCl gas is Po Tent! Watch yourself.
And, by the way ...
Nearby the Sweet Home Mine, across the ridge to the north is the Climax Molybdenum Mine on Fremont Pass just up the road from the Copper Mountain Ski Resort. This major mining operation is owned and operated by Climax Molybdenum Company, a subsidiary of Freeport-McMoRan. If you look at the image for a minute, perhaps you can see that most of Bartlett Mountain is gone. Just imagine laboring in a mine above the 11,000 ft altitude. I’d be dead by noon the first day …
Image from Google Earth. Up the canyon to the right of the mine as short distance are the East Fork headwaters of the Arkansas River.
The mineral of interest at the Climax is molybdenite, or molybdenum sulfide, MoS2. The deposit was discovered in 1879 by prospector Charles Senter who was actually prospecting for gold or silver. At that time there was no market for the moly. By 1895 Senter found a chemist who determined that the mineral contained molybdenum. In a few years steelmakers discovered that molybdenum had application in steel making and, with the onset of WWI. the mine went into full production after it was discovered that the Germans were using it to strengthen steel in their tanks and weapons.
Molybdenum sulfide is also valued as a dry lubricant for use in the temperature extremes and vacuum space. Dry, low vapor pressure lubricants are used to prevent evaporation and contamination of optical surfaces on a satellite.