"Rock of the Month # 22, posted April 2003" A specimen rich in the pink silicate mineral pyroxmangite (`rhodonite') makes a striking talking point, a sample from a gold mine noted for complex mineral assemblages. This particular sample (237) was collected loose on surface at the mine site, 13 September 1983. The estimated proportions of minerals in a polished thin section of this sample include 78% pyroxmangite (massive and subordinate crystalline material), 14% quartz, 6% yellow-brown sphalerite, 1% galena, 1% chalcopyrite, plus accessory pyrite, carbonate and feldspar. The appearance of the pink silicate suggests the more familiar rhodonite, but optical properties (see below) are consistent with pyroxmangite. The Sunnyside rhodonite was indeed found to be pyroxmangite by Tom Casadevall (1976: see also Cobban et al., 1997, pp.393,413).

This sample weighs 769.38 grams, mean bulk magnetic susceptibility estimated at 2.24x10^-3 SI units.

Rhodonite, ideal formula MnSiO₃, pyroxmangite and pyroxferroite are three kindred triclinic pyroxenoid minerals, of which rhodonite is the best-known. Manganese minerals such as these are often stained by black secondary oxides of manganese. Rhodonite is generally found as pink, massive aggregates, Moh's hardness 6, specific gravity 3.7. Rhodonite may be used as a semi-precious stone for lapidary purposes: it occurs at, e.g., the famous Mn-rich mineral locality of Franklin, New Jersey, and in association with Paleozoic chert sequences in British Columbia. It is much harder than the pink Mn carbonate, rhodochrosite. Pyroxmangite is known from the Mn ores of Orissa and Andhra Pradesh, in eastern and southeastern India. It is also known from localities in Japan, Austria, Spain and North Carolina. The Sunnyside material has also been identified as pyroxmangite. Pyroxferroite was first described formally in Moon rocks collected near Tranquillity Base, during the Apollo lunar landings of 1969-1972. Pyroxferroite or its decomposition products have also been reported in meteorites thought to originate on the planet Mars, and in metamorphic terranes on Earth, including Karnataka (south India) and northwest Queensland, Australia (the Cannington Pb Zn deposit on the southeast margin of the Mount Isa inlier).

Fig. 2. The hand specimen, with a "period" (1983) coffee mug
and a useful reference on state geology (Williams and Chronic, 2014).

The three minerals are part of the pyroxenoids, a family of chain silicates which include wollastonite and bustamite. The cations (metals) in the mineral formulae may include Mn, Mg, Ca and Fe. Rhodonite may include all of these and rarely Zn. Pyroxmangite is nominally MnSiO₃. Pyroxferroite contains Fe and Ca substituting for Mn. X-ray diffraction to determine the crystal structure is probably the most elegant way to distinguish these minerals. All three display biaxial positive interference figures, 2V near 70° for rhodonite, 40° for pyroxmangite and pyroxferroite. All three have high refractive indices, averaging 1.73-1.76, and upper first-order interference colours (birefringence 0.016-0.020).

The Sunnyside mineralization, as documented by Casadevall (1976) and Casadevall and Ohmoto (1977), is a complex multi-stage phenomenon with elevated values of Au, Ag, Cu, Pb, Zn, and Cd, associated with mid- to late-Tertiary magmatism. The ores were deposited in six stages: 1) pyrite-quartz ore, 2) banded quartz-sulphide ore, 3) massive galena- sphalerite- chalcopyrite- bornite- hematite ore, 4) Au- Te- quartz ore, 5) Mn ores and 6) quartz- fluorite- carbonate- sulphate ores. Casadevall (1976) considered that falling temperature was the prime cause of metal deposition.

Intrusive activity in the early Oligocene (32 Ma) was followed by the development of the San Juan - Silverton caldera (dated at 28.0-27.5 Ma, Oligocene), and then by the Sunnyside mineralization in Miocene time (circa 16.6-13.0 Ma).

Sunnyside is a noted locality for rhodochrosite, manganocalcite and `rhodonite' (pyroxmangite: Murphy, 1979). This particular sample represents Casadevall's period-V Mn-rich ore, which may be 20% of the entire mineralization in the Sunnyside system. The Sunnyside mine was a relatively gold-rich system amongst other vein mineralizations in the western San Juan Mountains, producing over 800,000 ounces Au and 14 million ounces Ag (Bartos, 1993). For an overview of Colorado geology, see Williams and Chronic (2014).

Fig. 3. Here is a superb example of a pyroxmangite ("rhodonite")-dominant vein cutting base-metal-mineralized host rock,
on display at the excellent museum of the San Juan County Historical Society in Silverton, September 2015.
The sample is approximately 1 foot (30 cm) long.

References

BARTOS,PJ (1993) Comparison of gold-rich and gold-poor quartz-base metal veins. SEG Newsletter 15, 1,6-11, October.

CASADEVALL,T (1976) Sunnyside Mine, Eureka mining district, San Juan County, Colorado: geochemistry of gold and base metal ore formation in the volcanic environment. PhD thesis, Pennsylvania State University, 146pp.

CASADEVALL,T and OHMOTO,H (1977) Sunnyside Mine, Eureka mining district, San Juan County, Colorado: geochemistry of gold and base metal ore deposition in a volcanic environment. Econ.Geol. 72, 1285-1320.

COBBAN,RR, COLLINS,DS, FOORD,EE, KILE,DE, MODRESKI,PJ and MURPHY,JA (1997) Minerals of Colorado by Edwin B. Eckel. Fulcrum Publishing, Golden, CO, 665+40pp.

MURPHY,JA (1979) Mineral collecting at the Sunnyside and Idarado mines. Mineral.Record 10 no.6, 385-392.

WILLIAMS,F and CHRONIC,H (2014) Roadside Geology of Colorado. Mountain Press Publishing Company, Missoula, 3rd edition, xvi+399p.