Rockhounds in Colorado and the American Southwest likely have a specimen or two of fluorite in their collections. More than likely the larger hunks are labeled “Blanchard Blue” the signature mineral mined from the Blanchard Group mines (and associated Mex-Tex, Desert Rose, and Royal Flush Groups of mines), Hansonburg District, Socorro County, New Mexico. In fact, many of the Colorado Clubs have visited the mines due to the generosity of the current owner, Ray Demark.
Microcubes of Blanchard Blue fluorite. Width FOV ~1.0 cm.
These mines are associated with activity in the Rio Grande Rift System (Colorado and New Mexico terminology), part of the Great American Rift System that extends from Mexico north along the Rocky Mountain Front to north central Colorado (and perhaps further). The Rio Grande River flows in the Rift System in New Mexico and southern Colorado while the upper reaches of the Arkansas River travel through the central part of Colorado. Movement of magma under the rift and release of crustal pressure most likely was responsible for this rift.
The Hansonburg Mining District is located in the Sierra Oscura Mountains near the tiny community of Bingham, New Mexico, on the eastern edge of the southern section of the rift system. The District is one of approximately 30 barite-fluorite-galena deposits in the southern New Mexico section of the Rio Grande Rift (Rakovan and Partey, 2009). The basement rocks in the Mountains are composed of Proterozoic granites and gneisses while Pennsylvanian marine rocks and arkose overlay the Precambrian rocks and host the mineralization. Evidently the hydrothermal fluids containing the minerals migrated from deep within the Jornada del Muerto Basin underlying basement rocks to the west of the Hansonburg District (Rakovan and Partey, 2009).
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The Blanchard and other nearby mines were established to produce copper, lead, silver, and perhaps barite and fluorite; however, it seems that a good profit was just out of reach. The mines today are major specimen producers of fluorite, linarite, brochantite, galena, and others. One of the less common minerals occasionally showing up from the mines is murdochite, a copper lead oxyhalide. The mineral seems interesting to me since it contains both chlorine and bromine anions. MinDat lists the chemical formula as PbCu6O8-x(Cl,Br)2x where x<=0.5).
Murdochite is usually black in color, a metallic black, with a metallic to submetallic luster; however, the crystal faces of the cubes and modified cubes (Isometric Crystal System) reflect light quite nicely and appear adamantine. Hardness is rated at ~4 (Mohs) and when rubbed on an unglazed porcelain plate, murdochite gives off a black streak. Like other metallic luster minerals, murdochite is opaque.     These enlargements of plattnerite and murdochite appear unfocused sine they are enclosed in secondary calcite. Note the rhombohedral crystal of calcite in the photo enclosing mucdochite crystals. I thought at first this secondary material might be gypsum; however, there is massive effervescence with a little acid. Murdochite is a secondary mineral found in the oxidized zones of copper-lead deposits. According to MinDat the primary hypozone lead mineral at the Blanchard is galena and the oxidized zone includes secondary lead minerals such as cerussite and anglesite. The copper primary minerals include the sulfide chalcopyrite and perhaps it provided copper for the several secondary minerals. Since secondary murdochite includes both copper and lead I suppose the metals must/might have oxidized from solutions passing through this sulfide and working their way up to the oxidized zone. At any rate,
murdochite is an interesting and not all that common mineral that displays
beautiful tiny crystals. My specimen is a micromount ex Art Smith 1979 and listed as Blanchard . |
What also makes this micromount interesting is the presence of tiny crystals and “slivers” of the lead oxide [PbO2] plattnerite. While the murdochite is identified (in my mind) by the cubic or modified cubic black crystals the plattnerite forms prismatic crystals or fragments of fibers, nodules, or other weird shapes, and sometimes is massive. They also have an adamantine or metallic luster, seem opaque, look brittle, and have a black or brownish black color. They would really be tough to visually identify if I was unaware of their presence at the mine. As with murdochite, plattnerite forms from the weathering of hydrothermal lead-rich minerals such as galena or secondary minerals like cerussite. Synthetic plattnerite is used in the construction of lead acid batteries.
The second interesting aspect of this tiny specimen is that the murdochite and plattnerite are enclosed in a secondary layer of calcite that rides on a quartz matrix. The calcite also captured a few butterscotch crystals and fragments of wulfenite [PbMoO4], a lead molybdate.
REFERENCES CITED
Rakovan, John; Partey, Frederick, 2009, Mineralization of the Hansonburg Mining District, Bingham, New Mexico, in: Geology of the Chupadera Mesa, Lueth, Virgil W.; Lucas, Spencer G.; Chamberlin, Richard M., New Mexico Geological Society, Guidebook, 60th Field Conference, pp. 387-398.
It's not an old book, or a treasure map. Nope. Staring up at me was a pile of rocks. Wendy Mass