BLANCHARD MINE, NEW MEXICO: MURDOCHITE AND PLATTNERITE

 STOP THE CAR, I SEE A ROCK!

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.

Map showing location of the New Mexico-Colorado section of the Rio Grande Rift and location (blue) of the Colorado River. Public Domain, https://commons.wikimedia.org/w/index.php?curid=98340367

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).  

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. Slightly different scales; however, the width of the specimen in the middle photomicrograph is ~6 mm.  Therefore the small cubes of murdochite are between .1 and .2 mm.  Very tiny.  The clear enclosing material (also reflecting some light) is calcite.  Plattnerite crystals are mostly prismatic. Note the butterscotch colored tab of wulfenite hidden under the secondary calcite. 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 

AFMS & RMFMS ANNUAL SHOW AND CONVENTION, BIG PINEY, WYOMING, JUNE 2021: PART FOUR: ACANTHITE, MIMETITE, WULFENITE, CALCITE

 Mexico is a mosaic of different realities and beauties.

Enrique Pena Nieto

My last post commented on several specimens of calcite crystals that came up from Mexico via a grandfather and granddaughters. Exact collecting information was very fuzzy; however, I expect they came from one of the mines at Santa Eulalia in Chihuahua.  My purchase plan was to give these specimens to some younger rockhounds.

I did come back from Big Piney with some nicer specimens containing collecting information.  The first is a really nice barite matrix covered with gemmy orange wulfenite crystals associated with green botryoidal mimetite.  I have written about these minerals before but here is a partial repeat.

Mimetite is a lead chloro-arsenate [Pb₅(AsO₄)₃Cl] often found with pyromorphite and/or vanadinite.  In the former mineral the phosphate radical, PO₄, replaces the arsenate radical (AsO₄) while in the latter the vanadate radical (VO₄) replaces the arsenate.  However, mimetite is also closely associated with wulfenite [(Pb(Mo₄)] since both form as secondary minerals in the oxidation zones of primary lead minerals.

Mimetite seems mostly yellow in color although other colors are common—green, red, colorless, brown, orange-yellow and shades in-between.  It usually appears as small barrel shaped prismatic crystals but may be tabular, mammillary, granular, rounded, botryoidal and others.

Wulfenite is a lead molybdate with colors ranging from orange (common) to yellow, yelow-orange or red.  Crystals typically are tabular and transparent, at times opaque, but may form pyramids or stubs.

The specimen I brought home came from the La Morita Mine, Ascención Municipality, Chihuahua, Mexico.  This former polymetallic mine is well known for its luscious silky wulfenite crystals and the unique mimetite.  MinDat noted that since 2018 the Mine has been operated for specimen production rather than metallic ores.  However, my specimen was collected/mounted on the 2^nd of January 1972, about 50 years ago.

Barite matrix with wulfenite tabular crystals and botryoidal mimetite.  Width FOV ~4.8 cm.

Wulfenite tabular crystals.  Width FOV ~1.4 cm.

Green botryoidal mimetite.  Width FOV ~1.5 cm.

The second specimen I brought home exhibits pointy scalenohedron crystals of calcite covered by a secondary calcite druse with all sitting on a base of small gemmy quartz crystals.  It almost appears that the quartz is/was slowly replacing the calcite forming quartz ps. calcite. The specimen was collected from a mine in Guanajuato, Guanajuato Municipality, Guanajuato, Mexico.  I am guessing either the La Sirena or Valenciana Mine.

Scalenohedron crystals of calcite covered with secondary calcite druse on a base of gemmy quartz crystals. Width FOV ~5.3 cm.

 

A stubby original calcite crystal covered with a secondary calcite druse with tertiary quartz crystal growing near the top.  Width FOV ~1.5 cm.

An original calcite crystal covered by a secondary shiny druse of calcite. Width FOV ~1.5 cm.

The Guanajuato mines are classified as “epithermal vein deposits” where a variety of minerals are located in veins that are probably the result of previous fractures and cracks in the host rock.  The hydrothermal brines, especially sodium-calcium-chloride brines, are effective solvents and are able to dissolve the primary sulfide ores and circulate through the host rocks.  As these brines begin to cool, usually at shallow depths, deposition of minerals take place and often these veins of minerals are quite rich.  In fact, epithermal gold deposits are some of the richest gold deposits in the world. At Guanajuato mineralization consists dominantly of silver sulfides and sulfosalts (a metal + semi-metal + sulfur), base metal sulfides (metal + sulfur) mostly chalcopyrite, galena, sphalerite, and pyrite, and electrum (alloy of silver and gold). Gangue minerals (the waste rock) are generally quartz and calcite; the host rocks (holding the minerals) are Mesozoic sedimentary and intrusive igneous rocks and Tertiary volcanic rocks (Morgan and others, 2014).

The Rayas Mine, one of the richest of the Guanajuato mines, was located atop the mother lode, or veta madre, set prominently in the hills above the city. In fact, the mine still partially operates and produces ore, as well as specimens. The Rayas has a long history of bringing to grass specimens of acanthite, a silver sulfide [Ag₂S] that crystallizes in the Monoclinic Crystal System. It is a low temperature sulfide and is the stable form below 177 °C. The high temperature silver sulfide is argentite, crystallizing in the Cubic Crystal System, and as it cools below 177 °C it transforms to acanthite. Rockhounds still may find old specimens labeled as argentite; however, the only form  stable in “normal” air temperature is acanthite.   

Acanthite is a common ore of silver, lead grey in color although it tarnishes black (as on Sterling Silver), has a metallic luster, and is soft (~2.0-2.5 Mohs). What make acanthite such a nifty display specimen is that it often appears as a “stack” of pseudo-octahedral and/or pseudo-cubic crystals (formed during transformation from cubic argentite).

A “stack” of pseudo-octahedral and/or pseudo-cubic crystals of acanthite.  length of "stack" ~2.0 cm.