SIDERITE RELATIVES? NO: PHARMACOSIDERITE & BARIOPHARACOSIDERITE

 

The iron carbonate siderite [FeCO₃] is a well-known mineral that contains about 48% iron.  In fact, the name siderite is derived from the Greek sideros (iron). Although the iron percentage is rather high, siderite is not an important ore since it is tough to smelt, and the ore bodies are typically small and discontinuous. Interestingly, siderite is found as crystals in metamorphic rocks, as a gangue mineral in hydrothermal rocks, in a variety of sedimentary rocks, and in igneous pegmatites.

 

Siderite and pyrite, Eagle Mine, Gilman District. Width ~ 4.6 cm.

Two tiny siderite rosettes situated on white calcite below a cross sectional view of pyrrhotite.  Width of each rosette ~2 mm.  Santa Eulalia District, Mun. de Aquilies Serdan, Chihuahua, Mexico (no exact mine listed).

Interestingly, mineralogists have co-opted the name siderite (sideros) for minerals that are not even related to the mineral siderite but do contain iron.  This naming has confused rockhounds since one tends to think that a mineral like phosphosiderite is related to, or allied with, siderite.  However, this is not the case since phosphosiderite is a hydrated iron phosphate [FePO₄-2H₂O] that is only related to siderite by the presence of iron.

 

Photomicrograph of lilac-colored elongated crystals of phosphosiderite collected from the Bull Moose Mine, Black Hills, South Dakota. Width of specimen at bottom of photo is ~ 1.1 cm.

For me the confusing minerals containing the word siderite are the iron arsenates lumped together in the Pharmacosideite Group: one or two cations (iron is always one) + the arsenate anion (AsO₄) + hydroxyl ion + water.  In the namesake mineral, pharmacosiderite, the two cations are potassium and iron— KFe³₄(AsO₄)₃(OH)₂-6/7H₂O.  The “pharma” is derived from the Greek word (φάρμăκου) pharmakou for poison or drug alluding to the presence of arsenic.  Other members of the Group have as their major cations: barium, cesium, sodium, lead, strontium, thallium, and a strange water H₃O (Hydronium). 

Pharmacosiderite crystals have a variety of colors--Olive to emerald green, reddish brown to yellowish brown, different shades of red, yellow.  The luster ranges from adamatine to greasy and often is tough to accurately place on such tiny crystals---at least with my camera equipment. However, there are some locations where the crystals are much larger. The crystals are brittle with a white streak and are soft (~2.5 Mohs). Pharmacosiderite is a secondary mineral usually forming in the oxidation zones of ore deposits containing iron and arsenic. 

Translucent green pharmacosiderite crystals from Wheel Gorland, Saint Day, Cornwall, England.  Ex Chris Christanson (CSMS) from Mitchell.  Width FOV ~ 5 mm.
 

Above four photomicrographs: tiny yellow-brown to green cubes, less than 1 mm. in size from Wheel Gorland.

The Gold Hill Mine (Clifton Mining District) in the Deep Creek Mountains in far western Utah was producing mine of As-Pb-Au-Cu-Ag-Zn-W-Ba until 1945.  Since then the mine has been a major producer of collectable mineral specimens, especially secondary copper minerals and arsenates; many are micromounts..

I have four specimens from Gold Hill that display very tiny crystals of the Pharmacosiderite Group.  Although they often are identified as pharmacosiderite, the potassium-iron arsenate, MinDat lists the mineral as bariopharmacosiderire [Ba_.05Fe₄(AsO₄)₃(OH)₄-5H₂O], an arsenate with barium replacing the potassium.  The crystals are generally translucent with a vitreous luster and a variety of colors—brown, green, yellow, yellow-brown, reddish brown, and sometimes bluish.  Now, here is the confusing part for me:  the crystals in my specimens appear as tiny yellowish cubes and I would assume they belong to the Isometric Crystal System (Cubic); however, Mindat notes they are Tetragonal, and the crystals are pseudocubic.  Without specific collecting data, my guess on any of these microcrystals would be assignment to the Pharmacosiderite Group.   

 

Yellowish cubes of bariopharmacosiderite from the Gold Hill Mine Mine in western Utah ~30 miles southwest of Wendover, NV.  Cubes are less than 1 mm. in size.

Unfortunately, the specimens display extremely tiny crystals (cubes) that are much less than 1 mm. in size and essentially beyond my photographic capabilities.  But, I learned much from this little experience. 

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