FAIRFIELDITE: UNCOMMON PHOSPHATE

 

As I have noted in several other posts, phosphates are among my favorite minerals (along with relatives vanadates and arsenates).  Phosphate minerals contain the anion PO₄ with an oxidation state of 3- arranged in a tetrahedron: 1 phosphate with an oxidation state of 5+ and 4 oxygens each with an oxidation state of 2- (for a total of 3-). In some phosphate minerals chlorine, fluorite, or hydroxide may be added as a negative anion as in the most common phosphate mineral group, apatite:   hydroxylapatite Ca₅(PO₄)3OH; fluorapatite Ca₅(PO₄)3F; chlorapatite Ca₅(PO₄)3Cl.

Fairfieldite is a uncommon phosphate that occurs as an accessory mineral in granite pegmatites, especially those that are rich in lithium—the Type Locality in Fairfield County, Connecticut (Fillow Quarry), Cleveland County, North Carolina (Foote Quarry), Custer County, South Dakota (Tip Top Mine and others).  Fairfieldite is a calcium manganese phosphate, [Ca₂Mn(PO₄)₂-2H₂O] with transparent to translucent euhedral crystals that leave a white streak. Crystals range from prismatic to equant and commonly appear as aggregates or foliated masses, rarely as radiating or fibrous.  Crystals have a sub-vitreous (mostly) to pearly luster and are soft at ~3.5 (Mohs).  Their color ranges from colorless to white to light yellow to perhaps greenish white.  It is sort of a nondescript mineral that sort of looks like gypsum.  The best bet to help identify fairfieldite is its occurrence in granite pegmatites, especially lithium rich forms. 

Numerous white to colorless of fairfieldite crystals.  Width FOV ~ 7 mm.     
I have not collected many minerals east of the Mississippi River—not an astounding statement when you consider that my only living stint east of the River has been about 15 feet while residing in La Crosse, Wisconsin.  I have traveled extensively in the eastern U.S.; however, it seems I was always fishing, hiking, camping, seeing the sights, etc. and missed most minerals.

During one sojourn through North Carolina I did travel through King Mountain and read about the nearby Foote Lithium Company Mine.  My interest in the mine, at that time, was in the lithium due to the many lithium mines in the Black Hills of South Dakota.  I really could not locate much information on the Foote Mine (partially due to self- quarantine and lack of access to research libraries) except that the mine has not been active for several years, is partially reclaimed (a park), and collecting is strictly prohibited.  According to MinDat, the granite pegmatite mined for lithium, tin, beryllium, niobium, tantalum, and “mica” is hosted in the Cherryville Quartz Monzonite, a Mississippian age batholith in the Carolina Piedmont Belt.  The Mine has produced 161 mineral species including the Type Locality of 15 minerals, many of which are rare phosphates.

SMALL SPECIMEN; TINY ZEOLITES FAUJASITE-Na AND PHILLIPSITE-K

 I came home from the Bridling Estate sales with several micromounts stuffed into plastic containers holding 25 boxes. Some of the boxes contained pretty ordinary mounts of minerals like calcite, dolomite, and pyrite. However, several of the mounts contain interesting, and sometimes rare minerals.  I am uncertain who set the zeolite mount described today: faujasite with phillipsite.

Zeolites have always been sort of confusing to me (mineralogically) although the group is quite large in numbers, many members are quite common, and they are very important as industrial minerals.  In addition, zeolites like chabazite and heulandite are usually seen at almost every rock and mineral show. Some zeolites, such as the acicular/prismatic crystals of mesolite, natrolite, and scolectite, make interesting museum specimens, as well as beautiful objects de art. 

I would have trouble defining zeolites to my fellow rockhounds.  So, I leave that task to Lauf (2014): the zeolites are a large and highly diverse group of (generally) aluminosilicate minerals in which SiO₄, AlO₄, or other tetrahedra are arranged into a three-dimensional structural framework having cavities and channels that can host ‘extra-framework” cations and water molecules.  The important item in this definition are the words about cavities and channels: 1) these open areas are able to hold a variety of cations which may be exchanged with other cations found in contacting solutions. For example, in water softeners/purifiers where zeolites are the "salts" the sodium in the zeolites may be exchanged for magnesium and calcite in the water (the “hardeners”); 2) the channels and cavities also may act, in industrial chemistry, as molecular sieves to remove specific sized molecules.  Zeolites are incredibly important in various industries.  Today there are over 200 synthesized zeolites; however, natural zeolites are usually found in cavities of volcanic rocks deposited via post-depositional contact of the rock with alkaline groundwater.

Faujasite is a rare zeolite, or actually a series of minerals with three endmembers named after the dominant cation: calcium, sodium or magnesium.  MinDat writes the chemical formula as:  M_3.5[Al₇Si₁₇O₄₈]-32H₂O where M is Ca, Na₂ or Mg present in different percentages while potassium and or strontium may partially substitute for one of the cations. Faujasite-Na is the most common composition with the type example from Sasbach, Germany; faujasite-Ca has a type example from Ilbeshausen, Vogelsberg, Hessen, Germany; and faujasite-Mg has the only known example from an “old museum specimen” collected from Sasbach, Kaiserstuhl, Germany (Faujasite Series, 2005).  

The specimen in my collection came from the Type Locality of faujasite-Na in the Linberg Quarries (seven of them),  Kaiserstuhl Volcanic Complex, Southwest Germany.  The volcanic rocks at the quarries are Miocene in age, ~19-16 Ma, and are a type termed limbergite: dark-colored rock resembling basalt but with an absence of feldspar. The glassy groundmass is composed of augite and olivine often with another generation of euhedral augite crystals. The zeolites usually appear in vugs of the volcanic rocks and are post depositional.

The crystals of faujasite are octahedrons, colorless to gray to white in color, transparent to translucent, vitreous and often are intergrown with other zeolites such as phillipsite and offretite.

Radial aggregates of phillipsite-K along with a mass of individual crystals. Width of vug with crystals ~2 mm.  Width of largest aggregate(middle left) is ~0.3 mm.

A second zeolite present in my specimen is phillipsite-K.  Originally there was a single phillipsite, essentially the potassium variety although the sodium species was the original Type. In 1997 phillipsite was divided into species with names based on the dominant channel cation. The type examples for the new phillipsite species compositions are as follows: phillipsite-Na has the composition from the original phillipsite type locality at Aci Castello, Sicily, Italy; phillipsite-K, Capo di Bove, Rome, Italy; and phillipsite-Ca, Lower Salt Lake Tuff, Puuloa Road, Oahu, Hawaii (Phillips Series, 2005).

MinDat.org writes the chemical formula for the potassium species as: (K,Na,Ca_0.5,Ba_0.5)_4-7[Al_4-7Si_12-9O₃₂] - 12H₂O.  So there is a mixture of potassium, sodium, and calcium cations combined with aluminum silicate and lots of water.  Also notice the barium (Ba) in the formula, an element present in all of the phillipsite series minerals.  In fact, if barium is the chief cation, the mineral is known as harmotome.

Phillipsite-K crystals are colorless and transparent to white, prismatic, vitreous, hard (4-5 Mohs), and often terminated.  They occur as terminated individual crystals or spherical radiating aggregates—both are visible in my specimen and often are intergrown with faujasite and another zeolite named offretite.

Phillipsite-K is the most common of the species and is found  in cavities of basaltic lavas and as a diagenetic alteration product in volcaniclastic sediment. The species of phillipsite formed is usually controlled by the type and mineral composition of the host rock, and the nature of the contacting solution Phillipsite Series, 2005).

  REFERENCES CITED

Faujasite Series, 2005, Commission on Natural Zeolites: IZA Commission on Natural Zeolites (iza-online.org).

Lauf, R.J., 2014, Collector's guide to the zeolite group: Schiffer Earth Science Monographs Volume 17.

Phillipsite Series, 2005, Commission on Natural Zeolites: IZA Commission on Natural Zeolites (iza-online.org).

The photomicrographs below are at the very limit of my skills with the digital microscope.  Many are somewhat fuzzy.  As you can observe these crystals are less than 1 mm in size. 

 

Aggregate of radiating phillipsite-K crystals with large faujasite octahedrons on either side of the mass.  Width of phillipsite ~0.3 mm.  

 

Radial aggregate of phillipsite-K water-clear crystals (pointing arrow).  The arrow bisects a large fuzzy crystal of faujasite. The aggregate is embedded in a mass of tiny, terminated phillipsite-K crystals along with nondescript faujasite.  Length of aggregate ~0.8 mm.      

Two "larger" octahedrons of faujasite situated in a vug on a layer of what appears to be an earlier generation of faujasite crystals.  Width: small, see below. 

 

Black, or darkest green, euhedral crystals of augite embedded in the limburgite groundmass.  The two vugs to the right hold distinct individual octahedrons of faujasite on an earlier generation of what I assume is faujasite. The photomicrograph above is an enlargement of the lower right quadrant of the complete vug. Length of vug ~ 2.7 mm.

Another vug containing faujasite octahedrons (F) and a sphere of perhaps offertite (O), another rare zeolite.  I am way out on a limb here with that designation, however, under my microscope it appears similar to those identified in MinDat.  Width of vug ~ 3.0 mm 

This is an interesting vugs with groundmass containing euhedral augite, numerous octahedral faujasite crystals and in the middle there are numerous small spheres that are most likely Opal-AN.  Also scattered around are individual prismatic crystals of phillipsite-K.  The orange mineral surrounding the vug is weathered olivine.  Width of vug ~4.0 mm.

 

BROCHANTITE, LINARITE & FLUORITE: BLANCHARD MINE

 

As noted in previous postings, Rebecca Nohe Estate Sales dispersed the rock and mineral collection of longtime Colorado Springs Mineralogical Society member Laurann Briding.  The sale attracted a large number of buyers wearing masks who were admitted via reservation (due to Covid-19 pandemic).  I came home with a few interesting specimens described in Postings on August 12 and 13. In addition, I have several Briding micromounts (~70) that were purchased at this sale, and at a later cleanup sale.  Among these mounts were several from the Blanchard Mine, New Mexico, and the subject of this Posting.

Dealers offering minerals “for sale” in many Colorado shows often display numerous chunks of fluorite. 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.  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 part 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 under-lying basement rocks to the west of the Hansonburg District (Rakovan and Partey, 2009).  

As for mineable metallic minerals it seems as if hard rock miners could not make a living from galena (lead), barite, or copper; however, many secondary and gangue minerals are of great interest to collectors and there are several specimen mines in the area. Perhaps the best known of the collectables is Blanchard Blue fluorite, specimens of which are found at hundreds of rock and mineral shows, especially in the southwest U.S.  Other “famous” collectables are the “world’s largest linarite crystals” [PbCu(SO₄)(OH)₂] (Delventhal, 2018). and colorful brochantite [Cu₄(SO₄)(OH)₆].

Crystals of linarite with submillimeter clear crystals of cerussite scattered about. Goethite "crust" noted on top specimen.  Width of both specimens ~7 mm.

Small cubes of clear fluorite with larger light tinted modified crystals of fluorite along with some quartz, what appears to be tiny cerussite crystals, sprays of brochantite sticking up in left bottom corner, and who knows what the material in lower right corner is.  Width FOV ~9 mm.

The handwritten note with the nine Briding micromounts states: “glory hole Blanchard cc bought 1968 fr Tony. linerite (sic),  malachite, fluorite, brochantite, barite, azurite.” I do not believe azurite is present and the blue specimens are linarite.  In addition, the bundles of green acicular crystals are probably brochantite rather than malachite.  There also are some nice gemmy, elongated, and striated crystals of cerussite.

 Brochantite sprays of acicular crystals.  Width of specimen ~5 mm.

Two cerussite crystals in center at a 45 degree angle pointing towards upper right corner.  Several smaller sprays of acicular brochantite. Width FOV ~7 mm.

A spray of green acicular brochantite and bundles of green and colorless brochantite. Cerussite lining right margin.  Maximum width of specimen ~7 mm.  

Sprays of prismatic brochantite of goethite.  Width FOV ~7 mm. 

This was originally identified as malachite but these are bundles of dark green brochantite on barite. Width FOV ~6 mm.

 

Sprays of brochantite on barite.  Width FOV ~7 mm.

Prismatic brochantite attached to goethite. Width of specimen ~ 6 mm.

Sprays of colorless and green acicular brochantite. Width FOV ~7mm.

Two other perky boxes came with the nine micromounts but are more macromounts than micro; however the specimens are mounted on posts with appropriate labels: 1) “Fluorite, clear xyls, Blanchard Claims, NM;” 2) “fluorite, quartz, pyrite, Blanchard Claims, N.M. Koelin.”  Now, I don’t have the slightest idea about the word Koelin.  Perhaps some reader will recognize the term as the “collector”? 

Mixture of transparent fluorite, mostly cubes.  Width FOV ~ 1.0 cm.


 

Mixture of blue to clear fluorite cubes, nice terminated quartz crystals.  The quartz in some areas is tinted sort of a light reddish brown. The faded yellow blogs are some sort of a clay mineral (I presume). The middle photo has a couple of pyrite crystals. Width FOV ~1.0 cm.
      

 

REFERENCES CITED

Delventhal, E., 2018, The Blanchard Mine, Hansonburg District, Socorro County, New Mexico: Minerals of the Metallic Ore Deposits of the American Southwest Symposium, August 4 - 5, 2018, Colorado School of Mines Campus, Golden, Colorado.

Rakovan, J., and F. Partey, 2009, Mineralization of the Hansonburg Mining District, Bingham, New Mexico: New Mexico Geological Society Guidebook, 60th Field Conference, Geology of the Chupadera Mesa Region.