BLACK HILLS PHOSPHATES: ROBERTSITE & WHITLOCKITE

 Back in the mid-1960s I was completing field work for my MS thesis in the badlands east of the Black Hills.  I certainly was not much of an igneous/metamorphic mineralogist/petrologist (AKA hardrocker) and often needed identification help with numerous specimens found in the Pleistocene stream sediments. So, off I went into South Dakota Tech looking for anyone who would indulge my numerous questions and help a poor ole grad student. One day a person said something like “go find Bill Roberts” and so I did.  At that time Bill was some sort of a research associate but at the time of his death in 1987 he held the rank of Senior Curator of Mineralogy and Invertebrate paleontology.  After locating him I was in awe of Bill’s knowledge of rocks and minerals around the Black Hills and was grateful for his time spent with a flatlander grad student. 

After leaving South Dakota for Utah, Kansas, Missouri and Wisconsin I paid little attention to the minerals and rocks of the Black Hills until settling in Colorado and reliving my youth (at least giving it a try but probably unsuccessfully).  As readers of these postings realize, I have taken a real shine to phosphates and arsenates, especially those from the Black Hills. I am constantly on the lookout for interesting minerals while wandering thru the Hills or perusing the offerings of show dealers.  I also check the internet for minerals proffered by Tom Loomis at Dakota Matrix. 

Only later in life, after I purchased used copies of his books (both written with George Rapp, Jr.) Mineralogy of the Black Hills (1965) and Encyclopedia of Minerals (1974), did I begin to take a stronger interest in Bill Roberts and his knowledge of minerals of the Black Hills—he was an amazing mineralogist. 

Norton (1989), in his memorial to Roberts, noted among many other achievements: 1) “that he added more than 100 species of minerals to the list known to occur in South Dakota; 2) he was involved with many new mineral discoveries and subsequent naming including---dernj'ite, ehrleite, fransoletite, jahnsite, johnwalkite, olmsteadite, metavivianite, pahasapaite, pararobertsite, perlofrte, robertsite (named after him), segelerite, sinkankasite, tinsleyite, tiptopite, walentaite, whitmoreite, wyllieite, a wicksite-like mineral that has been described but not yet named, and probably others that were overlooked when the list was assembled or that have not yet been studied; 3) his identification of carnotite in a specimen brought in by an amateur was the original discovery of uranium in South Dakota [and created a large-scale industry around the city of Edgemont]; and 4) their [his wife] private collection [of minerals] contained more than 30000 specimens.” WOW.  I also found it interesting (from the Memorial) that he studied “structural and field geology from A. J. Eardley.”  That really brought back memories since I also studied the same fields with Dr. Eardley (at the University of Utah), and he served on my Dissertation Committee.  What a serendipitous moment—ain’t life fun. 

I had been looking for a specimen of robertsite, a hydrated calcium manganese phosphate [Ca₂Mn₃(PO₄)₃O₂-3H₂O], for a few years and finally was able to nab a specimen at a recent show.  Actually, I purchased the specimen for the tiny cubes of whitlockite, a calcium magnesium phosphate [Ca₉Mg(PO₄)₆(HPO₄)], and the robertsite was a find when I returned home and examined the matrix under a scope.  Another great bit of serendipity. 

Mass of black, submillimeter in length, robertsite crystals (R).  Unfortunately a fiber (F) got stranded on the specimen: ~4 mm in length.

The specimen was collected from the Tip Top Mine near Custer in a pegmatite zone related to the Harney Peak Granite (~1.7 Ga forming the core of the Black Hills).  At last count 98 minerals had been identified from the pegmatite including 12 that count the Tip Top as their Type Locality; seven of these twelve have not been found anywhere else in the world!  The pegmatite is not “famous” for its large specimens but for microscopic crystals present within hydrothermally(?) altered beryllium and phosphatic rocks. Above information taken from a wonderful book by Lufkin and others (2009).

Black clusters of robertsite on a matrix of carbonate-hydroxylapatite (I think).  The spherical clusters are less than half a millimeter in size so the individual crystals are maybe a quarter? of a millimeter in length.  The upper photomicrograph shows a "worm" (length ~3 mm) of individual crystals.  I am uncertain about the "white cluster" but it may be some form of apatite.

Robertsite is a secondary phosphate mineral with tiny black to brown to red to bronze lath-like or wedge-shape crystals although at times (see photo) the individual crystals occur in botryoidal aggregates.  Crystals are soft at ~3.5 (Mohs), sometimes with a transparent to translucent diaphaneity but mostly seem opaque, and have a luster ranging from vitreous to waxy.  Nabbing a small crystal and rubbing on a porcelain plate will impart black-brown streak. 

Robertsite is closely related to another phosphate found in the Black Hills, mitridatite (see Posting Dec. 8, 2014), a hydrated calcium iron phosphate [Ca₂Fe₃(PO₄)₃O₂-3H₂O].  There seems to be some indication that an interchange between the iron in mitridatite and the manganese in robertsite—perhaps solid solution.

As previously stated, I purchased the specimen due to the gemmy, but tiny, rhombohedral crystals of whitlockite, a tricalcium phosphate (Ca₉Mg(PO₄)₆(HPO₄)] originally described from the Palermo Quarry in New Hampshire.  Whitlockite is, again, one of those secondary phosphates found (mostly, see below) in zoned granite pegmatites and is present at the Tip Top, Bull Moose and White Elephant mines.  The crystals from the Tip Top range from mostly transparent to translucent and are essentially colorless but also may tend to be white to gray.  Most of the Tip Top crystals are those nice rhombohedrals but in some places individuals are tabular to spherical and even?drusy.  Crystals are fairly hard, ~5.0 (Mohs), and the rhombs are vitreous if colorless and resinous if white.  The streak is also white.

Submillimeter clear crystals of whitlockite.

 

Whitlockite crystals (W), mass of black roberbsite crystals (R), drusy whitlockite (W?)--maybe, and unknown (?). 

The ?---> points to what appears to be larger whitlockite crystals covered by a druse.  Robersite (R) and a whitlockite (W) crystal.

 

Apatite (I presume), whitlockite (W) and the clusters of white (?)--maybe apatite.

A few interesting factoids about the mineral: 1) whitlockite has been identified in lunar samples and Martian meteorites and most mineralogists refer these extraterrestrial samples to the mineral merrillite; 2) whitlockite has been identified as forming in deposits of phosphate-rich bat guano; 3) magnesium-rich whitlockite can be found in many parts of humans and other animals ranging from bones to urinary stones to impediments in the blood vessels; 4) synthetic whitlockite is being tested for use in bone implants (it has a high compression strength); 5) the mineral bobdownsite is considered invalid and specimens should be referred to whitlockite.

So, there it is---more rare phosphates from the Black Hills, and especially the Tip Top Mine.  At many places in the Rocky Mountains old mines and their dumps are being reclaimed and vegetated and collecting nifty minerals is lost forever.  Fortunately, a geologist now owns the Tip Top and ensures its microscopic minerals, especially the rare phosphates, may be collected.  Tom Loomis is the resident expert on the phosphates and operates Dakota Matrix minerals out of Rapid City.

www.dakotamatrix.com

REFERENCES CITED

Lufkin, J.L., J.A. Redden, A.L. Lisenbee and T. Loomis, 2009, Guidebook to geology of the Black Hills, South Dakota: Golden, CO, Golden Publishers.

  

Nonrou, J., 1989, Memorial of Willard Lincoln Roberts February 12, 1923-March 23, 1987: American Mineralogist, Vol. 74.

Roberts, W. L. and G. Rapp Jr., 1965, Mineralogy of the Black Hills. South Dakota School of Mines and Technology Bulletin. 18.

Roberts, W.L., G. Rapp Jr., and J. Weber, 1970, Encyclopedia of minerals: Van Nostrand Reinhold Company, New York.

BLACK HILLS RARE PHOSPHATES: SINCOSITE & SINKANKASITE

I often travel to South Dakota, especially the Black Hills, trying to relive part of my youth.  Is that possible? Probably not for Henry Wadsworth Longfellow said youth comes but once in a lifetime, and Longfellow was a much smarter person than me.  But, one of the great things about being a geologist is that you may not be in the youthful stage of life but in looking at the rocks, memories come flooding back.  For remember, 50 years is not even the blink of an eye in geological time.  The outcrops that I have been looking at in the Hills---well pretty much the same as five decades ago .

However, I have noticed that my numerous joint replacements really slow down climbing and mobility. I blame those worn out joints on too many tumbles down hills and gullies while hunting for fossils in days gone by.  But, in all reality, my inherited genes are probably the culprit as I come from a long line of arthritic Danes. Many of my Scandinavian friends have mistaken me for a good ole Swede since my name, Nelson, ends in son rather than sen.  It appears my great grandfather did not like his Danish ancestry (Nielsen) so upon arriving at Ellis Island as a teenager (without parents) gave the authorities the Swedish spelling.  They hung a note around his neck saying something like “Ottawa County Kansas” and put him on a train heading west—he did not speak English--- and was heading to the plains looking for his 16-year-old Danish girlfriend (later my great grandmother).  The maternal side kept the Danish spelling, Bertelsen.  At any rate, I remember numerous relatives with gimpy knees and hips who loved pastries and “pickled fish,” and finished coffee by drinking leftovers from the saucer.  As a small boy I thought the center of the county was Minneapolis/St. Paul, Minnesota—my great uncles always ordered their “pickled fish” and stinky cheese from there.

So, with the artificial joints I now depend much more on locating interesting specimens at various shows, and occasionally at online venues.  At the summer Rocky Mountain Federation of Mineralogical Societies annual meeting and show (Rapid City; see Posting 7-28-18) I was able to visit with Tom Loomis of Dakota Matrix Minerals (www.dakotamatrix.com).  Tom is an online dealer with a large variety of interesting specimens and rarely sells at shows.  However, he did have a table at the Federation show and I was able to nab a couple of phosphates from mines in the Black Hills—sincosite and sinkankasite.

Sincosite [Ca(VO)₂(PO₄)₂-5H₂O] is a hydrous calcium vanadyl phosphate named after its type locality at Sincos, Peru, where the black, carbonaceous sedimentary shale rocks are phosphate- and vanadium-rich. Sincosite is never common either at the Peru mine or at a few other collecting localities.  It is probably best known from the Ross Hannibal Mine about three miles southwest of Lead (home of the Homestake Mine) in the northern Black Hills.  Martin and others (2004) mapped the area around the Ross Hannibal as phonolite sills and dikes intruded into Paleozoic sedimentary rocks. Loomis (www.mineralzine.chez.com/ancien/info1-7-b.htm) reported that sincosite was collected from gold-bearing siltstone of the lower Cambrian Deadwood Formation, a unit with pyritization and silicification.  He also noted that after collecting about 100 flats (this was 25 years ago) the mine was flooded by spring runoff and then backfilled, and the site is no longer available. At this locality sincosite is associated with minyulite (hydrated potassium aluminum phosphate) and hessite (rare silver telluride).

A mass of bright green tabular crystals of sincosite concentrated on matrix.  Width of photo ~2.7 mm. Collected by Tom Loomis from the Ross Hannibal Mine.

Photomicrograph of a section of above crystals.  Note the stacked bright green crystals on end.  However, the light reflects off the crystal or cleavage faces of the square-shaped crystals (each of these are ~1mm in size). 

Sincosite from the Ross Hannibal Mine occurs as very thin tabular crystals, square or rectangular in shape, that are always some shade of green in color.  At times these tabs are stacked together like a pile of pancakes.  On end the stack usually has a pearly to dull luster. The crystals have a perfect basal cleavage (like mica) and these surfaces are usually bright and shiny and vitreous in luster.  Crystals are quite small, a few millimeters or less in width, and amazingly soft (around 1-2 Mohs) and brittle.  The crystals are translucent to transparent.  I presume sincosite is a secondary phosphate meaning that that the phosphorus is recrystallized from a primary source.  

One of the icons of South Dakota mineralogy/petrology was the Curator of the South Dakota School of Mines Mineral Museum, Willard Lincoln Roberts—Bill Roberts.  I had been introduced to the gentleman back in the mid-1960s when he helped me with some rock identifications from my field area east of the Black Hills.

Bill was responsible, back in 1963, for “finding” a mineral in the Barker (aka Barker-Bergeson pegmatite) pegmatite (near Keystone in the Black Hills) that was described and named sinkankasite 20 years later by Peacor and others (1984). Sinkankasite is a very rare hydrated manganese aluminum phosphate [MnAl(PO₃OH)₂(OH)-6H2)] known only from three localities—the Barker pegmatite, Palermo No. 1 pegmatite, New Hampshire, and Sakha Republic, Russia.

Sinkankasite was named “in honor of Captain John Sinkankas [May 15, 1915 Paterson, New Jersey, USA - May 17, 2002 San Diego, California, USA] [US Navy Aviator #5390] innovator in faceting gem stones [the Smithsonian Institution has a 7,000 carat faceted quartz egg and a cut golden beryl of over 2,054 carats], author of mineralogical and gemological books, rare geoscience book dealer, mineral artist, mineral collector, and associate with Scripps Institute of Oceanography” (www.mindat.org). 

Sinkankasite usually occurs as tiny (a few millimeters) bladed or prismatic, colorless to white, transparent to translucent, brittle crystals often with a vitreous luster (although at times less “shiny”).  The individual crystals often occur together in sprays or bundles.

This mass of black, tiny (submillimeter) crystals? may be hessite (silver telluride). Photomicrograph width ~1 cm.  

Sprays of colorless crystals of sinkankasite associated with ?hessite, and muscovite above.  Width of photomicrograph ~9 mm. Collected by Tom Loomis from the Barker pegmatite.

Pecor and others (1984) described sinkankasite as a secondary phosphate formed in late-stage hydrothermal alteration of triphylite (lithium iron phosphate, the primary phosphate mineral).  The feldspars microcline and albite, as well as muscovite, supplied the aluminum. Evidently sinkankasite formed after other secondary phosphates including vivianite and hureaulite (see Posting 9-13-15). The Barker pegmatite is part of the Harney Peak granite complex of Early Proterozoic age (Precambrian ~1.75 Ga).

I want to thank Tom Loomis of Dakota Matrix for creating and encouraging my interest in the phosphate minerals of the Black Hills.

REFERENCES CITED

Martin, J.E., J.F. Sawyer, M.D. Fahrenbach, D.W. Tomhave, and L.D. Schulz, 2004, Geologic map of South Dakota: South Dakota Geological Survey, General Map G-10.

Peacor, D.R., P.J. Dunn, W.L. Roberts, T.J. Campbell, and W.B. Simmons, 1984, Sinkankasite, a new phosphate from the Barker pegmatite, South Dakota: American Mineralogist, v. 69.