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.