BLACK HILLS MINERAL
Collected Ross Mine
Unattractive graftonite
Is it a keeper?
I am always on the lookout for phosphate minerals and have documented several in this Blog. The Black Hills of South Dakota is among my favorite places, both to collect, and just to drive around and enjoy the scenery. Stay away from Mt. Rushmore and Deadwood and the causal collector/observer may be able to mosey along tens of miles of back country roads free of traffic and peppered with nice roadcuts. Most of the interesting phosphates come from the Precambrian Harney Peak Granite and associated pegmatites. Although mines, pits, and other diggings seem scattered everywhere, many are on private lands and/or are under claim so permission to explore off road is necessary. Often the desirable and collectable phosphates are secondary minerals and microscopic, so a loupe in the field and a good scope at home is necessary to enjoy their beauty.
During this winter break (whatever that means since being “retired” indicates my break is ongoing) I have been doing some specimen sorting and came across a Black Hills box containing a few unspectacular specimens. Some were collected back in the middle 1960s when wandering and the freedom to collect minerals were more common than today. So, I decided to bite the proverbial bullet, check labels with identifications and locations, and get them into the correct South Dakota drawer.
It turns out that all located specimens are massive phosphate minerals. However, my major problem with these “rather ugly” phosphates is their difficulty in identification. I probably should state it is “my difficulty in identifying these specimens with simple physical properties.” I am certain that my colleague Tom Loomis up at Dakota Matrix in Rapid City could take one peek with a loupe and spit out a name! It is just tough for me to make that final? identification.
One of these creatures is labeled graftonite collected from the Ross Mine in Custer County. I will stick with that identification from 50+ years ago and but it could also be from the nearby Bull Moose Mine. As an aspiring stratigrapher/ paleontologist I was not always accurate with collecting information on unspectacular massive and dark colored minerals. But I did note it was from the Ross.
Graftonite is a ferrous iron phosphate [Fe2+Fe2+2(PO4)2 ]—maybe. However, the IMA distinguishes between graftonite (Fe), graftonite (Ca) where calcium substitutes for some of the iron, and graftonite (Mn) where manganese replaces some of the iron. In the chemical formula just replace the second Fe with either Mn or Ca. In fact, the Handbook of Mineralogy defines graftonite as [(Fe2+, Mn2+,Ca)3(PO4)2]. In reality, most graftonite probably contains some iron, calcium and manganese. Unfortunately, my “lab” does not contain any electronic gizmos that could produce the percentage of the graftonite cations!
Graftonite is some shade of brown to reddish brown (depending on the amount of iron), depending on light conditions it may appear less-than-vitreous to greasy, has a white streak (mostly), and a hardness of ~5.0 (Mohs). It rarely exhibits crystals and, as a primary phosphate, is usually associated with triphylite [LiFe2+PO4] in complex granite pegmatites. Graftonite is not a really attractive mineral and most likely would be missed on a routine collecting trip.
Brownish graftonite ~3.0 cm x 1.4 cm. A cross sectional view (in one of my lost/misplaced files) would show bands of graftonite alternating with triphylite.
Not to be left out of the “not-too-attractive” mineral category is griphite, an extremely complex phosphate: Na4Li2(Mn2+,Fe2+,Mg)19Al8(PO4)24(F,OH)8. In fact, so chemically complex that it was named for the Ancient Greek term for puzzle: γρῖφος (grîphos). Griphite is another brown to yellowish brown to blackish brown phosphate; the exact color depends upon the mix of cations. It is brittle, has a vitreous to resinous luster, a hardness of 5+ and is translucent in thin sections: sort of non-descript, dark colored, primary phosphate!
The specimen I acquired several years ago has an older typewritten label (glued on the box)indicating it was collected in Custer County in the southern Hills. However, that is probably wrong as almost all griphite specimens in the Hills (and in the world) come from either the Everly Mine (Type Locality) two miles east of Keystone or, to a lesser extent, the Sitting Bull pegmatite one mile northwest of Keystone (both in Pennington County). In fact, my “Custer” was crossed out on my label and Everly was penciled in. That seems reasonable since Roberts and Rapp (1965) noted that “griphite occurs in dark brown resinous masses… in granitic pegmatite at the Riverton lode (Everly Mine) near Harney City two miles east of Keystone”. As Robert Lavinsky noted on MinDat, not pretty to look at but is a rare species.
And from the same collector as the previously described griphite is a specimen of lithiophilite from the Dan Patch Mine in Custer County-well maybe? First of all, the Dan Patch is in Pennington County ~ one mile west of Keystone near Mt. Rushmore. Feldspar and beryllium were mined from a pipe-like pegmatite body. Second, lithiophilite is unreported from the Dan Patch although triphylite is known. My guess is that the Dan Patch Mine is correct for the collecting locality since Roberts and Rapp (1965) noted, “ cleavable masses of greenish-gray unaltered triphylite up to 8 feet by 3 feet in size occur in great abundance at the Dan Patch Mine.” My specimen is a gray to greenish-gray color with a hardness of ~4.0 (Mohs); lithiophilite is usually some shade of brown or salmon in color. The luster is greasy to resinous, it is cleavable, brittle and has a colorless to off-white streak. I am sticking with triphylite as defined by Roberts and Rapp (1965).
What about the identification as the primary phosphate lithiophilite? Probably wrong but an easy mistake to make—triphylite (also primary), the lithium-iron phosphate [LiFe2+PO4], and lithiophilite, the lithium-manganese phosphate [LiMn2+PO4] are end members of an isomorphous solid solution series. I certainly have difficulty identifying either mineral and like many other rockhounds, refer to either/or as triphylite-lithiophilite. Throw in the alteration minerals sicklerite [Li(Mn3+Fe2+)PO4] and ferrisicklerite [Li(Fe3+Fe2+)PO4] and I am really confused!
But wait, there is more! Heterosite [(Fe3+,Mn3+(PO4)] and purpurite [Mn3+(PO4)] are secondary phosphate minerals that are the result of the oxidation of triphylite-lithiophilite and are themselves in a solid solution series and difficult to distinguish between. In both heterosite and purpurite the lithium is leached from the parent minerals. MinDat noted “that intermediate alteration products are part of a continuous process of alteration: ferrisicklerite and sicklerite are intermediate between the unoxidized and unleached parent minerals and the completely leached and appropriately oxidized end-products, but may not be valid species.” I have concluded that Tom Loomis, Mr. Phosphate, of Dakota Matrix may be the only rockhound/mineralogist who can properly identify these phosphate minerals of the Black Hills.
There are also light colored, primary phosphate minerals in the pegmatites of the Black Hills. For example, two minerals that confuse me (not hard to do) are the fluorophosphate amblygonite and the hydroxyl phosphate montebrasite. Both are lithium aluminum phosphates that lack iron and manganese and are in a solid solution relationship: LiAl(PO4)F—LiAl(PO4)(OH). Although Roberts and Rapp (1965) list a large number of collecting localities for amblygonite and only a few for montebrasite, they further state, “montebrasite, together with amblygonite, has been mined extensively in the state [SD] as an important lithium ore. Upon further investigation, many of the reported occurrences of amblygonite in this area [Black Hills] will in all probability, prove to be montebrasite.” In addition, MinDat noted, “montebrasite is, by far, the most common member of the group. Amblygonite is scarce.” However, MinDat localities for amblygonite are far more numerous than montebrasite! It appears that additional identifications are needed to shift many of the amblygonite localities over to montebrasite.
Both montebrasite and amblygonite are similar in appearance: massive, white streak with a color of white, milk white, colorless, pale yellow; mostly resinous to greasy luster maybe sub-vitreous; hardness of ~5.5-6.0 (Mohs); will cleave; translucent to transparent. Montebrasite may have a pink or brown tint. Amblygonite may be beige, salmon, light green, light blue in color.
I have only seen the massive to white specimens that could easily be mistaken for milky quartz and not worth a “bend over” to pick up. However, quartz is slightly harder than either, has a lighter heft (specific gravity of ~2.6 vs. 3.05), and a vitreous luster; all hard to distinguish “in the dirt” while sorting rapidly. Although I have never seen crystals, “montebrasite forms attractive, colourless, bright yellow-green, sometimes blue, pink and pale brown, equant to short-prismatic crystals” that may be faceted (Gemdat.org). Amblygonite crystals may be faceted but are rare. Care must be taken with either faceted gem due to the low hardness and poor toughness (Gemdat.org).
The specimen I picked up many years ago simply said “amblygonite, Keystone District, Black Hills.” However, I believe the correct ID is montebrasite from one of many mines in the District. I base this decision on: 1) previous discussions about the rarity of amblygonite; 2) most/many examples of amblygonite have some shade of color; my specimen is snow white; 3) an identical looking specimen of montebrasite is shown in MinDat photos as coming from the Keystone Mining District.
So, that is my take on some of the primary phosphate minerals from the Black Hills. They are not recent finds as I located them stuck away in one of my many boxes that I always intended to open up and be surprised. And that I was, and believe me, for an ole soft rocker I learned much. That is what life is all about at my age---keep on truckin'!
The love of learning,
the sequestered nooks,
And all the sweet serenity of books
Henry Wadsworth Longfellow
REFERENCES CITED
Roberts, W.L. and G. Rapp. Jr., 1965, Mineralogy of the Black Hills: South Dakota School of Mines and Technology, Bulletin Number 18.
All MinDat and GemDat references were accessed in January 2022.