PURPURITE (PHOSPHATE) FROM SOUTH DAKOTA AND COLORADO

For my part, I travel not to go anywhere, but to go. I travel for travel’s sake. The great affair is to move.

          Robert Louis Stevenson

This past summer I had an opportunity to revisit the Helen Beryl Mine in Custer County, South Dakota in the Black Hills.  I had first explored the area in the spring of 1966 as a geology graduate student enrolled in the University of South Dakota.  I, and a few of my geology friends, initiated a road trip to prospect for minerals in the Hills.  Our intention was to collect specimens for our introductory geology labs with the “leftovers” made into kits for sale to other interested students. 

If I remember correctly, we really did not sell any specimen kits back on campus.  But I do remember locating places and “things” in the Hills that I had never seen before.  We explored caves and mines and roadcuts and roadhouses with beer and country music and even had time to pound and collect minerals.  Growing up in Kansas I had never seen beryl before let alone spodumene.  The trip opened my eyes to the wonders of the Hills and was the highlight of the semester!

In revisiting the Helen Beryl Mine, I only had an opportunity to sort through part of the dump piles since I was hobbling around on my cane awaiting another knee surgery upon returning to Colorado Springs.  I did not find much of real interest but did bring home small specimens of purpurite and alluaudite, both phosphate minerals.

Purpurite [Mn⁺⁺⁺PO₄] is an interesting mineral that is the oxidation product of a lithium-rich mineral called lithiophilite [LiMn⁺⁺PO₄] and/or triphylite [LiFe⁺⁺PO₄].  There is a little confusion here and some of these minerals are difficult to distinguish between.  For example, purpurite is the manganese-dominant phosphate and is in solid solution with the iron-dominant phosphate called heterosite [Fe⁺⁺⁺PO4]; both are similar appearing to each other.  The parent of heterosite is triphylite with the latter in solid solution with lithiophylite.  So, nature has provided us with lithium-iron-manganese phosphates that weather to purple- to reddish purple- to rose- colored purpurite or heterosite.  During the weathering process, the lithium leaches away.  Can I tell the difference between purpurite and heterosite?  Maybe, but probably not.  My identification is based up two things: 1) the more purple to purple- red colored specimens are probably manganese-dominant purpurite since an increase in iron darkens the color; and 2) the locality mineral list published by MinDat!  

Purpurite (P) from the Helen Beryl Mine. A? may be alluaudite.  Specimen maximum width ~2.6 cm.

Purpurite seems never to form crystals but is always massive to granular.  Mineralogists with much more knowledge than me have placed the mineral in the Orthorhombic Crystal System.  I have described the color, and the hardness is about 4.5 (Mohs).  It has a dull or earthy luster and I obtained a red streak.  Once observed, purpurite/heterosite is easy to spot as it appears as a purple or purple-red “stain” on the matrix.

For purpurite/heterosite to form, a lithium(s) mineral must be present in the parent rock (precursor), usually igneous in nature.  As I understand the situation, lithium-rich minerals are not all that common in the rock record with the most abundant being “the lithium aluminosilicates spodumene,  petalite, and eucryptite , the phosphates amblygonite-montebrasite and lithiophilite-triphylite, several species of mica (mostly known as lepidolite), and the tourmalines (elbaite, rossmanite, liddicoatite)” (London, 2017).   It also seems these lithium-rich minerals are most abundant in pegmatites. My next question then was—what was the original source of the lithium? 

London (2017), in a fantastic article in Rocks and Minerals, explained the situation quite clearly: sediments containing fine-grained micas and clays that are lithium-rich collect in ocean basins and form a mud drape over the oceanic basaltic crust.  This mud lithifies into shale that later, during mountain building events, becomes a metamorphic mica schist.  During even later events the lithium in the mica was incorporated into molten granitic magma that upon slow cooling becomes pegmatites with lithium-rich minerals.   The formation of lithium-rich pegmatites is much more complex than this skeletal summary and I would suggest interested readers examine Professor London’s article.  

   

The Black Hills of South Dakota have numerous pegmatites containing many lithium-rich minerals.  For example, some of the largest spodumene crystals in the world have been identified in pegmatites of the Etta Mine near Keystone. 

The Helen Beryl Mine, southwest of Custer, is an oval mass of pegmatite about 250 feet long and 130 feet wide (Lufkin and others, 2009).  MinDat.org indicates the presence of lithium precursors spodumene, lithiophylite-triphylite and montebrasite-amblygonite (need chemical analysis to distinguish). London (2017), noted that “an abundance of amblygonite-montebrasite or lithiophilite-triphylite is indicative of the high phosphorus content of the marine shales from which most Li-rich pegmatites are derived.”  As the name implies, the Helen Beryl location was mined primarily for beryl.

The phosphate mineral alluaudite 

[(Na,Ca)(Mn,Mg,Fe⁺⁺)(Fe⁺⁺⁺,Mn⁺⁺)₂(PO₄)₃] also occurs at the Helen Beryl Mine.  This uncommon phosphate has a range of colors from green to yellow to tan to brownish yellow but is usually observed as a dirty yellow, opaque, earthy mass of tiny fibers and/or nodules---pretty nondescript; however, I have seem samples that are a dark greenish-black.  MinDat.org noted alluaudite is an alteration product of the complex phosphates varulite and arrojadite (see Blog posting April 2, 2013).

Photomicrograph of tan alluaudite from a second small specimen from the Helen Beryl Mine.  Length of tan spot ~2 mm.  The black matrix could also be alluaudite or some sort of phosphate.

My next question revolved around the “cause” for oxidation of divalent (++ charge) iron and manganese to the trivalent (+++ charge) forms.  At least in some cases the oxidation is due to bacteria.  However, I doubt that is the case in South Dakota pegmatites.  One of life’s persistent questions waiting for an answer!  

But, hold on, and paraphrasing the NPR show Wait, Wait Don’t Tell Me, London (2017) again provided an answer.  It seems as lithium-rich minerals in pegmatites decompose rather rapidly!  Toward the end of pegmatite formation, and in the presence of hot aqueous solutions, the early formed lithium-rich minerals undergo alteration: “spodumene and petalite alter to eucryptite + albite and to mica + albite. Montebrasite is commonly replaced by intergrowths of apatite + mica… Lithiophilite-triphylite alter to a large array of hydrous and more oxidized species of phosphates.”  In addition, the oxidation of lithium-rich minerals continues with surface weathering. At the Helen Beryl Mine the minerals include the oxidized (and lithium leached) heterosite, purpurite and sicklerite (intermediate solid solution mineral between unoxidized and oxidized end members). Ain’t learning fun?

I recently attended the Denver Gem and Mineral Guild spring show and picked up a nice specimen of purpurite collected from the Rainbow’s End Claim, Storm Mountain Pegmatite, Crystal Mountain District, Larimer County, Colorado: ~13 miles west of Fort Collins and Loveland.  The pegmatites seem related to the Silver Plume granites (Precambrian: ~1.4 Ga) and were intruded into schists of the Idaho Springs Formation (Precambrian: ~1.7 Ga   ) (Martin, 1993).  Jacobson (1986) was one of the last authors (I think) to report on the Crystal Mountain District and noted “blue apatite crystals, purpurite, spodumene, chrysoberyl and beryl are some of the choice mineral specimens available for collecting…This is one of the few pegmatite districts in Colorado where neither all the pegmatites have been found and studied or mapped nor all the minerals described.”  He listed 41 minerals of record.  Thirty years later MinDat.org has listed 55 valid minerals from ~ 60 claims, mines, prospects.  Most of the mining activity in the District, starting in 1884, centered around production of “mica” and beryl although most mines were rather unsuccessful (Thurston, 1952).  Jacobson (1986) stated that the pegmatites are beryl-rich and the rare lithium-rich minerals are in the most distal part away from the “parental granite.”

Purpurite (P) and alluaudite (A) on a "mica" schist (M) collected from the Rainbow's End Claim.  Width of specimen ~10.5 cm.

Purpurite is the most common phosphate mineral in the District and occurs in several of the prospects, mines, etc. (Eckel and others, 1997).    In addition, the purpurite specimens are a bright purple-lavender mass and are quite spectacular. Many/most purpurite specimens from the District are accompanied by the uncommon, tan phosphate, alluaudite 

[(Na,Ca)(Mn,Mg,Fe⁺⁺)(Fe⁺⁺⁺,Mn⁺⁺)₂(PO₄)₃].  Chemical analysis (EDS) by Modreski (Eckel, 1997) noted that some mines, prospects, etc. in the District produce purpurite while others offer heterosite.  At any rate, the Crystal Mountain District seems the only locality in Colorado that contains the phosphates purpurite/heterosite and alluaudite.

REFERENCES CITED

Eckel, E.B. and others, 1997, Minerals of Colorado: Fulcrum Publishing, Golden, Colorado. 

Jacobson, M.I., 1996, Pegmatites of the Crystal Mountain District, Larimer County, Colorado: in Modreski, P.J., ed., Colorado pegmatites---Abstracts, Short Papers, and Field Guides of the Colorado Pegmatite Symposium, May 30-June 2, 1986: Denver, Colorado Chapter, Friends of Mineralogy.

London, D., 2017, Reading pegmatites: part 3---what lithium minerals say:  Rocks and Minerals, vol. 92, issue 2.

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

Martin, C. M., 1993, Reconnaissance investigations of selected columbium and tantalum occurrences in Colorado: U.S. Bureau of Mines Open File Report 17-93.

Thurston, W. R., 1952, Pegmatites of the Crystal Mountain District, Larimer County, Colorado: U.S. Geological Survey Trace Elements Investigations 139.