Mt. Rushmore in the Precambrian Harney Peak Granite. The unit contains thousands of pegmatite
bodies.
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I recently received a small box of minerals from a
friend in Custer, South Dakota (June), and included was a specimen of
mitridatite. As usual, I was completely unfamiliar
with this calcium iron hydrated phosphate [Ca2Fe3+++(PO4)3O2-3H2O],
but decided to learn more. Perhaps the
most interesting aspect is that most of the time (from locations that I can
observe on MinDat) the mineral is found in granite pegmatites (usually late
stage). However, at its type locality in
the Crimea peninsula mitridatite is
found in oolitic sedimentary iron ores. The
commonality is that in almost all occurrences, mitridatite is an earthy green
(at least some stage of green from yellow-green to brown-green), compact mass
coating other [earlier stage] minerals. It is quite soft
(~2.5 (Mohs), and crystals are rare (certainly none on my specimen). At times, mitridatite may form pseudomorphs
after other phosphate minerals but any determination like that is above my pay
grade.
Bull quartz with a thin coat of green mitridatite --->
and M. Width FOV ~4.3 cm.
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Mitridatite seems in solid solution with both
arseniosiderite and kolfanite---just substitute the arsenate radical (AsO4)
for the phosphate radical.
Smear of green mitridatite om specimen above. Mineral FOV ~7 mm.
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My specimen is a hunk of bull quartz with a thin
coating of mitridatite on a broken surface.
The specimen came from the Bull Moose Mine in the Black Hills of South Dakota. Although substantial effort was exerted, I
could find very little information about the Bull Moose except that it is an
abandoned feldspar-rose quartz-beryl-muscovite-tantalum mine in a granite
pegmatite located 5 km SE of Custer (MinDat, 2014), and that the mine is
well-known for producing rose quartz and a variety of other rare phosphate minerals
(Loomis, 2014).
I have written numerous postings on minerals
associated with the Black Hills—they are among my favorite places to camp and
collect and sample microbrews. Lufkin
and others (2009) noted the “Black Hills is famous for its diversity in both
common and rare minerals…there are over 300 distinct species described in the
literature…The lithium-rich zoned pegmatites such as the Etta, Bob Ingersoll,
Tin Mountain, and Tip Top have the most mineral species…80 confirmed minerals
have been found at the Etta and Tip Top mines.” In addition, the pegmatites are famous for
their microspecimens of phosphates.
Large boulder of quartz and apatite once displayed
in Custer, South Dakota. Taken from the
Bull Moose Mine.
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In past years Scott’s Rock Shop in Custer displayed
a large boulder (~4 x 2 feet) in front of the store. I had glanced at the specimen several times
and simply assumed it was quartz since it had that “quartz look.” A couple of years ago I met the store clerk
(June) and she told me about the massive apatite coming out of the Bull
Moose. Of course I was interested and
she said something like “go take a look at the large piece in front of the
store.” Out I go but could not see
anything but the large “quartz” boulder!
OK, I made a big mistake---a substantial part of that boulder is massive
fluorapatite. What confused me in my
elementary understanding of mineralogy is that I always had seen apatite in
nice green, blue, yellow violet, etc. hexagonal prismatic crystals (or in
faceted stones). I had never observed
massive apatite--did not know about any form except those nice little crystals we
studied in mineralogy class, and later observed at most rock and mineral shows
and exhibits. What do you know! My friend was nice enough to haul a couple of
samples from her vehicle and send it home with me.
A yellow-green hexagonal crystal of apatite from Cerro
Mercado, Durango, Mexico. Many students
of minerals believe apatite is always found in nice crystals! Length of crystal ~1.2 cm.
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A specimen of massive apatite (and quartz) from the
Bull Moose Mine. Width of specimen ~7.5
cm.
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Fluorapatite [Ca2Ca3(PO4)3F]
is the most common phosphate mineral found in the Black Hills. Roberts and Rapp
(1965) noted that “olive-green to very dark greenish-brown masses of
fluorapatite up to one foot across have been observed at the Bull Moose Mine.” Alas, the boulder in front of the rock store now
has been reclaimed by the owner—but I have photos and samples.
I also have in my small collection a thumbnail-size
specimen picked up many years ago and labeled “Bull Moose Mine, Custer County,
SD, Black Hills.” In examining the
specimen I was able to recognize beautiful tiny violet spheres of radiating
crystals: strengite associated with
rockbridgeite. Strengite is an iron
phosphate [FePO4-2H2O] that is in solid solution with
variscite [AlPO4-2H2O].
The former is the iron-dominant series end member while variscite is the
aluminum-dominant phosphate at the other end.
However, I believe most strengite has small amounts of aluminum present
and small amounts of iron in variscite will allow the normally green mineral to
acquire a mild violet hue.
Violet spheres of strengite each less than 1 mm in
diameter. Note broken spheres composed
of radiating tiny crystals in upper photomicrograph. Collected Bull Moose Mine.
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Most strengite is found as spherical rounded
aggregates, radiating sprays, or as individual crystals. It is found in a wide variety of colors with
some sort of a violet color the dominant form although I have samples that are
transparent, gemmy and almost colorless.
It has a vitreous luster and if you can complete a hardness text the
result will come out ~3.5 (Mohs). The spheres are transparent to translucent
and quite brittle. When broken and
observed under high power binocular scopes, the spheres reveal tiny radiating crystals. However, there are instances where individual
prismatic, bladed or tabular crystals are noted.
Unknown tiny specks of a green mineral ---->
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Strengite also forms a solid solution series with
scorodite. When this happens the
phosphate radical is replaced by an arsenate radical and emerges as FeAsO4-2H2O.
The other interesting mineral on this thumbnail
specimen from the Bull Moose is rockbridgeite (I think), an iron phosphate hydroxyl. It is of interest to mineralogists for a
number of reasons, not the least of which is the presence of both ferric+++
and ferrous++ iron, an indication of organic material during
oxidation: Fe++Fe+++(PO4)3(OH)5. Usually rockbridgeite contains some manganese
(Mn++) substituting for the ferrous iron. If there is a complete substitution then
frondelite becomes the manganese-dominant end member of this solid solution
series. If zinc substitutes for the ferrous
iron then plimerite is the new mineral.
All of these minerals seem to form from the oxidation of primary iron
and phosphate minerals in granite pegmatites, or as oxidation minerals in “iron-bog”
sedimentary deposits.
My note: my specimen could just as well be frondelite, the manganese-iron phosphate in the solid solution series. It is so similar in appearance to rockbridgeite, and in some specimens the mineral in question seems to appear between rockbridgeite and frondelite. However, for convenience, I will call it rockbridgeite!
My note: my specimen could just as well be frondelite, the manganese-iron phosphate in the solid solution series. It is so similar in appearance to rockbridgeite, and in some specimens the mineral in question seems to appear between rockbridgeite and frondelite. However, for convenience, I will call it rockbridgeite!
Tiny mammillary structures
(~1 mm) composed of rockbridgeite covered by a white “crust” of an unknown
mineral. Collected Bull Moose Mine.
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Broken structures of: a
“rind” of rockbridgeite crystals parallel to rind surface, with a center core
of an orange-yellow unknown mineral (maybe mixed phosphates in center).
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Rockbridgeite occurs in a variety of dark and earthy
colors: brown, black, green,
yellow-brown. Most rockbridgeite is massive
or botryoidal and often forms a crust on other minerals. In my small specimen the mineral forms a
crust composed of “lumps” or mammillary structures that, when broken, display radiating
tiny crystals. In addition, the
rockbridgeite is present as small seams that show the crystals when observed in
cross-section. However, in many
instances rockbridgeite is just a number of small dark (brown or black) nondescript
lumps/bumps. The mineral observed on my
specimen is actually quite impressive.
Sylvan Lake in the
Black Hills—one of my favorite spots. Background
rock is Harney Peak Granite.
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The Black Hills are famous for phosphate
minerals, many of them tiny, colorful, and rare. Tom at Dakota Matrix is probably the expert on
these phosphates and I thank him some help with the minerals. Visit his web site noted in the References.
REFERENCES
CITED
Loomis, T.A., 2014, Pegmatites of the Black Hills,
South Dakota: www.dakotamatrix.com.
Lufkin, J.L., J.A. Redden, A.L. Lisenbee and T.A. Loomis,
2009, Guidebook to the Geology of the Black Hills, South Dakota: Golden
Publishers, Golden, Colorado.
Roberts, W.L. and G. Rapp, Jr., 1965, Mineralogy of
the Black Hills: South Dakota School of Mines and Technology, Bull. 18.
Mineralogy Database, 2014, www.mindat.org.
Strengite is one of my favorite minerals and Mike covers this subject well. Black Hills strengite is found in pegmatites as Mike covers in this blog, whereas many other localities it is found in "phosphorite" (sedimentary phosphates). Nice job Mike!
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