Several years ago, on one of my trips to Arkansas, I
stopped in a rock shop to check out the offerings and came across a specimen
with a few water-clear quartz crystals collected from a mine with a nifty
name—Stand-on-Your Head #1. They were
not large crystals but absolutely gemmy with nice terminations. The other side of the specimens was sort of
covered with crud that I assumed was maybe a mica or a clay of some sort. But, I purchased it for a few bucks since I
liked the quartz.
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Gemmy and well-terminated quartz crystals from
Stand-on-Your Head Mine #1 that first attracted my attention many years
ago. Length of right crystal ~1.3 cm.
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Well, several years later I was off on one of my
dreams wondering about the lithium minerals in the Black Hills of South
Dakota—things like spodumene, schorl tourmaline, amblygonite, triphylite and
lepidolite (and others). I was thinking
about the source of the lithium and decided that “someday” I would check on
this question. Those thoughts lead me to
check about other lithium minerals—what were they? In doing some reading I ran across the
mineral cookeite, a lithium, aluminum, magnesium silicate [LiAl4(Si3Al)O10(OH)8],
not exactly a true “lithium mineral”, but actually a member of the Chlorite
Group containing lithium —close enough! However, up popped another question---the
pegmatites of the Black Hills contain lots of lithium in several minerals and
one of the ways that cookeite forms is as an alteration product of lepidolite
and schorl, two very common minerals in the Hills. Why is cookeite really rare in the
Hills? A question for another day.
I
have always been much better at asking questions than knowing what the answers
were. Bill James
Although I had questions about lithium in the Hills,
the description of cookeite on quartz from Stand-on-Your-Head #1 Mine in
Arkansas rang a bell in my mind---the “crud” on my specimen picked up many
years ago. So, I pulled out the quartz
and examined the individual crystals and the “crud.” Yep, it was cookeite.
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Specimen of many quartz crystals (Q) with an
overgrowth of late mineralization cookeite (lower 40%) labeled C. Gemmy quartz crystals noted above are on
upper left section of aggregate. Width FOV
~6.2 cm.
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Cookeite is a rather uncommon to rare member of the
Chlorite Group, a group where individual mineral members are difficult to
identify since most are soft, micaceous, and “green” in color. Therefore, these different minerals are often
just passed off as “chlorite” which is, in fact, not a recognized mineral
species! The general formula for the
Chlorite Group is X4-6Y4O10(OH,O)8
where X represents one or more of aluminum, iron, lithium, magnesium,
manganese, nickel, zinc or rarely chromium. The Y represents aluminum, silicon,
boron or iron but mostly aluminum and silicon. (www.galleries.com).
The rare cookeite then is XLiAl4Y(Si3Al)O10(OH)8.
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Photomicrograph of quartz crystals and
cookeite. Arrow points to a tab of
cookeite showing perfect basal cleavage.
Individual tab is ~3 mm.
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Several tabs of cookeite on quartz, each ~ 3-4 mm.
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Cookeite comes in a variety of “soft” pastel-like
colors: yellow, green, orange, light brown, white, beige; however, if you can
obtain a streak it is always white. The
hardness is soft at ~2.5 (Mohs), typical for members of the Chlorite
Group. The small crystals commonly are
tabular pseudohexagonal stacks or worm-like aggregates but also are found as
botryoidal masses, rosettes, or mica-like flakes. They have a perfect cleavage along the base
of the tabs (like the mica minerals) and are transparent (in thin tabs). Cookeite has a waxy-like luster and actually feels
sort of greasy. Sometimes cookeite is
confused with something like muscovite; however, the cookeite tabs are flexible
(can bend once) but are not elastic (will spring back to the original shape as
happens in muscovite).
Cookeite is often a late stage mineralization
product in pegmatites such as noted in the many quarries in Maine where it is
associated with lithium minerals such as tourmaline. However, at the Stand-on-Your-Head Mine #1
(and several other small diggings) in Arkansas cookeite is found in “small
hydrothermal quartz veins, most commonly filling fractures in the Jackfork
Sandstone (Pennsylvanian)” (Arkansas Geological Survey, 2015).
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Photomicrograph of cookeite rosettes (B) and a tab
(A) collected from the Bennett Quarry, Buckfield, Oxford County, Maine. Width
FOV ~1 cm.
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Photomicrograph of individual rosettes from Bennett
Quarry. Width of two rosettes (between arrows) ~ 3 mm.
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What is the source of the cookeite lithium in
Arkansas? Since I am not a “rock person”
that is difficult for me to answer.
However, in the southwest part of the state the subsurface Smackover
Formation produces “bromine brines” that are quite rich in lithium. Perhaps these lithium-rich waters migrated
northward along fractures associated the building of the Ouachita
Mountains? The many seams of quartz in
the Mountains, for which Arkansas is famous, are of several different ages from
Permian to early Mesozoic (as I understand it).
Those fractures would seem a likely conduit, but that is a guess.
The Stand-on-Your Head Mine #1 is located west of
Little Rock near the community of Bland.
It evidently was a quartz specimen mine since MinDat noted the
occurrence of only four minerals: cookeite, quartz, rutile and rectorite. It “produced outstanding specimens in the
1970s and 1985.” My specimen was
collected sometime in the 1970s.
REFERENCES
CITED
Arkansas Geological Survey, 2016,
Minerals/Metallic/Mineral Descriptions: www.geology.arkansas.gov.
…attractive
quartz crystal may occasionally be recovered from any of the Paleozoic
units. The more than 25,000 feet of Paleozoic
rocks exposed in the Ouachita Mountains have been deformed into complex gently
plunging folds that trend nearly east-west.
Steeply dipping fractures, closely related to the major folds and faults
of the region controlled the location and deposition of most of the quartz.
Mike Howard