COOKEITE: CRUD ON THE QUARTZ

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.

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.

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.

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.

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.  

Photomicrograph of quartz crystals and cookeite.  Arrow points to a tab of cookeite showing perfect basal cleavage.  Individual tab is ~3 mm.

Several tabs of cookeite on quartz, each ~ 3-4 mm.

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).  

Photomicrograph of cookeite rosettes (B) and a tab (A) collected from the Bennett Quarry, Buckfield, Oxford County, Maine. Width FOV ~1 cm.

Photomicrograph of individual rosettes from Bennett Quarry. Width of two rosettes (between arrows) ~ 3 mm.

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