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

MAMMOTH MINE, UTAH: TYROLITE (ARSENATE)

One of the most famous mining districts in Utah is located about 50 miles south-southeast of Salt Lake City on the west central slope of the East Tintic Mountains.  The mountains are part of the Basin and Range Physiographic Province and connect the Oquirrh Mountains to the north (also Basin and Range mountains and the range visible directly to the west of Salt Lake City) and the Canyon Range to the south.  The highest point in the range is Boulter peak at 8,308 feet.  The "major" town is Eureka, now a much smaller community than the mining and financial center it was in the late 1800s.

Satellite image of Utah showing location of Tintic Mining District and Salt Lake City.  Image courtesy of Ray Sterner, Johns Hopkins University.

 

Google Earth image© of Tintic Mining District including Mammoth Mine.  Utah Lake is the large body of water in the northeast.

The Tintic Mining District was discovered in ~1869 and its geology/mineralogy was summarized by  Wilson (1995)  who noted the presence of ~175 species of minerals…”Much of the production of siliceous ore in the district [was] utilized by smelters in Tooele and Salt Lake City to mix with more iron-rich ores of Bingham.  Tintic has produced gold, silver, lead, copper, iron and zinc as its major commodities.”  Morris (1968) noted the primary ores consisted of “sulfides and sulfosalts of silver, lead, copper, iron, zinc and bismuth in association with jasperoid (silicified carbonate rock), barite, aggregates of quartz crystals, calcite, dolomite, and ankerite…gold is locally common…”  In addition, the Centennial Mine in the District is the type locality of eurekadumpite, perhaps my favorite mineral name!

Morris and Lovering (1979) described the general geology of the Tintic area as “… consisting of the eroded flank of a composite volcano, which, during the Oligocene, buried a preexisting mountain range that had been carved from folded and faulted Paleozoic sedimentary rocks.  During this volcanic episode many stocks, plugs, dikes and other intrusive bodies were injected into both the sedimentary and volcanic rocks.  As the Oligocene volcanic and intrusive episode subsided, great volumes of hydrothermal solutions coursed through the rocks, chiefly among faults and fissures eventually depositing large replacement ore bodies and veins that occur almost exclusively in the sedimentary rocks underneath the hydrothermally altered lavas.”

So, several large volcanoes erupted in the Tintic area (and much of western Utah) during the early Oligocene (Hintze and Kowallis, 2009) and covered Paleozoic rocks that were folded and faulted by an earlier mountain building event termed the Sevier Orogeny (Cretaceous).  During the later Oligocene these volcanoes begin collapsing and large calderas formed.  The hydrothermal solutions associated with the volcanics followed the cracks and faults in the Paleozoic rocks and helped dissolve portions of the limestones.  As these solutions cooled the minerals begin to crystallize forming the ore bodies in the limestone.

There are several mines in the Tintic District, including the Mammoth Mine.  At one time this mine supported the community of Mammoth, now essentially a ghost town.  The Mammoth produced silver and gold but also a host of unusual and rare minerals including tyrolite.  For many years the dumps at the Mammoth were available for collecting by rockhounds; however, I am uncertain about current collecting access, rules and regulations.

Tyrolite is an uncommon to rare calcium copper arsenate carbonate with the chemical formula of Ca₂Cu₉(AsO₄)₄(CO₃)(OH)₈-11H₂O.  Tyrolite is very soft (1.5-2.0 Mohs) and forms glassy to silky to pearly, blue to blue-green to turquoise to light green, feathery to botryoidal masses of radiating crystals.  In thin layers the crystals are translucent.  It does effervesce in hydrochloric acid indicating the presence of carbonate (CO₃).  Tangdanite is a similar mineral where the sulfate radical (SO₄) replaces the carbonate.

Specimen of leafy green tyrolite along with the azure blue azurite; broad view.  Width of view ~1.6 cm.

Photomicrograph of radiating tyrolite crystals; width of crystals ~2 mm.

Tyrolite is a secondary mineral (supergene) found in the oxidized zones of copper sulfide deposits where the ore contains both arsenic and copper and perhaps oxidized from something like the hypogene ores tennantite (Cu₁₂As₄S₁₃) or enargite (Cu₃AsS₄). Tyrolite often is associated with azurite, a copper carbonate.

Mostly, tyrolite is just one of those uncommon, colorful, and somewhat “strange” arsenates that I love to collect.  In addition, it is part of an entire suite of rather rare minerals coming from the Mammoth Mine and its associates in the Tintic Mining District. 

REFERENCES CITED

Hintze, L.F. and B.J. Kowallis, 2009, Geologic History of Utah: Brigham Young University Geology Studies, Special Publication 9.

Morris, H.T., 1968, the Main Tintic Mining District, Utah in Ore Deposits of the United States, 1933-1967: American Institute of Mining Engineers, Graton-Sales Volume, New York, v. 2. 

Morris, H.T., and Lovering, T.S. (1979), General Geology and Mines of the East Tintic Mining District, Utah and Juab Counties, Utah: USGS Professional Paper 1024.

Wilson, J.R., 1995, A Collector’s Guide to Rock, Mineral & fossil Localities of Utah: Utah Geological Survey, Misc. Publication 95-4.