SCAPOLITE: A SORT OF FORGOTTEN MINERAL

Why should things be easy to understand?

Thomas Pynchon

Scapolite is one of those minerals that sort of rings a bell somewhere in the recesses of your mind; however, you cannot quite pinpoint the location! About the only thing that finally surfaced in my mind came from basic mineralogy and pointed out that scapolite is usually an alteration product of feldspar (which one?), and is a metamorphic mineral (which facies?)!  I sort of left it at that point until a few years ago when I saw some beautiful faceted gemstones labeled “scapolite.”  Perhaps my mineralogy factoids were a figment of my imagination for those faceted gems looked nothing like some less-than-spectacular specimens I remembered from class.  

Perhaps I could forgive my mind since I was a third-year college student trying to reconcile memorizing mineral crystal systems with understanding the bombing of the 16^th Street Baptist Church in Birmingham and the assassination of President Kennedy in Dallas.  In fact, the assassination of Kennedy is one of those moments in history that persons of my age have imprinted on their minds--- I was heading to Mineralogy class!  Why did the crystal systems matter when young girls and presidents were being murdered?  I guess the short answer is that I did not want to return to my home town and work in my father’s gasoline station.  And then, there were rumors about “goings-on” in southeast Asia with the military draft picking up and men of my age learning a new trade.  So, back to learning about Monoclinic and Hexagonal minerals (and I never really understood the Systems and became a paleontologist).  And, scapolite became lost!

Age is an issue of mind over matter.  If you don’t mind, it doesn’t matter.

Mark Twain 

Scapolite reappeared in my mind back in 2012 when I was working on a post describing idocrase/vesuvianite.  The latter mineral was named by the famous German mineralogist Abraham Gottlob Werner and an informal variety of scapolite is called werernite.  Long story---read the November 18, 2012 Blog posting.  At any rate, I then took scapolite from the back recesses and shoved it toward the front of my mind and four years later am finally getting around to describing some specimens that I picked up along the way!

Scapolite is a silicate but is not really an individual mineral!  It is a solid solution series between end members marialite (sodium chloride rich) and meionite (calcium carbonate rich): Na₄Al₃Si₉O₂₄Cl to Ca₄A_l6Si₆O₂₄CO₃.  The sodium and calcium are interchangeable with each other as are the chlorine and the carbonate radical, therefore leaving an infinite number of chemical compositions. In addition, the calcium may include some strontium while the sodium may include potassium. And SO₄ may substitute for some CO₃ (Evans and others, 1969).  It appears that “pure” end members never occur in nature so intermediate compositions are the norm; however, these intermediate members vary considerably in chemical composition and remain unnamed.  Members of the solid solution series are essentially indistinguishable (visual) from each other and therefore scapolite is simply used for all. 

Scapolite comes in a variety of spectral colors ranging from colorless to white and yellow, purple, blue, red, green, pink, brown, gray, orange and various mixed compositions.  However, all varieties have a white streak. The transparency ranges from completely opaque to translucent to completely transparent while the luster ranges from vitreous to dull and pearly.  As scapolite weathers to “mica” the luster becomes dull and the diaphaneity becomes opaque. The hardness of ~5.5-6.0 (Mohs) makes gemmy varieties more suitable for pendants rather than rings.  Scapolite crystals are Tetragonal and generally come in two distinct forms: short and fat, or long and prismatic.  Gemmy varieties are usually prismatic and commonly striated.  A couple of my specimens show masses of non-gemmy and opaque crystals.  Many times, crystals fluoresce under both short and long wave UV.

Scapolite is one of the few minerals that have a “square” cross-section that helps in identification.  Compare photos below of a weathered crystal from Monmouth Township, Ontario, Canada, with a crystal diagram from the Goldschmidt atlas and found on www.mindat.org and courtesy of www.smorf.nl.

Cross-sectional view of scapolite crystal, non-terminated, collected from Grenville Terrane near Bancroft, Ontario.  Note square shape of crystal and compare with sketch below.  Width of crystal ~1.6 cm; length ~2.3 cm. 

Crystal diagram of scapolite from the Goldschmidt atlas and found on www.mindat.org and courtesy of www.smorf.nl. Note square shape.

I thought scapolite was perhaps a mineral indicative of a specific metamorphic facies.  However, I have learned the “mineral” occurs in a variety of metamorphic conditions ranging from regionally metamorphosed schists and gneisses to higher temperature and pressure amphibolites and granulites (usually as an alteration of feldspar minerals and producing non-gemmy crystals).  In addition, scapolite, at times gemmy, is found in marble produced by contact metamorphism. At other times scapolite in these calc-silicate rocks contain inclusions of clinopyroxene, quartz, titanite and calcite (Ocean Drilling Program).   It is also found, at times, in pegmatites associated with contact metamorphism, and basalt ejected from volcanos.  I certainly am far from a mineralogist/petrologist but have spent numerous hours reading “lots of articles” concerning scapolite, and trying better to understand the chemistry and genesis.  I have somewhat failed in my understanding and concluded that it is a very complex mineral found in several different environments and is quite difficult to identify as to a specific mineral.

Just because we don't understand doesn't mean that the explanation doesn't exist.       Madeleine L’Engle

My collection includes two specimens composed of a non-gemmy mass of opaque crystals collected from around Bancroft, Ontario, Canada.  Also from near Bancroft is a single, squat weathered crystal. 

Above two photos are masses of opaque, non-gemmy scapolite crystals.  Note nice terminations on crystals with T pointer and nice square shape with SQ pointer..  Width FOV top ~4.0 cm, bottom ~4.3 cm.  Both specimens have tiny crystals of an amphibole (katnophorite/hornblende??) and ferroan phlogophite on reverse.

  

Map of proto-North America showing addition of crust (yellow) to continent in late Precambrian by plate collision tectonics (Grenville Orogen).  High temperature and pressure accompanies these collision events and creates large expanses of metamorphic rocks and allows for the formation of minerals like scapolite. Map from Karlstrom and others (1999).

The Bancroft area of Ontario, part of the Grenville Province, is thought to have been the margin of North America during the Proterozoic part of the Precambrian.  The rocks are composed of two tectonic elements: 1) high-grade gneisses that were part of the 1.7-1.4 Ga continental margin; and 2) a package of volcanic, plutonic, and sedimentary rocks that are thought to be a collage of arc components accreted at ca. 1.17 Ga (island arc material stuck onto the early continent by plate collision) (Keck Geology Consortium, 2011).

From the Dara-i-Pech pegmatite field, Chapa Dara District, Konar Province, Afghanistan, I have several small gemmy crystals lavender in color.  The crystals are prismatic in nature and have at least one terminated end.  The location of the crystal mine is in the northeastern part of the country where lower Paleozoic rocks are intruded by Cretaceous-Tertiary granite and granodiorite intrusions (creating contact metamorphism--cooking the limestone).   Due to political instability in Afghanistan, specifics about gemstone localities are difficult to ascertain.  

Nice gemmy scapolite crystals.  length of longest crystal is ~1.1 cm.

I also have a partial violet crystal from the Marble Occurrence, Morogoro Region, Uluguru Mountains, Tanzania.  As best that I can determine, the area is the site of plate collisions in the latest Precambrian.  Metamorphism and thrust faulting left small patches of marble on older rocks (Fritz and others 2009).  If you have the inclination to read about some really complex geology, check out the Fritz article!

Partial crystal of gemmy scapolite with undetermined inclusions.  maximum width of crystal ~1.1 cm.

And finally, I have a beautiful, free form cab of crystal-clear, gemmy scapolite collected from Espirito Santo, Brazil (along with a second specimen, a nice gemmy, prismatic crystal).  Espirito Santo is a coastal Brazilian state north of Rio de Janeiro and east of the famous mineral-producing state of Minas Gerais.  It was difficult to acquire much information about the area except that really gem quality aquamarines are mined from the Mimoso do Sul Mine.  The gem bearing rocks are latest Precambrian in age (100 Ga to 54 Ga) and seem related to the Aracuai Orogeny and include a wide variety of metamorphic rocks and igneous intrusions.  The Aracuai Orogeny added crustal rock to the local Brazilian Craton. I presume, but remain uncertain, that the gem scapolite came from some of the marble units.

Prismatic, gemmy, clear with yellow tint, scapolite crystal. Length ~3.0 cm.

Gemmy, clear with yellow tint, free-form cab of scapolite. The X is beneath the cab to show the transparent nature of the crystal (thickness 6 mm.).  Length ~2.3 cm.

So, when it comes to scapolite:  I don't think I'm old enough or experienced enough to give anyone any guidance. All I would like say is that as long as you're having fun, I think you're doing the right thing.                Sania Mirza

REFERENCES CITED

Evans, B.W., D.M. Shaw, and D.R. Haughton, 1969, Scapolite stoichiometry: Contributions to Mineralogy and Petrology, v. 24, issue 4.

Fritz, H., V. Tenczer, C. Hauzenberger, E. Wallbrecher and S. Muhongo, 2009, Hot granulite nappes—Tectonic styles and thermal evolution of the Proterozoic belts in East Africa: Tectonophysics, v. 477.

Karlstrom, K.E., S.S. Harlan, M.L. Williams, J. McLelland, J.W. Geissman, Karl-Inge Åhäll, 1999, Refining Rodinia: Geologic Evidence for the Australia–Western U.S. connection in the Proterozoic:  GSA Today, v. 9, No. 10.

Keck Geology Consortium, 2011, Anatomy of a mid-crustal suture: Geology of the Central Metasedimentary Belt boundary thrust zone, Grenville Province, Ontario:  http://www.keckgeology.org/2011-ontario-canada.

Ocean Drilling Program, Unknown date, Macroscopic description of calc-silicate rocks:  http://www.odp.tamu.edu/publications/161_SR/chap_18/c18_3.htm

MORE SOUTH DAKOTA FAIRBURN AGATES

Memories, pressed between the pages of my mind
Memories, sweetened thru the ages just like wine

Elvis

As many/most readers know, I have a very soft spot in my heart for the State of South Dakota.  I suppose that comes from my attendance, in the mid-1960s, at the University of South Dakota.  During those two years of completing a graduate degree, I became fascinated with the diversity of the state’s geology from the glaciated eastern half to the Missouri River Trench to the “badlands” of the west and finally to the mountains of the Black Hills.  In “those olden days” collecting minerals from outcrops and mine dumps was fairly easy as the land was open or land owners were very accommodating to student collectors.  Today much has changed as land owners are increasingly frightened by liability lawsuits, and unscrupulous collectors (I use that term loosely) have ruined landscapes, left open pits, and swiped vertebrate fossils.  Field collecting is getting difficult. 

As usual, my early fall trip to the Black Hills of South Dakota included a couple of jaunts to the agate beds---locations where collecting is still possible.  There are several postings on this Blog that emphasize the geology of the beds so I will not repeat that information here.  I essentially want to post a few photos to emphasize that the agates are still out there, both in the source beds at Teepee Canyon, and out on the plains where rocks were transported by ancient streams.  Collecting the former requires some large crack hammers, arm strength, gloves and eye protection while collecting on the plains emphasizes “lots of” walking away from the main roads.

Teepee Canyon is located approximately18 miles west of Custer, South Dakota, about 2 miles west of Jewel Cave National Monument off U. S. 16.  As soon as travelers leave the Monument they should look to the west, up slope, to spot piles of broken rocks.  Sawmill Spring Road, (FS 456) leads off to the west and about a mile further West Teepee Canyon road takes off.  My best advice is to follow one of these roads/tracks and look for quarries where past prospectors have tried their luck.  The land is managed by the U.S. Forest Service and there are mining claims---I think.  It is best if rockhounds stop in the USFS office in Custer and discuss your plans with one of the friendly employees. 

The agates are encased in chert nodules housed within the lower Minnelusa Formation (Paleozoic: Pennsylvanian).  I suppose these nodules are the result of silica-rich meteoric waters circulating through the unit with resulting diagenesis producing the chert.  Why some nodules are agatized—I don’t have the slightest idea.  Just as I am uncertain how/why agates really form!  The formation of agates in several types of rocks is extremely complicated, even for the “experts”.

Teepee Canyon agate.  Width ~3.1 cm.

Teepee Canyon agate.  Width ~3.9 cm.

Teepee Canyon agate.  Width ~3.2 cm.

Fairburn agates are perhaps the “most famous” agates found in the Great Plains and are valued for their colorful fortification patterns with an abundance of reds (iron oxide), oranges (iron oxide) and blacks (manganese oxides).  There are several localities where agate hunters have collected a variety of stones but the easiest spot for collectors to locate is the “original Fairburn Beds” near the small community of Fairburn, located south along I-90, ~25 miles, of Rapid City near SD 79.  After reaching the community of Fairburn, agate hunters should travel east along French Creek Road (good gravel road) for about 12 miles to a sign locating the original collecting area managed by the Buffalo Gap National Grasslands.  Although known to collectors for decades, these Fairburn beds still yield an occasional agate, and as many colorful specimens of jasper, quartz and chalcedony as can be carried out in your collecting bag.  There are also occasional pieces of petrified wood, and brachiopods replaced by silica.

Fairburn Agate (obverse).  Width bands ~1.1 cm.

Fairburn Agate (reverse).  Width bands ~1.4 cm.

The really interesting story about the Fairburns involves their relationship with the Teepee Canyon Agates described above.  Most geologists now believe that the Minnelusa Formation in the Black Hills is the source of the Fairburn Agates and the siliceous pebbles were transported out to the plains by Tertiary streams draining the Hills.  The agates may be found within conglomerate beds of the Chamberlain Pass Formation and overlying Chadron Formation (both are Eocene in age and part of the White River group). Perhaps agates are most easily observed in the lag gravels covering many outcrops where the finer sediments have eroded away from the Eocene formations leaving behind a veneer of pebbles. 

At any rate, 2016 was again a successful season of locating a few agates, and that was all I asked.  Just hobbling around in the agate fields brought back a flood of pleasant memories from 50 years ago. See below.

I love those random memories that make me smile no matter what is going on in my life right now.  Unknown

I would suggest that if you are interested in Fairburn Agates---pick up a book or three written by Roger Clark, the premier expert on the agates.  Book one, South Dakota's Fairburn Agate, is only available on the used book circuit.  Book number two, Fairburn Agate: Gem of South Dakota, and number three,  Fairburn Agate: South Dakota State Gemstone are in print and published by Silverwind Agates. 

ROCHE MOUTONÈE: EVIDENCE OF GLACIAL MOVEMENT

No stress here.

For the last few years I have taken a late summer trip to a remote lake in Ontario, Canada---to slay the mighty walleye.  Well, not really slay them as most of our fishing is “catch and release.”  We keep a couple of small fish, under16 inches, to coat with Shore Lunch© and throw them in the hot bacon oil for a few minutes.  In another skillet, a can of potatoes is frying away and then come the eggs.  The coffee is hot and black.  It adds up to a day’s worth of fat and cholesterol; however, it is one of those bonding experiences that only comes around once per year.  Included in this experience: catching between 300-400 fish, drinking coffee from a Thermos© jug, plugging along in an aluminum boat, belching without saying excuse me, napping in the sun while waiting for “a bite” and never hearing a vehicle, eating salami sandwiches (and belching some more), having a cool one on the deck, ambling over to the small lodge for a magnificent evening meal including fresh homemade pie (talk about more fat and calories), watching the sun set over the lake with vibrant colors, hearing the loons chatter to each other, and sometimes catching a dazzling display of northern lights.  The quietness of the cool nights is indescribable.

Can't beat the view at sunset.

Nice wildlife including moose.

My bonding partner is a Ph.D. biologist who knows much about plants and animals living in the water.  His favorite trick is to pluck a scale off a large fish, pull out his hand lens (people other than geologists carry these gadgets), count the growth rings, and give me an age for the fish.  Pretty nifty.  I also quiz him about all of the water plants, those things that back in my Kansas home are generally called “moss.”  I have learned much about the aquatic world in my 16 years of bonding.

The geology in this part of the world has two classes of rocks/outcrops: 1) Precambrian igneous and metamorphic rocks; 2) Quaternary glacial crud covering most of the landscape.  However, down in the river valley that holds the natural lake (about 26 miles long) one is able to observe results of glacial scouring.  In fact, the river follows a channel that was “dug out” by the glacier.  It is not a smooth channel as noted by the numerous rocks sticking above the water’s surface and then plunging down 60-80 feet in a short distance.  The best fishing seems to be locating the “humps” where the rocks do not break the surface but where small baitfish congregate and therefore attract large northern pike (40-inch range) and walleye (up to 31 inches). 

Notice the glacial striations or grooves.

The glacier came from "that-a-way" and created this Roche moutonnée.

 At any rate, our conversations in the boat are not always piscatorial but tend towards nature and the environment---what would one expect from a couple of ole academic scientists.  One of my contributions to the discussions has been to explain the formation of a small island in the lake--see photos above. I had been studying the island every time we were near the place since the fishing was exceptional off one end.  And then it hit me—from the back recesses of my mind and my junior-year course in geomorphology I realized the island was a Roche moutonnée.  So, my friend, which way do you think the glacier was moving?  Let me explain how I know the answer (hoping my answer was at least as impressive as the scale aging of fish).  Mosey the boat over by that smooth end of the island and note the smooth but striated surface—but don’t bend the prop in the shallow water.  Now, on the other end where we catch the fish and the water is deeper note the rough surface of the island and the scattered pieces of the bed rock.  The glacier came from “that-a-way” (the shallow end) and smoothed off the knob and left the striations.  However, as it crawled over the knob and begin to descend it plucked off pieces of the bedrock leaving behind the rough surface of the leeward side.  Neat, huh?  Some geologists restrict the term Roche moutonnée to large scale features and term the small features like this island “stoss and lee.”  Personally, I like the French term better.

Cartoon showing formation of a Roche moutonnée or stoss and lee.  Public Domain sketch created by Jasmin Ros.

Cruising up an outlet stream to a kettle lake where a "hump" once produced 42 walleye in one hour.

Unfortunately, that was about the extent of exciting geology around the lake.  We did observe smaller lakes draining into the large lake that were kettles, basins created by large hunks of ice breaking off the main glacier and then melting.  I would have liked him to have observed the Matterhorn, an erosional glacial feature, as I did on a full moon night in Switzerland.  However, I suppose that would have started him on his stories about spending a summer cruising up the Volga River in the USSR studying plankton, or perhaps relaying information about flying (as a pilot) his research crew up to Hudson Bay (and seeing two wolverines as a bonus). 

If readers want to view a large-scale, mountain Roche moutonnée visit one of my favorite Bogs, In the Company of Plants and Rocks, at www.plantsandrocks.blogspot.com (Oct. 5 post).

I cannot even imagine where I would be today were it not for that handful of friends who have given me a heart full of joy. Let's face it, friends make life a lot more fun.          Charles R. Swindoll

I think we have reached the head of the lake!  I am pretty certain that we cannot push the 20 HP motor through those rapids!

COLLECTING STRIPED ROCKS IN UTAH

Google Earth© image of area around Milford in southwestern Utah.

Several years ago, I had the opportunity to consult on a project involving a pipeline bringing natural gas from southwestern Wyoming to southern California.  My job was to examine outcrops along the Wyoming and Utah portions and make decisions about the need for mitigation to protect fossils.  That distance is a long way to walk; however, I and my crew examined every rock outcrop along the route and consumed many cans of tuna, sardines, and Vienna sausage.  As we traveled along trying to unravel the geology we had the opportunity to notice some great scenery.  One place that still stands out in my mind was exploring the Mineral Mountains near Milford in southwestern Utah.  I was quietly leaning against a tree perched along a very large canyon, eating my can of sardines, and sort of daydreaming about what a great spot I had chosen.  Furthermore, I imagined that Native Americans also had enjoyed the view since it was easy to spot large mammals moving up or down the canyon.  In addition, a geologist exploring a mountain range named Mineral seemed like some sort of a nirvana had been reached.

There are certain half-dreaming moods of mind in which we naturally steal away from noise and glare, and seek some quiet haunt where we may indulge our reveries and build our air castles undisturbed.             Washington Irving

But, times marches on and we needed to get the pipeline route cleared so a call to duty was sounded.  But then we had a serendipitous moment for just on the other side of the tree was a large area covered by black obsidian flakes.  A spectacular site that confirmed our idea that Native Americans also had enjoyed our view! 

Now finding black obsidian in southwestern Utah is not an unusual occurrence; however, these flakes had been worked and were the “leftovers” from the making of projectile points.  We found the flakes since loose sand was been removed from the area by shifting winds.  We examined a few of the flakes and then left them undisturbed in their natural state.  No need to disturb the Gods.

Another event, perhaps as exciting as the chipping area, was noting the power station near Milford generating electricity from geothermal resources.  Neither I, nor my crew, had seen such a creature.  The Roosevelt Hot Springs Geothermal Area was commercially developed in the early 1900s, mostly as bathhouses, hot pools and adjacent hotels.  In 1984 the power plant was brought on-line and uses steam powered turbines created by bringing up hot brines via wells.  The area is perched on top of a fractured and faulted subsurface intrusion allowing circulation of the hot brines. At times, hot water followed a fracture and created a surficial hot spring (and filled a swimming pool).  At other places, the hot water deposited amorphous silica and opal.  One such place we observed was Opal Mound along the Opal Mound Fault.  Opal Mound is essentially a venting area for silica-rich hot water; however, the deposit is rather common opal and non-precious.

I had sort of forgotten about Milford and the opal and moved on to various other projects and interests.  However, a few years ago while touring the Electric Park venue, one of the Tucson shows, I ran into a couple who were selling Utah Lace Opal (ULO).  I am always interested in Utah minerals and therefore had a nice conversation Larry and Joyce Wright, the owners of Aspen Rock and Gem.  It seems the couple had been rockhounding and looking for Opal Mound but found it claimed and so headed out to hunt for obsidian.  What they discovered on their hike was not obsidian, but a banded “hard foamy rock” that could be stabilized and then cut, polished and used as jewelry in cabs or wire wrapped.  The trade name for this rock is Utah Lace Opal. 

Utah Lace Opal, non-stabilized.  Width ~5 cm.

It appears that the ULO was deposited via silica-rich mineralized thermal water migrating along a fracture system that Aspen Rock and Gem calls a silica splinter seam.  This seam goes down about 70 feet and all mining of the ULO is done by hand.  The Company also states the ULO was formed 2000 years ago over a 300-year period. I was unable to verify those dates but presume they have a strong basis for the results.

I purchased a small sample of ULO that was not stabilized but simply cut.  Without stabilization, the ULO has numerous vugs that create the “foamy” appearance.  So, Utah Lace Opal is a proprietary name but what about the description of the opal?

Much opal in the world can trace their origin of silica to marine or fresh water single-celled organisms called diatoms and radiolarians (both have siliceous skeletons).  However, the ULO traces its origins to silica-rich mineralized hot water where the solution captured the silica from rocks in the subsurface.  As these high-temperature fluids reached the surface they experienced rapid cooling and an amorphous silica precipitated as opal (sometimes called silica sinter or geyserite).

Although opal is commonly called a mineral, it is a mineraloid since it varies in chemical composition and is microcrystalline and/or amorphous since the atoms do not occur in a regular structure; however, there is an orderly arrangement of closely packed spheres of silica.  One distinguishing feature of opal is the amount of water in the atomic structure, perhaps up to 20%:  SiO₂-nH₂O where n represents a variable amount of water. 

The Quartz Page (http://www.quartzpage.de/gen_mod.html) noted that opal is classified as:

• Opal-C - microcrystalline opals made of cristobalite [ high temperature polymorph of quartz].  Transitional state in the formation of diatomites [rock formed mostly of diatom tests] and radiolarites [rock formed mostly of radiolarian tests] from opaline skeletons. Found in nodular concretions in sediments,

• Opal-CT - microcrystalline opals made of intergrown cristobalite and tridymite [another high temperature polymorph of quartz].  Found in common opal, as well as a transitional state in the formation of diatomites and radiolarites from opaline skeletons. Found in opaline concretions in sediments

• Opal-AG - amorphous opals with a gel-like structure. This is the structure of potch [common] opal, precious opal and fire opal.

• Opal-AN - amorphous opals with a network-like structure.  A common example is hyalite or glass opal.

As best that I can determine, the geothermal solutions rising along faults zones near Milford deposited both common opal and hyalite; however, Aspen Rock and Gem noted “it [ULO] went from one silica phase to another forming Opal-C to Opal-CT to quartz and is seen in various stages and is still maturing.  Vugs with Botryoidal Hyalite Opal clusters form stalactites and stalagmites giving lacey layers to the formation.”   Since neither cristobalite nor tridymite may be identified with ordinary microscopy, I was unable to note Opal-C or Opal-CT in my specimen.  It appears that I have common opal (Opal-AG) and glassy hyalite (Opal-AN).

The vibrant, and distinguishing, bands of ULO are created by common and hyalite opal with various impurities of aluminum (blue-green), magenta (manganese), orange (iron), gray (magnesium), yellow (titanium) (Aspen Rock and Gem).  

Photomicrograph common opal.  Width FOV ~1.2 cm.

Photomicrograph glassy and colorful hyalite opal.  Width FOV ~1.2 cm.

The Mineral Mountains, those of leaning trees great for a nap, are a complex and rugged range.  Structurally the range is a large horst (upthrown block bounded by faults) located in the transition zone between the Basin and Range Physiographic Province to the west and the Colorado Plateau Physiographic Province to the east.  Most of the exposed rocks consist of a intrusive pluton of mid-to late Tertiary age (Mineral Mountain Pluton).  The range is well known for producing gold, silver, copper and lead from contact metamorphic zones associated with Paleozoic rocks and the intrusive rocks.  In addition, the metamorphic heat has cooked a Late Paleozoic limestone and fluids have filled some of the small fractures creating a beautiful rock marketed as Picasso Limestone.  Crosby (2014 in MinDat.com) stated that the carbonate is the Permian age Toroweap Limestone while Stibbett and Nielson (1980) believed the small outcrops need additional study and the carbonate could be as old as the Mississippian Redwall Limestone.

Polished specimen of Picasso Marble so named due to the abstract nature of the veins.  Width ~ 7 cm.

It is tough to locate much confirmable information on the geology and mineralogy of the Picasso Limestone.  A stray BLM report noted that a few tons of the material was blasted and hard sorted by Penny’s Gemstones LLC from the Silver 1-2 and Silver 3-4 mines in the southern part of the Range.  Crosby (2014 in MinDat.com) stated that all outcrops of the Marble are under claim by David L. Penny of Beaver, Utah.  I presume David Penny and Penny’s Gemstone are synonymous.  Some banter on a chat room type of site indicated the Marble is mined out and now only available from the stock at Penny’s shop in Beaver, Utah (http://www.ltagallery.com).  At any rate, Picasso Marble is a beautiful rock and is valued for carving (especially bear fetishes), slabbing, and cabochons since it easily takes a very high polish (note light reflections on photo).  I don’t have the slightest idea about the mineral composition of the cross-cutting veins in the Marble but have seen undocumented chatter about silver sulfide, serpentine and various copper minerals. 

Another popular Utah rock with even less provenance information (at least that I can locate) is Zebra Stone or White Tiger Stone---or you can substitute rock or marble in place of stone!  There seem to be several localities in the world that produce a “zebra stone”; however, most pieces quarried from Utah are labeled as such.  It seems that any rock with black and white stripes from limestone to quartzite to sandstone to granite to gneiss etc. may be termed zebra rock.  I have noticed zebra rock for sale at numerous rock shows and usually it is simply identified as being collected in Utah.

Photomicrograph dolomite "stripe" in Zebra Rock.  Width FOV ~1.6 cm.

Zebra Stone composed of crystalline dolomite.  Width ~10 cm.

 The piece in my collection was purchased several years ago at a rock shop in southern Utah.  When asked about locality data all the proprietor would part with was a claim and quarry west of Sevier Lake in Millard County.  Per the geologic map of Utah, the rocks exposed along the west side of the Lake are Cambrian and Ordovician in age.  However, a stop at the BLM office in Fillmore indicated that zebra stone can be found in Lawson Cove, also in Ordovician rocks but southwest of Sevier Lake in the Wah Wah Mountains. My specimen is composed of both white and dark (blackish) crystalline dolomite. Zebra Stone or White Tiger Stone is a rock with an interesting pattern and certainly catches the attention of collectors and rockhounds. 

My last striped rock from Utah has very good locality data since it collected several decades ago near Vernon, Utah, in the West Desert (Tooele County).   In fact, Vernon Hills Wonderstone is one of Utah’s best known collectable rocks.  It is a welded, glassy, volcanic tuff (air fall ash, rhyolitic in composition) in which the glassy particles were “welded” together by heat and compaction.  In fact, mid-Tertiary welded tuffs are very common in western Utah; however, the Vernon Wonderstone is different from most in that it displays a variety of colorful concentric or layered patterns.  These vibrant colors are due to post-depositional, circulating hydrothermal waters depositing pyrite (mostly) that was later oxidized by rainwater creating hematite, goethite and other iron oxides.  Lapidaries love Vernon Hills rocks since the non-porous nature of the rock, along with a high percentage of silica, creates a suitable hardness for polishing.

Polished Vernon Hills Wonderstone.  Width of polished surface ~5 cm.

Wonderstone is sort of an overused term.  Besides the rhyolitic wonderstone, many types of “picture sandstone” and “picture jasper” have been stuck with the same moniker.  However, I suppose the rhyolitic wonderstones of Utah and western Nevada are the best known.

As far as I know, the Vernon Hills location is on BLM land and is still available for collecting.  In fact, during my last trip the area was covered by literally tons of pieces.  There may be mining claims; however, they are well marked.

I am not a big collector of striped rocks or any of the “picture types;” however, since they were collected in Utah, home of most of my professional work, I picked them up at shows or on the ground. 

REFERENCES CITED 

Stibbett, B.S. and D.L. Nielson, 1980. Geology of the Central Mineral Mountains, Beaver County, Utah: Department of Energy Division of Geothermal Energy Contract DE-AC07-78ET28392, DOE/ET/28392-40.

 As for reaching into the back recesses of my mind and remembering the Mineral Mountains, I am always reminded of our recent Nobel Laureate in Literature:  Take care of all your memories. For you cannot relive them.