Friday, November 30, 2012

CINNABAR and MERCURY


TWINNED CINNABAR CRYSTALS IN GROUND MASS OF CALCITE AND QUARTZ.  WIDTH OF SPECIMEN IS ~ 6.5 CM.  THE CINNABAR CRYSTAL ON THE RIGHT IS ~ .7 CM AND DISPLAYS A SCARLET END SECTION FACING CAMERA.

Mercury (quicksilver) is one of those interesting minerals that I vividly remember from my college days in mineralogy and chemistry.  However, one must remember that those days were long ago and generally before anyone thought too much about mercury’s toxicity.  But in those “olden” days mercury was “fun” to play with since it is the only mineral metal that is liquid at room temperature.  I distinctly remember “defacing” (in those days a federal crime I believe) copper pennies by subjecting them to a bath in nitric acid.  We then smeared these reduced-size pennies with mercury and tried to pass them off as dimes (when dimes were worth “more”).  We also played games on the black lab tables with moving liquid globs of mercury!  In mineralogy class, we loved to heat up ore and watch the mercury bubble up on the surface.  Have a time machine take us back to the late 1990’s when a mercury thermometer broke on the floor in the hallway of the science building.  In a few minutes the building was evacuated and the fire team arrived in space-like hazmat suits (we soon replaced all of those mercury thermometers).  In addition, when living in Wisconsin I watched my dietary intake of fish caught in lakes contaminated by mercury. My teeth have numerous amalgam fillings from 50 years ago.  However, they are slowly breaking apart and being replaced by quite expensive ceramic caps.  As Bob Dylan crooned, The times they are a-changin’!

The other day, after an appointment with a dentist to replace an amalgam filling, I was “thinking” about mercury.  Did any of that metal really leach into my system from over 50 years of having that mixture in my teeth?  Did I eat too many walleye in past years?  What about the chemistry labs, did I absorb the liquid?  I don’t seem to have any symptoms of mercury poisoning so maybe I am “OK”.  I am hoping that the odds for contracting mercury-related problems are sort of like the odds in the recent Power Ball lottery ($550 million) where a person was approximately 100 times more likely to be killed by a swarm of killer bees than win the grand prize. I figure my chances are about the same with the mercury.

After pondering these deep thoughts I decided to check my collection since I knew that at least one specimen had some nice crystals of cinnabar, the major ore of mercury.  I don’t know an awfully lot about cinnabar except that it is a scarlet color, quite soft, and mines in Nevada had produced some nice crystals.  I picked up this specimen at an auction and it was unlabeled but I assumed Nevada (I learned that from Brian P).    After some detective work on the internet and in the library, I now am certain the specimen is from Nevada, most likely from the Antelope Springs District in Pershing County, and quite possibly the Red Bird Mine.

As far as I can tell, there are no operating mines in the U.S where mercury is the primary objective; however, there may be mercury produced as a byproduct of mining other metals.  In past years Nevada was a (?the) major state for the production of mercury and as a result parts of the state, especially in western and central regions, are littered with abandoned mines.  In the 1990’s the USGS begin a long-term study examining the effects of the abandoned mines on the surrounding ecosystems (Gray and others, 1999).  They noted: “Mercury is a heavy metal of environmental concern because highly elevated concentrations are toxic to living organisms, and thus, the presence of these abandoned mercury mines is a potential hazard to residents and wildlife when drainage from the mines enters streams and rivers that are part of local ecosystems…At the abandoned mercury mines in Nevada, the presence of cinnabar remaining in ore and calcine piles (roasted ore), and any elemental mercury around the mill and retort areas are environmental concerns. For example, in all the districts studied, there is cinnabar visible in the area of the open pit cuts and trenches, ore piles and tailings, as well as in the calcine piles…Detrital cinnabar and cobbles containing cinnabar visible in streams drainages below the mines indicate that mercury present at these sites is eroding down gradient from the mines.”  That sounds like pretty messy stuff to me and I remain uncertain about cleanup efforts, if any.

Mercury was mined in Nevada from about 1907 (discovered by then at Antelope Springs and with mining beginning in 1914) until the early 1990’s. The District mines produced from veins in Triassic limestone, dolomite, conglomerate, and shale (Gray and others, 1969). Evidently these veins were emplaced during the Miocene as a result of extensional magmatism (Noble and others, 1988).  That is, Miocene extensional tectonics involved the stretching of the earth’s crust producing what we know today as the Basin and Range physiographic province.
Cinnabar, the major source of mercury, often is “massive”, with poorly-formed crystals; however, there are exceptions and one of those crystal localities is found in Nevada.  Here the individual crystals are large (for cinnabar), very soft (easily scratched by a fingernail, 2-2.5 Mohs), and their scarlet color is often somewhat masked on the surface and they seem to display a submetallic luster.  However, underneath the surface the beautiful scarlet color stands out with an adamantine luster.  The Antelope Springs crystals are well known among collectors as individuals are often twinned (penetration twins) with six-sided crystals surrounding a top pyramid.  The twins are two “penetrated” individual crystals with a common C-axis rotated 180 degrees from each other.  At Antelope Springs the ground mass is composed of calcite and quartz, some with nice crystals.  
PHOTOMICROGRAPH OF BROKEN CRYSTAL OF CINNABAR SHOWING FACE VISIBLE IN MACROPHOTO.

PHOTMICTOGRAPH OF CRYSTAL OF CINNABAR WITH A NICE GEMMY CRYSTAL OF  QUARTZ.
As a point of interest (to me anyway) is that Meriwether Lewis took along, as a medicine, substantial amount of mercury and mercurial compounds to fight rampant outbreaks of venereal disease (and others) among the boatmen.  Some brought it along as a pre-existing condition while other crew members picked it up along the way from Native Americans who in turn had contracted  it from British traders.  The most famous of the mercury pills were the Bilious Pills of Dr. Benjamin Rush.  These powerful pills, termed Rush’s Thunderbolts or Thunder Clappers acted as a laxative and a body purger; they really cleaned out the digestive system!  The major ingredient of the pills was mercury chloride.  So, if the syphilis didn’t get you, the gums bled and your teeth loosened and fell out.  And if things were really bad the expedition leaders had packed several urethral or penis syringes in order to inject mercury solutions directly into the urethra.  Ouch! Those men were one tough breed. Today modern historians are able to accurately locate many campsites of Lewis and Clark since the ground still retains mercury---in elevated amounts! 

In today’s hectic and uncertain world (the Fiscal Cliff) I  continue to remember Dylan’s words:
Come senators, congressmen
Please heed the call
Don't stand in the doorway
Don't block up the hall

REFERENCES CITED
Gray, J.E., M.G Adams, J.C. Crock, and P.M. Theodorakos, 1999, Geochemical Data for Environmental Studies of Mercury Mines in Nevada: U. S. Geological Survey Open-File Report 99-576.  

Noble, D.C., J.K. McCormack, E.H McKee, M.L. Silberman, and A.B. Wallace, A.B., 1988, Time of Mineralization in the Evolution of the McDermitt Caldera Complex, Nevada-Oregon, and the Relation of Middle Miocene Mineralization in the Northern Great Basin to Coeval Regional Basaltic Magmatic Activity: Economic Geology, v. 83

Tuesday, November 27, 2012

HIKING PULPIT ROCK


Pulpit Rock, a recognized Colorado Springs beacon, is composed of the late Cretaceous Dawson Formation.  The "andesitic facies" forms the lower vegetated slope and indicates the presence of eroded volcanoes to the immediate west of Colorado Springs in the Rampart Range.  The upper "cliff forming facies" represents deposition by energetic streams flowing off the Rampart Range depositing eroded granite.  Photo courtesy of Gilbert Davis.
One of the major landforms that visitors observe while traveling along I-25 through Colorado Springs, other than Pikes Peak, is the Austin Bluffs--Palmer Park Divide, a major landform trending north-south through the northern and central parts of the City.  The Divide, located just east of the Interstate, actually “hides” the eastern part of the city from the western locales.  The parks/open spaces all beckon visitors for hikes to observe their many geological features; however, perhaps the most intriguing trek is up the prominent landmark termed Pulpit Rock.  This Park may be accessed from a frontage road just east of the terminus of North Nevada Avenue as it merges with I-25.  A small parking lot is available and several trails lead to the summit.

Before beginning the hike visitors should look directly westward across Monument Creek and I-25 to gaze upon the nearby sandstones and shales of the late Cretaceous Laramie Formation that are well exposed in the Popes Bluff/Popes Valley area (see 4/6/11 blog).  These exposures of the Laramie Formation represent the final regression of the vast Western Interior Seaway (WIS) that flooded what is now Colorado during much of the Cretaceous Period, and whose sediments were deposited in a series of stream channels, coal swamps, and lagoons bordering the seaway.  The rising Rocky Mountains to the west were responsible for providing the sediments of the Laramie Formation and for driving the WIS from the interior of the continent. 

Stratigraphically above the Laramie Formation is the Dawson Formation, a unit spanning the Cretaceous-Tertiary (K-T) boundary (~65.5 my). The Dawson Formation may seem to represent continuous sedimentation from the Laramie Formation but actually there is a break in deposition and the missing rocks (unrepresented geologic time) are represented by an unconformity.  This boundary between the Laramie and the Dawson is not exposed at Pulpit Rock but is buried in the valley of Monument Creek.

At Pulpit Rock the Dawson consists of two major rock units, or facies, and both are synorogenic in nature, that is the deposition of the sediments was occurring at the same time (syn) as the mountain building (orogeny) to the west.  The sediments were being shed off the rising Rocky Mountains into the subsiding Denver Basin east of the mountains.

The lower exposed facies of the Dawson is a “greenish-gray to olive-brown pebbly sandstone composed almost exclusively of andesitic material” interbedded with siltstone and sandy claystone.  Locally, near the base of the andesitic unit, lenticular beds of pebbly sandstone and/or conglomerate, and sandy claystone representing reworked beds of the older Pierre Shale and Fox Hills Sandstone are exposed (Thorson and others, 2001).  This lower unit forms the vegetated slopes at the base of the Pulpit Rock cliffs.

Andesite is a gray, fine-grained volcanic rock, with a high percentage of plagioclase feldspars (named after rocks well exposed in the Andes Mountains); therefore, andesitic sediments contain a high percentage of fragmental volcanic andesite.  However, when one examines the current mountains west of Colorado Springs large exposures of andesite are essentially absent!  The question then becomes---what was the source of these sediments?  Robert Raynolds and Kirk Johnson from the Denver Museum of Nature and Science have studied extensively the sediments and rocks of the Denver Basin and believe this episode of sedimentation represents “uplift of a portion of the Front Range bounded by the Golden and Rampart Range faults…[where] andesitic volcanic rock that covered much of the Front Range was stripped from the uplift and deposited” in the Basin (Raynolds, 2002).  The Rampart Range fault forms the eastern boundary of the Rampart Range near Colorado Springs.  Thorson and others (2001) noted that large boulders (up to three feet) and logs (up to eight feet in length) in the andesitic sediments indicate deposition in a very energetic stream environment.  All of this evidence points to a large system of overlapping braided streams and fans radiating off the rising Rampart Range, stripping off the andesite, and depositing the resulting sediments to the east of the mountains.  This is a great example of sediments and rocks informing geologists about the past presence of volcanoes to the west of Colorado Springs although physical remnants (the landforms) of these volcanoes are now absent.

The upper unit of the Dawson exposed at Pulpit Rock is mostly a white to light-gray, crossbedded to massive, coarse-grained arkosic (feldspar-rich) sandstone or pebble conglomerate.  Locally there are gray, massive mudstones representing ancient mudflows, and brownish-gray, organic-rich siltstones to claystones representing deposition in ephemeral swamps (Thorson and others, 2001).  This upper unit forms the massive cliffs and spires of Pulpit Rock proper.

There is a distinct change in composition of the rocks forming the lower unit of the Dawson from rocks in the upper unit.  While the lower unit is composed of andesitic material as previously described, the upper unit is almost devoid of andesitic sandstone.  Instead of the gray plagioclase feldspar common in the lower unit, the upper sandstones have a high percentage of pink feldspars characteristic of a granitic source terrain.  This arkosic sandstone would seem to indicate that that a “wedge or lobe of andesite-free debris was shed eastward from a granitic source along the western edge of the basin” (Madole and Thorson, 2003).  Perhaps enough of the Paleozoic and Mesozoic rocks had been stripped from the rising Front Range so as to expose the Precambrian Pikes Peak Granite?

Although the Dawson Formation spans the K-T Boundary (~65.5 my) in the Denver Basin, the rocks exposed at Pulpit Rock are latest Cretaceous in age at approximately 66 my (Johnson, personal communication, 2008).  Vertebrate fossils, including dinosaurs, have been collected at other localities in the Dawson Formation; however, bones seem rare or non-existent at Pulpit Rock.  On the other hand, researchers from the Denver Museum of Nature and Science (with appropriate collecting permits) have excavated numerous late Cretaceous leaves and other plant material from the Park.  In addition, any causal hike through the Park reveals numerous pieces of petrified wood scattered about on the surface.

It has been common practice in past years to assign the lower andesitic unit of the Dawson Formation to the Denver Formation and one still hears that term used.  However, Madole and Thorson (2003) have shown that the Denver Formation of the north pinches out and intertongues with the Dawson near Colorado Springs.  In addition, the source areas for the Denver Formation and the andesitic facies of the Dawson Formation are different.  Therefore, the use of the name Denver Formation for rocks at Colorado Springs is invalid.

The hike to the summit of Pulpit Rock is invigorating and geologically interesting.  In addition to the exposures of the Dawson Formation, one has a great view of Pikes Peak and Cheyenne Mountain, the Rampart Range, Monument Creek, Rockrimmon, and Popes Bluff/Popes Valley.  Time your hike in the evening and the sunset is spectacular.

      REFERENCES CITED

 Madole, R.F. and Thorson, J.P. Geologic Map of the Elsmere Quadrangle, El Paso County, Colorado. Denver: Colorado Geological Survey. Open-File Map and Report 02-2, 2003.

Raynolds, R.G. 2002, Upper Cretaceous and Tertiary Stratigraphy of the Denver Basin: Rocky Mountain Geology, v. 37, no. 2. 

Thorson, J.P., Carroll, C.J. and Morgan, M.L. Geologic Map of the Pikeview Quadrangle, El Paso County, Colorado. Denver: Colorado Geological Survey, Open-File Map and Report 01-3, 2001.

Sunday, November 25, 2012

DIOPTASE vs. DIOPSIDE


I cannot pretend to be partial about the colours, I rejoice with the brilliant ones…
Like Winston Churchill quoted above, I am quite partial to bright colors found in some minerals, but this trait is likely the norm among most collectors.  And, I am especially fond of green minerals—peridot, some garnets, some tourmaline, malachite, emeralds, and several others.  So, in perusing my collection, I came across green examples of dioptase and diopside and decided to further investigate.  In fact, I knew little about either mineral except several decades ago I attended a summer geology field camp and was shown chrome diopside from diatreme rocks in what is now the State Line Diamond District of Colorado and Wyoming.   And, I probably thought the two minerals were somehow related due to their color and their “diop…”.   However, I have found that diopside is named for the Greek prefix di, or two, while dioptase is for the Greek dio, or through.  And, they are not related.

BLOCKY, DARK GREEN DIOPTASE CRYSTALS FROM NAMIBIA.  WIDTH ~1.0 CM.

Dioptase is a hydrated copper silicate (CuSiO3-H2O) whose emerald green color is due to the copper cation.  The mineral is translucent to transparent, the individual crystals are mostly six sided and capped by a rhombohedon.  Although fairly soft at 5 on the Mohs scale, some cutters facet the gemmy crystals but must install the finished stone in a pendant—it is too fragile for a ring or bracelet.
NICE, SEMI-GEMMY CRYSTALS OF DIOPTASE FROM ALTYN-TYUBE, KAZAKHSTAN.  WIDTH OF PHOTO ~4.0 CM.
Dioptase is a secondary mineral that is found in oxidized zones associated with hydrothermal replacement in copper sulfide deposits.  One specimen is my collection came from the Christoph Mine, Kaokoveld Plateau, Kunene Region, Namibia.  The best I can determine is that dioptase, and several other oxidized secondary minerals, are found in limestones associated with the Otavi Group (late Proterozoic [Precambrian] ~600-800 Ma).  The mine, and adjacent area, is “famous” for producing dioptase crystals.

MASS OF VUGGY, MOSTLY CERUSSITE, CRYSTALS FROM ARIZONA.  SEE PHOTOMICROGRAPH BELOW.  WIDTH ~ 2.1 CM.  NICE, SEMI-GEMMY CRYSTALS OF DIOPTASE FROM ALTYN-TYUBE, KAZAKHSTAN.  WIDTH OF PHOTO ~4.0 CM.
The second specimen is from the famed Mammoth-St. Anthony Mine (Tiger) located about 50 miles northeast of Tucson in Pinal County.  A variety of metals, including gold, has been mined from the shafts but all activity ceased many years ago.  “Mineralization in this district is a series of veins within shear zones…Their gangue consists of brecciated country rock, cemented and replaced with quartz and calcite together with some barite and fluorite” (www.mindat.com).  My particular specimen is a mass of vuggy cerussite crystals (clear and gemmy) with sparse tiny crystals of green dioptase, and tiny arborescent “growths” of the mineral.  Very small partial crystals of wulfenite may also be present.

PHOTOMICROGRAPH OF GEMMY AND CLEAR CERUSSITE CRYSTALS WITH TINY GREEN CRYSTALS OF DIOPTASE.  WIDTH OF PHOTO ~2.45 MM.
Diopside is a calcium magnesium silicate (CaMgSi2O6) and a member of the pyroxene family. It is the magnesium-rich end member of a solid solution series with augite and hedenbergite (iron-rich).  Prismatic crystals are rather square in cross-section.  Most “common” diopside is green or sometime colorless/white while chrome diopside is a nice gemmy stone with chromium imparting a deep green color.  At times rutile is included and cat’s eye or star diopside (four-rayed) is produced.  Most specimens of diopside have a vitreous luster and are translucent to transparent with a hardness of 5-6 on Mohs scale.  Most gem cutters prefer to facet chrome diopside although some “common” diopside is cut, especially if the crystals are gemmy and clear. 


Diopside may occur in a variety of environments-- in hornfels associated with regional and contact metamorphic zones, in kimberlites associated with diatremes, in metamorphic gneiss and schist, and in skarns.
GEMMY CRYSTALS (PROBABLY CHROME-RICH) FROM MINAS GERAIS, BRAZIL  WIDTH ABOUT 3.8 CM.
I have two diopside specimens, one from Minas Gerais, Brazil (Aracuai [Arrassuai], Jequitinhonha Valley) where very gemmy crystals (probably chrome-rich) are associated with quartz.  The second specimen includes several (some quite large), non-gemmy blocky crystals from the York River Skarn, Hastings Co., Ontario.  


LARGE DIOPSIDE CRYSTALS FROM HASTINGS. CO., ONTARIO.  WIDTH OF TOP CRYSTAL ~2.4 CM.
So, although diopside and dioptase seem like they might be closely related, they are really “far apart”.  One is a hydrated secondary copper mineral while the other is a magnesium-iron mineral usually found in metamorphic rocks (or sometimes peridotites).   
Churchill finished by his thoughts about color by saying, and I am genuinely sorry for the poor browns!


Sunday, November 18, 2012

IDOcrase: WHERE HAVE YOU GONE ?

CRYSTAL OF VESUVIANITE FROM MEXICO (DESCRIBED BELOW).  LENGTH~1.4 CM., HEIGHT ~1.2 CM.

Ido, as a male name, is of Germanic or Dutch origin and was more popular in long past years. Today the name, for new born children, is almost non-existent with only three U.S. babies born in 2011 awarded that moniker.  Likewise, it appears the mineral name idocrase has gone by the wayside after being “replaced” by vesuvianite.  Perhaps that is not quite correct as noted below; however, for some reason my mineralogy class had these nice crystals of “idocrase” in the collection, and that is what I learned to identify.  I really did not recognize the name vesuvianite until years later when I examined beautiful faceted specimens in a jewelry shop.  And, it was not until several years later that I connected idocrase and vesuvianite as being different names for the same mineral.

Vesuvianite was actually named “first”, in 1795, by Abraham Gottlob Werner for a mineral found near/at Mt. Vesuvius.  As best I can tell, Professor Werner was working to distinguish grossularite, a garnet, from this “other thing”---vesuvianite—that were found in metamorphosed limestone blocks blown out by the volcano. Werner was not all that healthy in his adult life and did not travel outside of Germany; therefore, some collector brought the specimens from Italy to his lab at Freiberg (I presume at the Freiberg Mining Academy).

Essentially every geology student studied Werner in their historical geology class because of his erroneous theory about Neptunism—that all rocks formed when minerals crystallized in oceans of the primitive Earth.  However, he did redefine the term "geological formation" to mean rocks deposited/crystallized at the same time.  Previously “formation” was used to define the chemical makeup of a rock.  On the other hand, he tried to establish “universal formations” to indicate that certain rock layers were deposited at the same time all across the surface of the Earth.  But, the 1700’s were a different time and a different place and geology has advanced!
Students of geology also know Werner as the “Father of Mineralogy”. Werner was a classical descriptive mineralogist and in 1774 published Von den äusserlichen Kennzeichen der Fossilien.  The book offered a classification of minerals along with techniques for identification.  Wernerite is a variety of scapolite, an intermediate member of the marialite (Na4Al3Si9O24Cl)---meionite Ca4Al6Si6O24CO3 solid solution series (www.mindat.org).
Rene Just Haüy is known, as least by some scientists, as the “Father of Modern Crystallography” and is the author of Essai d'une Théorie sur la Structure des Cristaux (1794), and five volumes of the Traité de Mineralogy (1801).  His ideas about crystals and crystallography came about in a serendipitous moment.  He dropped a piece of calcareous spar (calcite) and it broke into many “little” rhombs.  Voila, the small broken pieces looked just like “mom and dad”--same cleavage faces and same face angles.  Haüy had a personal collection in excess of 12,000 specimens but nearly died in the French Revolution (as an ordained Catholic priest he did not want to swear allegiance to the Revolution).  He taught at the Sorbonne and the Muséum d'Histoire Naturelle but was fired from those jobs by members of the Bourbon Restoration Government. It was a really tough time to live in France, but he did retain his “head” and died more peacefully in 1822.
Haüyne is a feldspathoid, sodalite group (often a beautiful blue color), [(CaNaCa)4-8(Al6Si6(OS)24)(SO4Cl)1-2] named after this mineralogist (www.mindat.org). At any rate, in about 1799, Haüy decided the mineral named vesuvianite by Werner should be named idocrase!  Why, I have not been able to determine.  I don’t know if the naming was due to a lack of communication among the few mineralogists of the world, or maybe Haüy just liked the name better!  Whatever the case, there has been some minor confusion about this mineral for a couple of centuries.  It is my understanding that gemologists prefer idocrase while crystal collectors like vesuvianite.  However, the names seem to be used interchangeably in the popular literature, BUT Mineral Data (www.rruff.geo.arizona.edu), the International Mineralogical Association, and MinDat (www.mindat.org) use vesuvianite as the “official” mineral name with idocrase listed as a synonym.  I don’t have the slightest idea why my mineralogy class preferred idocrase except that this offering was a long time ago!  So, from now on, for me, it is vesuvianite!

Vesuvianite has a very complex, at least for me to understand, chemical formula and interestingly contains both neosilicate (SiO4 with a silicon anion and one tetrahedron) and sorosilicate (Si2O7 with two tetrahedra) groups:  Ca10(Mg, Fe)2Al4(SiO4)5(Si2O7)2(OH)4.   The nicest specimens (and I have not seen many) that I have observed are green in color but specimens also might be brown to yellow to even a nice blue-purple in color.  If in crystals, they are usually prismatic and “eight-sided” with one set (of sides) being quite dominant so the prisms look square in cross section.  They are commonly terminated with a four-sided pyramid.  Vesuvianite is a fairly hard mineral at ~6.5 (Moh’s) with a vitreous to “greasy” luster.  Gemmy vesuvianite is transparent to translucent while other specimens are less than translucent (not quite opaque but something).
GEMMY AND FACETED VESUVINIATE, 1.25 CT, 8 x 6 CM.  PHOTO FROM EBAY.

Other than Mt. Vesuvius in Italy, vesuvianite, including gemmy varieties, is found in a number of localities world-wide.  Cyprine is a sky-blue vesuvianite (copper impurities) first reported from Norway but now known from other locations.  Californite is a compact (non-crystalline) and massive type of vesuvianite often cabbed and sold as California Jade.  The Jeffrey Mine in Asbestos, Quebec, produces a very rare violet to magenta variety (manganese impurities).  Egeran is browner in color and reported from the Czech Republic.   


   
TOP CRYSTAL: DOUBLE TERMINATED BICOLOR MANGANOAN VESUVINIATE CRYSTAL.  SIZE: 1.5 X 1.0 X 1.0 CM.  PHOTO FROM EBAY.

Most vesuvianite occurs in skarn deposits, especially in contact metamorphism of limestones.  However, at times it is found in regionally metamorphosed schist and serpentine.   I have never observed vesuvianite in the field (I was always looking for fossils) but Eckel and others (1997) have documented several localities in Colorado.  Perhaps the most interesting statement: “Park County—Badger Flats District.  Grayish green vesuvianite is an abundant constituent of calc-silicate gneisses of the Tarryall Springs District”. Maybe a field trip is in order!
The lone specimen that I have in my collection is a nice, but non-gemmy, crystal from “Lake Jaco, Mexico”.  However www.mindat.org noted that the locality named Lake Jaco is really a misnomer since the early mineralogist collecting crystals did not want competition at the mine!  So, as best I can tell the locality is Sierra de la Cruz (Lake Jaco), Mun. de Sierra Mojada, Coahuila, Mexico.  At least that is how MinDat lists the name.
In three words I can sum up everything I’ve learned about life: it goes on.  Robert Frost
REFERENCES CITED
Eckel, E. B. (and others), 1997, Minerals of Colorado: Fulcrum Publishing, Golden.

Post Script:  I recently acquired two green, gemmy crystals collected several years ago from the VAG Mine, part of the Belvidere Mountain Quarries. The quarries produced chrysotile asbestos from the early 1800's until 1993.
LENGTH ~1.2 CM.


   

LENGTH ~ 2 CM.


 

Monday, November 12, 2012

BLUE-PURPLE HALITE

HALITE FROM DELAWARE BASIN, NEW MEXICO. CUBE ~2.5 X 2.5 CM
I have always associated the mineral halite (NaCl) with white-colored table salt, the shimmering small crystals on the “Bonneville Salt Flats” in western Utah, the salt anticlines and salt valleys in the Paradox Basin of the Four Corners, and the massive subsurface deposits left behind by the evaporating Permian cratonic seas.  These thick subsurface beds of Permian age are abundant in my native Kansas and, in my youth, could be observed by descending into one of the mines and picking up a few crystals.  Although mines at Hutchinson and Kanopolis are still producing “rock salt” for a variety of uses, getting a free visitor trip into the diggings is virtually impossible. 
HALITE CRYSTAL FROM PERMIAN SALT, KANSAS. CUBE ~ 2.5 X 3.0. 
But even if you cannot descend into a mine there are numerous opportunities to observe natural halite on the surface--the above mentioned Bonneville Salt Flats, for example, and certainly the vast Searles Lake region in California where nice pink crystals are exposed over an area of about 20 mi2.
HALITE CRYSTALS FROM SEARLES LAKE, CALIFORNIA.  WIDTH ~9 CM.
We all realize that halite easily dissolves in ordinary water, H2O.  This process is “good” in some cases, for example in flavoring our foods.  But in other instances, the results are badly degrading the environment---“road salt” runoff certainly contaminates local streams and in some cases even ground water.  The solubility of halite is also responsible for the large number of sinkholes in western Kansas (and many other states).  Although Kansas is not known as a “cave state” where carbonate rocks dissolve to form voids, many people are surprised about the subsurface solution of Permian-age halite and gypsum with resulting collapse of overlying rock layers. A couple of examples include the Ashland Basin in southwestern Kansas, perhaps 10-12 miles long, and representing a series of coalescing sinkholes.  The Basin lies in the High Plains Physiographic Region and that upland is perhaps 400-500 higher than the bottom of the sinkhole.  In Clark County, also in southwestern Kansas, the circular Big Basin is a collapse structure about one mile in diameter and dissected and drained by the Cimarron River.
GOOGLE MAP VIEW OF BIG BASIN, A SINKHOLE LOCATED IN SOUTHWESTERN KANSAS.  THE CIRCULAR STRUCTURE IS ABOUT ONE MILE IN DIAMETER.  NOTE ROAD DISSECTING THE FEATURE.
In central Kansas, where I spent my youth wandering hills of the Dakota Formation (Cretaceous), I was fascinated with the amount of halite leaching out of the rocks.  The Jamestown Wildlife Refuge is a major stop for migrating waterfowl in the Central Flyway, and a tremendous place to “bird watch”.  The refuge is a large marsh with a series of salt water springs and seeps issuing from the upper part of the Dakota.  As the water evaporates halite crystals are constantly being produced (and re-dissolved) along water’s edge.
I grew up on the Saline River below its contact with the Dakota and the water was highly charged with sodium chloride and could not be used for direct irrigation of plants and crops (several thousand milligrams of chloride per liter).  The Saline is a fairly long river at ~400 miles (entirely in Kansas) but is actually quite small in size---except during the numerous floods!  There are at least two “Salt Creeks” flowing into the Saline and French explorers noted the “briny” water as early as 1724.

So halite, a somewhat interesting mineral, is similar to many of the other evaporates--they seem rather dull as a collectable mineral. Halite comes in a variety of colors, mostly light in nature, with the tint commonly due to small amounts of impurities.  However, at a recent show my eyes about popped out when I discovered a dealer with several specimens of blue to purple halite, a really bright-colored halite! At first I thought perhaps this was simply a crystal constructed from a halite-saturated solution with food coloring added in.  But, I was assured the crystal was natural.  I had really never seen blue to purple halite before but thought it would look nice in my collection for a couple of dollars.  After returning home I begin a literature search to try and locate information about this halite and came across several specimens listed for sale in the 50 to 500 dollar range.  That aspect made me feel good about my frugal purchase!  

There are some rather famous collecting localities for colored halite in Poland; however, it appears that all blue to purple halite collected in this country comes from potash mines in the Delaware Basin of southeastern New Mexico: the upper Permian McNutt member of the Salado Formation Bickham, (2012).  As for origin of the blue to purple color, many/most geologists believe the coloration is due to gamma-ray bombardment of halite with the rays coming from radioactive potassium-40 found in associated minerals (like sylvite: KCl and isomorphous with halite).  K-40 is rare but does occur in some instances.  The gamma rays then disrupt the lattice structure of the halite and force the displaced electrons to reflect the blue to purple wavelengths from the visible light.  I am not enough of a mineralogist to vote yay or nay on this thought but it sounds good to me.  Bickham (2012) has some other ideas that seem worth exploring.  Whatever the reason for the bright color, these specimens make very nice displays and certainly generate many questions from visitors.

REFERENCES CITED

Bickham, M., 201, Chemical Analysis of Blue Halite [abs]: Geological Society of America Abstracts with Programs. V. 44, no. 1.

Wednesday, November 7, 2012

SPINEL: THE FORGOTTEN GEMSTONE



THE BLACK PRINCE'S RUBY DISPLAYED ON THE FRONT OF THE IMPERIAL STATE CROWN OF GREAT BRITAIN.  PHOTO PUBLIC DOMAIN FROM THE CROWN JEWELS OF ENGLAND.
I attended a recent show and one of the vendors had a very nice selection of faceted, gemmy spinels.  In fact, several displayed spectacular cuts.  I was further impressed by the specimens when I learned they were natural spinels rather than laboratory-produced crystals.  But, I really had to rattle the back part of my mind to even recall something about the mineral!  I actually had not thought much about spinel for years.
I don’t remember much about spinel from my undergraduate mineralogy class except seeing some very hard octahedral crystals rolling about in a white specimen box.  At first glance it was easy for me to get the crystals mixed up with magnetite octahedrons and I sort of wondered if there was a relationship.  But then, something in my brain said “wake up Mike, use a magnet” to weed out the spinel.  I never used the magnet since I don’t recalling needing to identify spinel on any test!  In fact, I don’t believe that I ever observed or identified the mineral in the field.

Later in life essentially the only thing I knew about spinel was that for years the royal British jewelers were a bit confused about gems in the crown jewels!  The Black Prince’s Ruby and the Timur Ruby (~350 carats), part of the crown jewels, are actually red spinels; however, before modern gemological tests were readily available most red–colored gems were termed rubies.  The Black Prince’s Ruby is ~170 carats in weight but is more of a cab or blob and is not faceted.  It prominently sits on the front of the Imperial State Crown, which in turn may be seen by visitors in the Tower of London Jewel House.  The Samarian Spinel is part of the Iranian Crown Jewels and may be the largest spinel gem in the world (~500 carets).  Legend has it this particular spinel adorned the biblical Golden Calf, constructed by the Israelites (for worship) while spiritual leader Moses was receiving the Ten Commandants.

A recent advertisement came across my computer screen extolling the thought that black spinels were a wonderful substitute for more expensive “black diamonds”.  That blurb intrigued me even more so I begin to dig through the literature, and also remembered that somewhere in the recesses of my collection was a specimen of spinel.  
 
Spinel, a magnesium aluminum oxide (MgAl2O4), comes in a variety of natural colors with nice stones being colorless (rare), red to pink (probably due to trace chromium), orange to purple (probably due to trace iron and chromium), black (probably due to trace iron), and blue (probably due to trace iron and cobalt).  A jeweler told me that very few faceted spinel gemstones are enhanced; occasionally, some are heated to improve the color. As noted above, red spinel is often confused with ruby, a red aluminum oxide (Al2O3) with trace chromium supplying the color in both.  In fact, they often occur together with ruby forming after the available magnesium is “used up”.
GEM RED SPINEL FROM THE GEM AND MINERAL COLLECTION OF THE SMITHSONIAN'S NATIONAL MUSEUM OF NATURAL HISTORY. PHOTO COURTESY OF DANE A. PENLAND.
 
Spinel is a hard mineral coming in at 7.5-8 on the Mohs’ scale and forms in the isometric system; most crystals are octahedrons.   It is translucent to transparent to even opaque and has a vitreous luster.  It does not cleave. As I understand it, most gem spinels are found in alluvial gravels with the finest red, pink and orange specimens coming from Myanmar (Burma).  Sri Lanka (Ceylon) is a major producer of blue and violet stones; however, it appears that new deposits have been located in Tanzania.

One of the unfortunate facts (or depending upon your point of view, fortunate) is that spinel is easily synthesized and colored and many people simply do not appreciate the beauty of natural spinel; they may not even realize that “natural” spinel exists!  In fact, some of the colors in synthetic stones do not appear in nature and virtually all clear crystals marketed as spinel are synthetic.  Synthetic spinel also is often used to replicate other gemstones such as diamond, ruby and sapphire---know your dealer! Have an old high school class ring with a big colored stone?  There is a good chance the colored stone is synthetic spinel.

Spinel is a member of the “Spinel Group” of minerals crystallizing in the isometric system and with a general formula of AE2O4.  A represents various metallic cations.  Minerals in this group include the Spinel or Aluminum Series where E is Aluminum, the Magnetite or Iron Series where E is iron (Fe), and the Chromite or Chromium  Series where E is chromium..  The Aluminum Series includes the gem spinels, but at times the magnesium of spinel (the A) may be replaced by zinc (the mineral gahnite), iron (the mineral hercynite), or manganese (the mineral galaxite).  The best known mineral of the Iron Series is magnetite (Fe3O4) while chromite (FeCr2O4) is a representative of the Chromium Series.
MAGNETITE OCTAHEDRONS FROM MAGNET COVE, ARKANSAS. WIDTH OF SIGLE CRYSTAL ON RIGHT ~1 CM.
Spinel is found in both igneous rocks (basalts, peridotites, kimberlites) and high temperature contact metamorphic rocks (especially hornfels and marbles).  Some of the most beautiful specimens that I have seen in museums are where a crystal of red spinel is perched on a metamorphic white marble.

The specimen in my collection (black and opaque probably due to iron) was purchased at a show and labeled “Huntington Lake, Fresno County, Calif.”  As best that I can tell this spinel occurs in “a deposit of pre-Cretaceous crystalline limestone changed to marble and containing several smaller bodies of calc-silicate hornfels… It is in the high Sierra Nevada, 3 miles north of the east end of Huntington Lake, at an elevation of 8500 to 8800 feet. As mapped, this limestone pendant is over 10,000 feet long and from 1250 and 3000 feet wide… the contact-metamorphic rocks formed where the Sierra Nevada batholith intruded a region deeply covered by older sediments, of which the original upper portions have been removed by erosion, including glaciation” (Logan, 1947).  Pink, green and lavender crystals also have been collected from this locality.
SMALL MASS OF BLACK SPINEL OCTAHEDRONS COLLECTED FRESNO COUNTY CALIFORNAI.  WIDTH OF SPECIMEN ~5.2 CM.  LARGE CRYSTAL UPPER RIGHT IS BELOW.
 
ONE HALF OF A SPINEL OCTAHEDRON FROM CALIFORNIA.
I certainly have not collected crystalline spinel (Spinel Series) anywhere in Colorado and Eckel (1997) stated that “spinel…has seldom been observed in Colorado”.  However, he also noted that Spinel Series minerals “are common accessory constituents of mafic igneous and metamorphic rocks… and are much more widespread than indicated by the literature.  Much of the spinel…occurs associated with Precambrian massive sulfide deposits”.
Gahnite, the zinc-dominant member of the Spinel Series, “is commonly found in Colorado in Precambrian metamorphosed, base-metal sulfide deposits and in a few pegmatites of granitic composition…It is moderately common in such deposits in the Front Park, and Sawatch Ranges”.  One of the local rock and mineral shops has several specimens of gahnite on display and for sale; however, the mineral is non-crystalline and is a granular (small) blackish (maybe dark, dark green), smear on country rock.  I certainly would not be able to identify it as gahnite without its label, or perhaps by using a microprobe stuck in my back pocket.

Several gem dealers refer to spinel as the “forgotten gemstone”.  It certainly was an import gem at one time---witness the crown jewels.  However, with the advent of synthetic spinel the stone seemed to decrease in value.  My suggestion is that readers ask their dealers for a peek at the real thing---you will be surprised at the gem’s beauty.

Rich and rare were the gems she wore,
And a bright gold ring on her hand she bore.
Thomas Moore
 
REFERENCES CITED
 
Eckel, E. B. (and others), 1997, Minerals of Colorado: Fulcrum Publishing, Golden, Colorado.
 
Logan, C. A., 1947, Limestone in California: California Journal of Mines and Geology, v. 43, no.3.