MORE SERPENTINE!

I recently acquired a couple of nifty little specimens that continued to pique my interest in serpentine and hydromagnesite (see Blog posting June 14, 2013).  The first specimen was “collected” from a recent estate auction and originally was taken from the Hunting Hill Quarry near Rockville, Maryland, opened in the 1950’s as a site to obtain crushed stone for road building.  Although I have never visited the site my contacts in the area tell me the quarry is a popular collecting site for rockhounders in the local Washington, DC, area.  Parker (2005) noted that at least 60 mineral species have been documented from the quarry including such rare types as desaultelsite [Mg₆Mn₂(OH)₁₆[CO₃]-4H₂O],  pokrovskite [Mg₂(CO₃)(OH)₂], tochilinite [Fe_5-6(Mg,Fe)₅S₆(OH)₁₀],  mcguinessite [(Mg,Cu)₂(CO₃)(OH₂)], and coalaingite [Mg₁₀Fe₂(OH₂₄[CO3]-2H2_O)].  Of course, I am unfamiliar with all of these!

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The Hunting Hill Quarry lies within the Piedmont Physiographic Province of the Appalachian Mountain Chain.  The history of the Piedmont Uplands is extremely complex and I have a great deal of respect for the geologists, especially the early ones, who tramped through the woods and shrubs fitting the pieces together.  In summary, the area now termed the Maryland Piedmont was originally sedimentary rocks deposited in marine waters perhaps beginning ~1.6 Ga (Precambrian: Proterozoic).  These rocks were then deformed and altered (lots of gneiss, quartzite, marble, peridotite, and pyroxenite; Parker, 2005) during continental collisions termed the Grenville Orogeny (~1.2-1.0 Ga).  The result of these collisions was the formation of a supercontinent termed Rodinia.  We often thing of the late Paleozoic supercontinent called Pangaea but here was an earlier event creating a super land mass.  The plates of the Earth are in constant motion so Rodinia begin to break apart in the latest Precambrian (~1.0-.54 Ga) and eroding sediments accumulated along what was then the eastern coast of Laurentia (ancestral North America).   These sedimentary rocks were again altered during the Ordovician by collisional events termed the Taconic Orogeny.  In addition, collisional events in the Devonian (Acadian Orogeny) and late Paleozoic (Alleghenian Orogeny) left their imprint on the rocks and contributed to the amazingly complex geology of the region. 

 

Serpentine with small amounts of talc? (T), hydromagnesite HM), aragonite (A), and magnesite (M).  Specimen width~5.5 cm.

The Hunting Hill Quarry is located in the Hunting Hill pluton of Ordovician age (igneous event associated with the Taconic Orogeny--intruded into pre-existing sediments/sedimentary rocks, the ones associated with the erosion of Rodinia).  Today the rock is mostly a serpentinite (various serpentine group minerals)---see Blog posting June 14, 2013) that originally was a dunite (plutonic rock more than 90% olivine) and which oxidation (termed serpentinization---common along plate boundaries) converted to minerals of the serpentine group.  As noted in a previous blog posting, it is very difficult, at least for me, to differentiate various minerals of the group.

OK, wow. 

The specimen in my collection has the massive dark green serpentine associated with lighter green talc (I think) [Mg₃Si₄O₁₀(OH)₂], coated with a white hydromagnesite [(CO₃)₄(OH)₂-4H₂O] with an aragonite [CaCO₃] spray heavily covered with magnesite [MgCO₃] (rather than calcite I believe).  There may also be a few other strange minerals that I lack the skills to identify. 

 

Specimen of green serpentine with molybdenite and dirty marble.  Width of specimen ~6 cm.

The second specimen of serpentine was found while I was rummaging around the cases at Ackley’s Rock Shop here in Colorado Springs.  All of a sudden the rock in question sort of attracted me since I have been looking for other examples of serpentine, and in addition, the specimen contained crystals of molybdenite.  This specimen has been a tough one to track down much information about the geology.  I know that it came from the old Royal Green Marble Quarry, near Phillipsburg, Warren County, New Jersey.  Warren is adjacent to Sussex County to the north where the famous collecting localities of Ogdensburg and Franklin are located.  It appears they might share some of the same rocks.  As best that I can determine the quarry is in the New Jersey Highlands, a physiographic province trending northeast-southwest in western New Jersey containing Precambrian igneous and metamorphic rocks—mostly granite, gneiss, and some marble.  They are the oldest rocks in New Jersey, perhaps late Precambrian  in age (Proterozoic:  ~1.3--.8 Ga).  MinDat noted the quarry was established in 1880 to produce marble and dimension stone and operated until 1941.  Mineralization is the Precambrian marble. Correlation with the Franklin Marble cannot be made with precision. However, as currently interpreted all the marble along the northwestern margin of the Reading Prong highlands represents a shelf sequence along the margin of a back arc basin. Therefore, the marble in the Easton quadrangle is, at least, broadly correlative with the Franklin Marble.  The USGS geological quadrangle GQ 594 maps the area as Precambrian “dolomite, and lesser calcite marble, largely altered to serpentine, tremolite, and talc” (Drake, 1967).

Crystal of molybdenite enclosed in serpentine.  Width of photomicrograph ~1.2 cm.

 

Photomicrograph of platy and micaceous serpentine group mineral (??lizardite).  Width of photo ~1.2 cm.

The specimen I purchased for a couple of dollars has massive serpentine (??lizardite) that is light green in color, micaceous and platy, with scattered crystals of molybdenite.  A stringer of dirty white marble is attached.

So, in my curious mind serpentine is becoming really quite interesting.  I only wish that my knowledge of metamorphic petrology was sufficient to better understand the situation.

ADDENDUM:  I recently picked up a couple of more specimens from the Royal Green Quarry.  They include "serpentine" of a different sort, much less micaceous than the specimen above.  In addition the specimen contains fibrous ?tremolite.

Fibrous tremolite? [{Ca₂}{Mg₅}(Si₈O₂₂)(OH)₂], an amphibole, with pyrite and molybdenite, Royal Green Quarry, Warren County, New Jersey.  

 

REFERENCES CITED

Parker, F. J., 2005, The Minerals of the Hunting Hill Quarry, Rockville, Maryland: The Mineralogical Record,

Drake, A.A. Jr., 1967, The Geologic Map of The Easton Quadrangle, New Jersey-Pennsylvania: U. S. Geological Survey Quadrangle map GQ-594, scale 1:24,000.

PECOS DIAMONDS: AUTHIGENIC QUARTZ

The rockhounder’s world of terminology is often filled with rather misleading names, for example the notation “diamonds”.   I suppose that just about everyone with at least the slightest bit of knowledge concerning minerals knows about the “scarcity”, cost, hardness, mining, etc. of these precious gems.  What I find most interesting about the gems is how a tightly controlled cartel, led by the DeBeers Company, managed a majority of the diamond market (for decades) and essentially established price (mostly inflated) and supply---at the demand and conditions of the market.  However, in the 1990’s their influence started to wane as mines in Australia and Russia started to sell diamonds on the open market and new gem mines opened in Canada.  Today the diamond market is quite competitive and prices are volatile; however, aggressive marketing techniques by retailers have driven prices even higher than the somewhat steady cost when the market was controlled by De Beers in the 1980’s (and before).

But there are other types of diamonds, besides the gem variety,  floating around the mineral auctions and markets!  As a kid growing up in Kansas we hunted the sand and gravel “pits” looking for Kansas Diamonds.  What we actually collected were pieces of pebble-size clear quartz that were rounded due to their transportation in the late Cenozoic from parent localities in the Rocky Mountains.  I suppose many of these pebble were brought east as part of the Ogallala Group sediments and then re-transported by modern streams dissecting Ogallala outcrops.  The pebbles were also quite frosted due to banging around during transportation and needed to be whacked by a rock hammer, or cut by a saw, to see their internal beauty.  I have seen many very nice stones faceted from these gemmy clear Kansas Diamonds.

As a bit of a sidebar, I always had my students examine the “gravel” very closely and identify the major constituents.  They certainly noticed that quartz was present in large amounts and individual grains (some of pebble and cobble size) were quite rounded.  However, the feldspar particles were mostly cleavage fragments, somewhat angular, and rather small in size.  In addition, the feldspar—quartz ratio was just about the opposite of what one would expect from the feldspar---quartz ratio in the parent rock (mostly granite or metamorphic rocks).  Of the ferromagnesium minerals generally found in granite, biotite was very rare and hornblende uncommon in the gravel pit sediments.  A nice little lesson on weathering and cleavage!

Later on in life I was sort of fascinated by Herkimer Diamonds, the doubly-terminated, crystal-clear quartz from New York.  Now, I could easily see how these beautiful crystals received their name.  Although I have a couple of specimens in my collection, I picked up another nice crystal at the recent RMFMS show in Sandy, Utah.

A beautiful clear, gemmy Herkimer Diamond.  Length ~1.45 cm.

Another addition from the show was a specimen that I had been waiting to purchase—a nice euhedral, doubly-terminated, quartz crystal termed a Pecos Diamond.  Wow, another diamond!  And, this got me to thinking about the geological environments in which quartz forms.

There are a gazillion web pages espousing information about quartz but one of the best is a site called The Quartz page (www.quartzpage.de), a “work in progress” constructed by A. C. Akhaven (in Germany, hence the .de on the address).  This tome is well-illustrated, well-written, and chock full of information about all things quartz (macrocrystalline, microcrystalline, you name it).  And, I love the disclaimer—I am not an expert, so information given on this page could be wrong.  And if I were an expert, I could still be wrong.

Akhaven describes the occurrences of quartz as follows; 1) Vein Quartz—where hot, silica-rich waters deposit quartz in cracks or fissures of pre-existing rocks.  Much vein quartz is milky or bull quartz and good crystals are not common; 2) Gangue Quartz—quartz precipitating along and with hydrothermal ore veins; 3) Quartz Veins and Pockets in Carbonate Rocks—formation of quartz in sedimentary rocks (especially limestones and dolomites) related to low-temperature hydrothermal environments.  The hydrothermal waters percolated through the carbonates mostly depositing calcite crystals but at times quartz, usually either druzy or isolated stubby crystals.  In some instances the crystals grew in isolated gas cavities and crystals may be bright and shiny---Herkimer Diamonds; 4) Authigenic Quartz—often produces well-developed crystals that grow within a solid rock.  In soft sediments, many bazaar shapes of minerals form---for example, the barite and gypsum roses.  In more indurated rocks, crystals with well-formed faces often form, such as pyrite cubes in limestone.  More on authigenic quartz later; 5) Concretions in Sedimentary Rocks—in early diagenesis of sedimentary rock, minerals precipitate from solutions in the pore spaces of sediments.  The common minerals are quartz and calcite but also include barite, pyrite and a host of others; 6) Pegmatites—these rocks form from hot fluids and often form gas pockets that might contain well-formed quartz crystals (just ask some of our CSMS crystal hunters); 7) Miarole Pockets---quartz forming in gas pockets during the solidification of igneous rocks (similar to pegmatites); 8) Geodes and Cavities in Volcanic Rocks—quartz (often chalcedony and agate), and a number of other minerals, are common constituents in geodes.  These include such items as thundereggs and various agates; and 9) Skarns---quartz may form as igneous magma intrudes carbonate rocks. 

So, quartz forms in a great variety of different geological environments.  It is also quite resistant to both physical and chemical weathering and therefore is a very common mineral in the earth’s crust (second only to the feldspars).

A Pecos Diamond.  Length ~ 2.9 cm.

A Pecos Diamond, the specimen that got me started on this journey, is authigenic quartz.  That is, the crystal was generated “in place” in the Seven Rivers Formation of Permian age that now crops out along the Pecos River in southeastern New Mexico.  The Pecos continues into Texas; however, as best that I can determine most of the better “diamond” specimens come from New Mexico.  But, and this is a big but based mostly on personal observations, many “diamonds” sold at various venues have a locality listed as “Texas”.  Ask the dealers about specific locations and the answer is Texas—somewhere, or maybe New Mexico.  I suppose the Pecos is more romantic in Texas due to the infamous Judge Roy Bean, the only “law west of the Pecos”.  The Judge became “popular” with the 1972 release of “The Life and Times of Judge Roy Bean” starring Paul Newman (an earlier version starred Walter Brennan).

Movie poster advertising The Life and Times of Judge Roy Bean.  Photo public domain.

The Pecos Diamonds are euhedral, doubly terminated, quite colorful, quartz crystals that formed in evaporitic salt pans situated in a larger sabkha environment—now turned to dolomite rock (Albright and Lueth, 2003).  At some localities along the Pecos large dolomite crystals also occur with the quartz---and some rockhounds also hang the Pecos Diamonds moniker on these crystals. 

Albright and Lueth (2003) have produced a wonderful description of the Pecos Diamonds, including collecting localities:  size of the crystals range from microscopic to perhaps 6.5 cm along the C-axis although individuals larger than about 2.5 cm often are distorted.  The diamonds occur in a wide range of colors reflecting the colors of the enclosing gypsum matrix.  Clear and transparent crystals, when found, are very small, usually no longer than ~4 mm.  Most are opaque to translucent.  The Pecos Diamonds seem unique in that there is a great variety in crystal form compared to other occurrences of authigenic quartz.  Most are prisms terminated on both ends by hexagonal pyramids.  However, there are a number of other forms including some that are equant pseudocubic.


A small Pecos Diamond ~1 cm. long.

Sabkhas are evaporitic pans of saline water formed adjacent to arid coast lines but above the tidal zone.  They are very complex environments and are somewhat rare in the modern world---the Persian Gulf area being the poster child.  Halite is commonly precipitated on the surface and gypsum and aragonite form by capillary action in the subsurface.  Dolomitization, a diagenetic (secondary) process, often then turns the aragonite into the rock dolomite.  Long story but dolomite, common in the rock record today, almost always is secondary and rarely formed in a primary environment.  Geologists really do not understand the entire dolomite story and perhaps ancient dolomites formed in several different environments (such things as high temperature vs. low temperature; the role of bacteria; etc.).  

A couple of final comments: 1) I have used the term authigenic in a fairly strict sense.  There are numerous other uses employed by sedimetologists, metamorphic petrologists, petroleum geologists, etc.  I am referring to nice euhedral quartz crystals forming in solid sedimentary rocks; and 2) a long time ago I was wandering around in Mesozoic sedimentary rocks north of Fort Collins and remember picking up nice authigenic quartz crystals.  Currently not a single person knows what I am talking about.  Who knows, my memory could be shot!

Pecos Bill was quite a cowboy down in Texas
The Western Superman to say the least
He was the roughest, toughest critter
Never known to be a quitter
'Cause he never had no fear of man, nor beast

          As presented by Riders in the Sky.

REFERENCES CITED

Albright, J.L., and Lueth, V.W., 2003, Pecos Diamonds---Quartz and Dolomite Crystals from the Seven Rivers Formation Outcrops of Southeastern New Mexico: New Mexico Geology, v. 25.

UVITE: AN UNCOMMON TOURMALINE

Green crystals of uvite on quartz with scattered crystals of clear magnesite.  Note tiny magnesite crystals perched on large uvite crystal.  Length of specimen ~4 cm.

Readers of this little blog know that I am always on the lookout for nice specimens, especially those displaying rather unfamiliar minerals.  That is, since I am not a mineralogist I usually know almost nothing about “lots of” minerals; however, I am always up to a challenge of “trying to find out” additional information!  So, at the recent RMFMS show in Sandy, Utah, I was intrigued by a specimen labeled “Uvite Tourmaline”.  I am somewhat familiar with many Tourmaline Group members such as elbaite, indicolite, rubellite, schorl, and dravite, but uvite?  What is the world was that?  So, I pulled my handy-dandy mineral guide out of my backpack---but no uvite listed.  That little bit of information prodded me to purchase this specimen, and besides, it was a beautiful green crystal perched on some quartz and what appeared to be magnesite.  It was a reasonable price, three dollars, so it was wrapped and on the way to Colorado Springs.

I was like a kid in a candy store (or a rockhound in a mineral store) upon returning home and checking out all my “hot buys”.  There it was, the uvite!  Immediately I checked out my other mineral books and about the only reference located was in Roberts, Rapp, and Weber (1974) who noted that uvite was a hypothetical end member of tourmaline.  Now, what does that mean?  So, on to the internet with a short stop in Eckels and others (1997) where they stated that “black-brown crystals of tourmaline from Italian Mountain [Gunnison County] were identified as uvite…by XRD [x-ray diffraction]”.

What I found out, after extensive reading, was that uvite is a valid mineral name, sort of, that was redefined by the International Mineralogical Association in 2011 as being two different mineral species.  There is a uvite series (two members) with one end member being a hydroxyl-dominant mineral and the other end member being a fluorine-dominant uvite:

The general chemical formula for the series is usually written as: [Ca(Mg₃)MgAl₅(Si₆O₁₈)BO₃)₃(OH)₃(F/OH)]. Note the fluorine or the hydroxyl at the end of the formula.  The hydroxyl-dominant form is referred to as uvite (to avoid confusion with the original use of the name) while the latter is called fluor-uvite. 

It seems like all/most of the Tourmaline Group minerals have fluorine-dominant analogues, for example fluor-schorl (fluorine-dominant) to go with schorl (hydroxyl-dominant).  MinDat (www.mindat.org)  furthermore goes on to note that an analysis really needs to be completed before many specimens of uvite (sensu lato) can be assigned to one of the end members.  Since I don’t carry an XRD in my back pocket, I could be in trouble when it comes to correct identification!

Uvite is often green in color, as in my specimen, but at times it may be black, brown or even colorless or perhaps white.  I suppose, but am guessing here, that the color range may be due to the magnesium?  The crystals seem to be short and stubby compared to other tourmaline species, for example schorl and elbaite.  In fact, they almost look flattened (along the C-axis) or tabular.  Good crystals are complexly terminated and my specimen is transparent, vitreous, and gemmy although others may be translucent. Tourmaline Group minerals are harder than quartz coming in at about 7.5 (Mohs).

The specimen that I acquired came from Brumado (Bom Jesusdos Meiras), Bahia, Brazil.  I don’t know much about the local geology; however, magnesite (one of the largest deposits in the world) and talc are mined in the region.  Cassedanne and Cassedane (1978) published an article on the Brunado District in the Mineralogical Record; however, I have been unable to locate this vintage article.  I do believe the uvite is associated with the magnesite deposits that in turn are located in Precambrian rocks partially subjected to replacement by hydrothermal solutions.  Henry and others (2011) made the Brumado District the type locality for uvite (sensu stricto); however, MinDat notes “it is not possible to assign 'uvites' from Brumado to the correct species without a reliable analysis”.  

In summary, I listen to music while writing, mostly the “oldies station,” and often remember the words of the world’s greatest rock and roll band, the Rolling Stones: You can't always get what you want, but if you try sometimes, you might find, you get what you need.  So, I “got what I need”, a nice crystal of uvite.

REFERENCES CITED

Cassedanne, J.P. & Cassedanne, J.O., 1978,  Famous Mineral Localities: The Brumado District, Bahia , Brazil: Mineralogical Record v. 9, p. 196-205.

Eckel, E. B., 1997, Minerals of Colorado: Fulcrum Publishing, Golden, Colorado.

Henry, D., Novák, M., Hawthorne, F.C., Ertl, A., Dutrow, B.L., Uher, P. & Pezzotta, F., 2011, Nomenclature of the Tourmaline Supergroup Minerals: American Mineralogists, v. 96, p. 895-913.