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.