AEGIRINE: INTERESTING MINERAL

AEGIRINE AND FELDSPAR.  WIDTH ~4 CM.

 At a small rock and mineral show a few days ago I saw a specimen that sort of intrigued me since the presence of numerous large crystals sort of reminded me of epidote, or something close.  However, the label stated “aegrine” that I took to mean aegerine or aegirine.  It was an older label (Albuquerque Gem and Mineral Club) and the seller informed me that it was purchased as part of a previous collection.  At any rate, it was not very expensive and I was interested.

After returning home I rattled my mind to recall something about aegirine (preferred spelling of MinDat.org) but that was not too successful except to remember that it may be a pyroxene.  After that knowledge, “things” got complicated for a non-mineralogist!  

I know that the pyroxenes have a general formula of (Na,Ca)(Mg,Fe,Al,Ti),(Si,Al)₂O₆, or something close.  In the clinopyroxene subgroup diopside is the calcium/magnesium-rich end member and at the other end of the solid solution series is the calcium/iron-rich member, hedenbergite.  Somewhere in the middle is the very common rock forming mineral, augite.  In addition, jadeite is a sodium/aluminum end member.  What I needed to better understand was the relationship of aegirine to augite and other pyroxenes.  So, I decided to contact  Pete, my “go-to answer man” up at the USGS.  What I found out is that in aegirine (NaFe(+3)SiO₂O₆) there is a coupled substitution where Na replaces the calcite and Fe(+3) replaces the magnesium.  And, the possibility exists that aegirine may be in solid solution transition with any of the other  members with the most common transition being aegirine-augite (and that is what the hybrid is called).  Those tidbits are about all that I pretend to understand, or want to understand  when Pete tells me the mineralogy is somewhat more complicated!

AEGIRINE CRYSTALS.  WIDTH PHOTO ~3.5 CM.

At any rate, I have a nice little specimen with prismatic crystals (Monoclinic) of vitreous aegirine, black in color (may be sort of a greenish-black), and a hardness of 6 (Mohs).  Some of the crystals are terminated and they come in all sizes.  They are associated with large feldspar crystals, orthoclase, and very tiny crystals of zircon and ?quartz.  The specimen comes from Mount Malosa, Zomba District, Malawi, where outcrops of “alkali pegmatites [contain] unusual REE minerals [and are] famous for excellent and large aegirine crystals, feldspars, arfvedsonites and rare Be minerals” (MinDat.org).

I love to learn and specimens like this, along with a “go-to answer man”, make life quite enjoyable.

PHOTOMICROGRAPH OF ZIRCON CRYSTAL ~54 MM.

PHOTOMICROGRAPH OF ZIRCON.  TOP CRYSTAL ~.51 MM.

PHOTOMICROGRAPH OF CRYSTAL ~1.2 MM.  INITIALLY I THOUGHT INCLUDED QUARTZ BUT NOW UNCERTAIN.

GILSONITE, A COLLECTABLE HYDROCARBON

GILSONITE VEIN NEAR BONANZA, UTAH 

One of the major subdivisions of the northern Colorado Plateaus is the Uinta Basin, a large basin associated with the Sevier/Laramide (Cretaceous & early Tertiary) Orogeny and located in eastern Utah south of the Uinta Mountains.  The Basin is a structural basin, as opposed to a topographic basin, meaning that it is a very large syncline, and a compliment to the anticlinal Uinta Mountains.  The structure is related to the Piceance Basin of northwestern Colorado, and the Fossil and Green River Basins north of the Mountains in Wyoming.  The rocks in the Basin are an interesting sequence of latest Paleocene (~58 my) to early Oligocene (~28my) basin-fill sediments consisting of, in ascending order, the Colton Formation (stream and flood plain deposits), the Green River Formation (Lake Uinta, part of a large fresh water lake system), the Uinta Formation (lake edge and lake filling sediments), and the Duchesne River Formation (stream and flood plain sediments on top of the lake sediments.

The Basin is well-known for its production of oil and gas, as well as for interesting examples of solid hydrocarbons--kerogen-rich mudstone (oil shale) and bitumen-impregnated sandstone (tar sands).  But perhaps the most interesting hydrocarbon in the Basin is one that a few years ago (pre-1995) would not be classified as a ”mineral” and that is Uintaite, referred to in this article as the trade name Gilsonite.  In 1995 the International Mineralogical Association adopted a new definition of a mineral as “an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes” (Nickel, 1995).  This definition opened the way for an organic class of substances that included hydrocarbons of which Gilsonite is a member.  Gilsonite is a solid hydrocarbon that comes from the solidification of petroleum.  It is usually a dull black in the field and resembles coal; however, some fresh surfaces are quite shiny with a conchoidal fracture and superficially looks a little like obsidian.

"FRESH" GILSONITE.  PHOTO AUTHOR UNKNOWN.

Gilsonite in the Uinta Basin occurs in long veins (measured in miles) from a few inches to several feet thick and hundreds of feet in a vertical direction.  Most thick veins occur in the Green River and Uinta Formations, both units are Eocene in age.  In fact, the veins seem “rooted” in the oil shales of the Green River Formation.  Tripp (2004) believes the Gilsonite had its beginning in the large amounts of organic debris that accumulated in the sediments of tropical Lake Uinta.  The burial of these sediments created heat and pressure and the Green River oil shales were formed.  Again, burial of the oil shales created water and hydrogen and this explosive mixture was expelled and created fractures in the surrounding rock.  These fractures were later filled with petroleum whose viscosity disappeared with desiccation.  Gilsonite is essentially solid “oil”.

ABANDONED GILSONITE PROCESSING PLANT.

ABANDONED ELEVATOR IN GILSONITE VEIN

Gilsonite was “discovered” by Caucasian settlers in the 1860’s although most did not know of a good use for the substance as it melted and ran from the stoves when used as a substitute for coal.  In the 1880’s a Mr. Samuel Gilson begin to market (as St. Louis Gilsonite Company) “asphaltum” as a waterproof substance and as an electrical insulator.  However, two major problems arose: 1) many of the thick veins of Gilsonite were on the Uintah-Ouray Indian Reservations; and 2) a railroad did not extend into the Uinta Basin and Vernal (and even today the Basin is without rail transportation).  Gilsonite and Uinta Basin boosters took care of the first problem as the U. S. Congress simply took away 7000 acres from the Native Americans (Burton, 1996)!  By around 1900 the Gilson Asphaltum Company (now called the American Gilsonite Company) was the major player in Gilsonite and had consolidated several claims; however, “trucking” (used freely since horsing may be a better term as horse-drawn wagons were used) the Gilsonite 80-100 miles to a Utah rail head was not a very viable option.  So, as with so many projects, “necessity is the mother of invention” (att. To T. Veblen).  The necessity was transportation and the “invention” was the Uintah Railway--started in 1903 and completed in 1911.  The Uinta Basin terminal was at Rainbow near the mines while the southern terminal was at Mack, 22 miles west of Grand Junction, CO, on the Rio Grande Western Railway.  In between those two points was a three foot wide track (narrow gauge) traversing Baxter Pass (8473 feet) and approximately 63 miles of crookedness, curves and grade. The first 28 miles, coming from the south, had 36 bridges while at some of the steepest grades the brakeman could walk faster than the train.  On a few curves the engineer could shake hands with people in the caboose, or so it was said (The Uintah Railway, no date).

The Uintah also hauled passengers, freight, mail and livestock but was abandoned in 1939.  Perhaps its best known cargo---the Dinosaur bones Earl Douglas quarried from what is now Dinosaur National Monument but were then shipped to the Carnegie Museum in Pittsburg.  Today a few grades remain and I have done some hiking along the railway route.  In the 1970’s spikes associated with the tracks were common; today, rare. 

TIMBERS SHORING UP OLD GILSONITE VEIN

After abandonment of the railway, Gilsonite was shipped by truck from a new processing plant near Bonanza, UT.  In the 1950’s a slurry pipeline was built from Bonanza to near Mack, CO, and gasoline was distilled from the mineral until the 1970’s.  Today, American Gilsonite Company continues to mine away in the Basin and sells its product for use in “160 products, primarily in dark-colored printing inks and paints, oil well drilling muds and cements, asphalt modifiers, foundry sands additives and a wide variety of chemical products” (American Gilsonite Company at www.americangilsonite.com).  If visitors travel to the Basin on U. S. Highway 40 it is a very worthwhile trip to make the detour to Bonanza and the abandoned mines.  The deposit is unique and the mines and abandoned facilities are “ghost-like”. 

And, for a special treat, check out the video of the railroad at:
http://www.youtube.com/watch?v=x7IkujWEpm8

REFERENCES CITED

Burton, D. K. 1996, A History of Uintah County: Scratching the Surface in Utah Centennial County History Series: Salt Lake City: Utah State Historical Society, Salt Lake City. 

Nickel, E. H., 1995, The Definition of a Mineral: The Canadian Mineralogist, v. 33, no. 3.

 

The Uintah Railway, 2009: http://home.bresnan.net/~bpratt15/

Tripp, B. T., 2004, Gilsonite, an Unusual Utah Resource: Utah Geological Survey Notes, v. 36, no. 3.

VALENCIANITE: ADULARIA (K-FELDSPAR)

ADULARIA VARIETY VALENCIANITE,   LENGTH 7.5 CM.

The other day I had an opportunity to attend a small rock and mineral show.  Actually it was not really a show, as I had assumed from the advertisement, but simply about 15 dealers displaying wares on a number of tables.  Whatever, I am always game for looking at new specimens.

One particular dealer caught my eye since it appeared that he had purchased, at various times, the stock of several different dealers or individuals.  Many of the tags appeared quite old, and some were handwritten in ink---great place to see something interesting!  I was not disappointed and purchased a couple of specimens.

One specimen that caught my eye was labeled Calcite, Valencianite, Valenciana Mine, Guanajuato State, MEXICO. Now, I didn’t have the slightest idea what valencianite was but presumed the mineral had something to do with the small yellow-gold “spots” on the specimen.  Meanwhile, I could see the massive calcite on the “bottom” of the specimen but was uncertain about the major mass of crystals making up the specimen.  It actually looked like feldspar.  At any rate, my interest was piqued and a purchased price was negotiated.  It always seems nice to have a mineral from the “type locality”.  See Zunyite posting on Feb. 21, 2013.

Upon returning to my references I was amazed to learn that valencianite is actually a variety of adularia, which in turn is a type of orthoclase (potassium or K-feldspar) which belongs to the large group known as feldspars.  Wow, this was going to get interesting.  About the only type of adularia that I knew about was the transparent and somewhat opalescent moonstone that is popular ---or so I thought.  MinDat.org states that although some moonstone may be a K-Feldspar the real moonstone (my term) is a variety of microcline.  Now, Gemdat.org notes that moonstone is composed of thin layers of orthoclase (K-feldspar) and albite plagioclase feldspar).  

  

It is no wonder that I get confused, and that our hobby is so interesting and enticing!  Now, as I understand the K-feldspar mineralogy, and that sometimes is a stretch, microcline (triclinic) is a low temperature version (polymorph) of potassium feldspar mostly found in pegmatites while sanidine (triclinic) is a high temperature version common in volcanic rocks, especially things like obsidian and rhyolite.  Adularia (monoclinic) is also a low temperature polymorph from volcanics, especially in hydrothermally altered rocks. But, MinDat.org also states adularia may be partially disordered microcline (see moonstone above)!  Orthoclase is also a low temperature polymorph common in granites and pegmatites but is in the monoclinic crystal system.  If this isn’t confusing enough there is always the many solid solution series in the group; the feldspars are complex and volumes have been written.

So anyway, back to adularia.  It seems as though the variety valencianite was first described at the Valencianite Mine in Mexico and seems to be the major location for collecting the varietal mineral.  MinDat.org lists a location in Calaveras County, California, from seemingly old references, and another from Owyhee County, Idaho.  Neither of these latter localities provided photographs.  The specimens from Mexico, both in MinDat.org photos and from my acquired piece, are not transparent like the gem adularia but do have that opalescent sheen usually termed adularescence.

OK, so now I have a nice specimen of a varietal adularia called-valencianite (KAlSi₃O₈) but what about the little yellow-gold spots?  That is a tough question to answer.  I am sort of convinced, at least halfway, they may be a fairly rare mineral called milarite (K₂Ca₄Al₂Be₄Si₂₄O₆₀0-H₂O).  I say that only because my back pocket microprobe is not working, and that tiny yellow-gold crystals appear in MinDat.org photos from the mine.  There are other of the spots that appear composed of tiny translucent clusters of cubes but they could be some sort of twined crystals.  To top it off, several of these tiny spots have “very tiny” cubes present with a metallic luster.  I would like to think these might be acanthite (Ag2S) since this is a silver and gold mine.

I am sometimes amazed, but always delighted, to locate a nice specimen at a good price that leads me to more questions than answers.   These sort of dilemmas keep me intellectually alive and like Guy Noir I am one man…trying to find the answers to life's persistent questions.

COULD THESE METALLIC CRYSTALS BE ACANTHITE?  WIDTH OF PHOTOMICROGRAPH 2.40 MM.  ARE THE CLEAR CRYSTALS FORMING THE "BALL" CALCITE?  DOES "IRON" STAIN SOME AREAS?    

     PERHAPS MILARITE IN ABOVE THREE PHOTOS??