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??