ZUNYITE: A COLORADO TYPE MINERAL

PHOTOMICROGRAPH OF SPECIMEN FROM ZUNI MINE SHOWING TINY CRYSTAL FACES OF ZUNYITE.  EACH SIDE OF THE EQUILATERAL TRIANGLE IS ONLY ~2.5 MM.  MATRIX INCLUDES QUARTZ AND KAOLINITE WITH VERY TINY PYRITE CRYSTALS.   

I love the world of minerals, and as John Lennon sang Let me count the ways… beautiful, colorful, and exciting to collect (and even purchase).  However, what I really like about minerals is the element of surprise.  I suppose the “average” layperson might be able to name 10 different minerals, maybe the “average” geologist a hundred on any given day.  But to me almost every week brings out a serendipitous moment when a mineral name that is totally unfamiliar pops out from somewhere!

Recently I was down visiting my favorite mineral store in Colorado Springs, Ackleys on Stone Street (is that a good location for a rock store?).  I noted a small sample in a box labeled “Zunyite from Zuni Mine, San Juan Mountains, Colorado”.  That was new one for me so I shelled out the two bucks and went home to explore the references—actually I thought it might be mislabeled or misidentified or a substitute name.  But then I found a reference in Minerals of Colorado (Eckel and others, 1997) explaining that the Zuni Mine near Silverton is the type locality for the mineral and that it is relatively rare in the record.  OK, so back to the store to check on other specimens and I found one additional one to take home.

TOTAL SPECIMEN FROM ZUNI MINE SHOWING LOCATION OF ZUNYITE CRYSTALS, 2.5 MM.

The type locality is located northwest of Silverton maybe three miles at an altitude approaching 12,000 feet on Anvil Mountain.  The Zuni was originally a silver mine and zunyite was first described clear back in 1884 (Hillebrand).  How he determined the chemical composition of a chlorinated silicate, I don’t have the slightest idea!  Some (many) of those early workers were just pretty darn “smart”.  Guitermanite (Pb₁₀As₆S₁₉(?) was also described, with zunyite, from the mine; however, Eckel and others (1997) noted this mineral from the mine is actually jordanite (Pb₁₄(As,Sb)₆S₂₃).

Zunyite is an aluminum sorosilicate:  Al₁₃Si₅O₂₀(OH,F)₁₈Cl and is generally found in hydrothermally altered rocks (Eckel, 1997), especially volcanic rocks.  The mine is located the San Juan Mountains, an area known to most geologists as a volcanic terrane since there is a tremendous amount of evidence pointing to numerous volcanic eruptions in the Tertiary (last 66 million years or so). The San Juans are also home to perhaps 60 volcanic calderas, usually circular or oblong collapse features indicating ancient volcanoes that “blew their stack” (see blog posting Calcite from La Garita; Oct. 4, 20120).  Associated with the volcanics are igneous intrusions, including the Oligocene Sultan Mountain Stock along the edge of the Silverton Caldera (Hon and Lipman, 1989).  Molenaar and others (1968) believed the altering hydrothermal solutions came up along the faults associated with the intrusion. 

The crystals of zunyite are generally quite small, less than one cm., usually tetrahedrons and in cross section appear as tiny equilateral triangles, sort of a grayish-white to “transparent” color ( I know that is not a real color), have a vitreous luster and are about the hardness of quartz (~7 Mohs).  Evidently a few localities (and there are not many localities producing specimens of the mineral) sometimes produce reddish crystals.  www.MinDat.org has some wonderful photos with a good depth of field achieved with CombineZM.  Hlava and others (1994) have produced a number of very nice drawings of zunyite crystals, and have a list (quite extensive) of associated minerals at the Zuni Mine.

So, as a collector of mundane facts and minerals, I was quite happy to acquire these specimens.

ADDENDUM 15 APRIL 13:  I recently obtained a small specimen of pyrophyllite [Al₂(Si₄O₁₀)(OH)₂] that contained tiny crystals of light brown zunyite crystals.  The specimen was collected at the Big Bertha Mine, Middle Camp-Oro Fino District, Dome Rock Mountains, La Paz County, Arizona.

Photomicrograph of Arizona zunyite.  Length largest crystal ~1.8 mm, far right crystal ~1 mm.

 

REFERENCES CITED

Eckel, E. B. and others, 1997, Minerals of Coorado

Hlava, P. F., A. G. Hampson, and W. P. Moats, 1994, Zunyite and other minerals of the Zuni Mine, San Juan County, Colorado: Proceedings of the 5^th Annual New Mexico Mineral Symposium.

Hillebrand, W. F., 1884, On zunyite and guitermanite, two new minerals from Colorado: Colorado Scientific Society Proceedings, v. 1.

Hon, K., and P. W. Lipman, 1989, Western San Juan caldera complex: in Chapin, C. E., and J. Zidek, (eds.), Field excursions to volcanic terranes in the western United States, Vol. I: Southern Rocky Mountain region: New Mexico Bureau of Mines and Mineral Resources, Memoir 46.

Molenaar, C. M., D. L. Baars, J. Mayor, and V. C. Kelly, 1968, Road log from Ouray, Colorado to Farmington, New Mexico via Silverton, Eureka, Durango, and Aztec: San Juan-San Miguel-La Plata region: New Mexico Geological Society, Guidebook to 19th Field Conference.