DUFTITE: A NICE GREEN ARSENATE

Got to love those Arizona sunsets!  West face of Superstition Mountains near Apache Junction.

I can always rely on my friend, Mr. Rockhounding the Rockies, to supply me with some rather odd and sometimes unique crystals. Those who read his Blog (www.rockhoundingkw.blogspot.com) know this prospector is interested in “big” crystals of amazonite and smoky quartz.  He might stoop to collect some brightly colored fluorite or some nifty goethite but his heart and soul are dedicated to those “really” big pegmatite crystals near Lake George, Colorado.  Someday I hope to transport him to the Black Hills of South Dakota to check out spodumene and feldspar crystals that measure in tens of feet!  In the meantime, I continue to accept what scrapes he throws my way---I just posted a new photo of “zinnwaldite” collected near Lake George (Blog 11/12/13).  Mr. Rockhounding has never collected in Mexico but recently he tossed me some microcrystals of really curious arsenates, including duftite, that I believe came from the Ojuela Mine, Mapimi, Durango, Mexico.  I eagerly accepted since I am a sucker for bright-colored minerals. Got to love those Arizona sunsets!  West face of Superstition Mountains near Apache Junction.

Dufitite [PbCu(AsO₄)(OH)] is a lead copper hydroxyl arsenate found as a secondary mineral in the oxidized zone of sulfide ore deposits.  The arsenates contain a suite of beautiful and often confusing (to identify) minerals.  Adamite is perhaps the best known of the group but others include cornwallite, erythrite, olivenite, legrandite, chalcophyllite, scorodite, etc.  In textbooks, the arsenates (contains the AsO₄ radical) are often grouped together with the phosphates (PO₄ radical) and the vanadanates (VO₄ radical) since the radical often can interchange with each other and therefore create new minerals.  All together the three groups form an amazing number and variety of often brightly-colored minerals that are exciting to collect and observe but tough to identify.  It seems best to always carry a microprobe in your back pocket in order to obtain a specific identification!

Green duftite botryoidal duftite on "limonite."  Width of specimen ~3.1 cm.

Green duftite botryoidal duftite on "limonite."  Width of specimen ~2.5 cm.

As stated many times, I am not a mineralogist but do enjoy learning as I muddle through the minerals.  I am familiar with the arsenate and green encrusting mineral conichalcite since it has been found at Gold Hill in western Utah (see Blog posting Dec. 13, 2011).  In fact, duftite and conichalcite [CaCu(AsO₄)(OH)] form a solid solution series with the calcium of conichalcite substituting for the lead of duftite.  And in fact, austinite [CaZn(AsO₄(OH)] is also thrown into the soup with the zinc substituting for the Cu, or parts thereof.  At the type locality of duftite at Tsumeb, Namibia, a detailed 1980 study of the solid solutions in the arsenates found that “things” are not as straightforward as seemed—perhaps the original type duftite is not really “all” duftite: “the specimens are commonly zoned in color and composition. Microprobe analyses and X-ray powder-diffraction studies indicate extensive Cu-Zn and Pb-Ca substitutions, which represent solid solution among conichalcite… austenite… and duftite” (Jambor and others).  Green duftite botryoidal duftite on "limonite."  Width of specimen ~3,3 cm.

Photomicrograph of green botryoidal duftite on "limonite."  Width of width of photo ~1.1 cm.

The Handbook of Mineralogy (www.handbookofmineralogy.com) noted that conichalcite is “an uncommon secondary mineral in the oxidized zone of copper deposits, typically an alteration product of enargite” [copper arsenic sulfide] while duftite is “an uncommon mineral in the oxidized zone of some hydrothermal base-metal deposits.”  So, copper needs to be present in the solution for conichalcite to crystallize while lead must be around for the formation of duftite. 

 

So, what is an ole duffer like me going to call a specimen of green botryoidal crystals?  I don’t have a probe or FTIR or powder x-ray apparatus so must rely on physical appearances!  Yes, I know, that is the first thing instructors in Geology 101 tell students---don’t rely on color.  However, the physical properties of duftite and conichalcite are very, very similar.  I am calling my specimen duftite since: 1) some obscure pencil marks on a small slip of paper provided by the original owner labeled it duftite; 2) it “looks like” photos of duftite displayed on Mindat.org rather than conichalcite; 3) it “looks different” from the specimens of conichalcite in my collection.  That is the best that I can accomplish.  One great “thing” about a personal blog is that the editor easy to please.

As with much of what I write: The more I live, the more I learn. The more I learn, the more I realize, the less I know.  (Michel Legrand).

REFERENCES CITED

Jambor, J.J.. D.R. Owens, and J.E. Dutrizac, 1980, Solid Solution in the Adelite Group of Arsenates: Canadian Mineralogist, v. 18.