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| Specimen of carminite on druzy quartz. Width of specimen ~3.7 cm. I am certain that other minerals are present (see photomicrographs below); however, my knowledge certainly does not allow much identification. |
Among the specimens tossed to me (see posting on Duftite)
by Mr. Rockhounding the Rockies (he of the large crystals) was one that had a
druzy quartz matrix with “blobs” of this gorgeous tiny red mineral. Who could turn down such a beautiful-colored
sample? Since this suite of specimens,
including the duftite, came from the oxidation zone of some lead deposits, I
begin to check the references listing possible associated minerals that were
red in color, and up popped carminite [PbFe2(AsO4)2(OH)2,
a lead iron arsenate and a fairly uncommon alteration product of arsenopyrite,
an iron arsenic sulfide. In fact, arsenopyrite [FeAsS]
is probably the most common arsenic mineral and in many instances represents a major cause of pollution
by acid mine drainage where the arsenic (As) and sulfur (S) ions are released
into the water. I certainly wish that I
had remembered more of my chemistry class to better understand what happens in
the oxidation zone of sulfide ores so that the silver colored and shiny
crystals of arsenopyrite are transformed into these carmine red crystals and encrustations.
This identification may be a little suspect but to me the color seems very
diagnostic. I believe, but would not bet
the farm on it, that the specimen came from the Ojuela Mine, Mapimi, Durango, Mexico.
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| Photomicrograph showing tiny crystalline carminite. FOV ~.8 cm. |
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| Photomicrograph showing tiny crystalline carminite. FOV ~1.0 cm. |
Nature composes some of her loveliest poems for the
microscope…
Theodore
Roszak
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