Mimetite (?) and Pyromorphite
Redmond Mine
Haywood County, North Carolina
February 2015
By Mike Streeter
(mcstreeter@charter.net)

When stuck inside on a frigid winter day, I will sometimes pull out previously collected rock and mineral specimens to study. While closely re-examining a few specimens from inside Haywood County, North Carolina's Redmond Mine, a few tiny flashes of light emanating from the surface of one of the rocks caught my eye. I knew the specimen contained grass-green pyromorphite, but didn't know what mineral would be reflecting light like a faceted diamond. Upon close inspection with a 10X loupe, I discovered a scattering of incredibly small (up to 0.3 mm long) pale orange prismatic crystals with adamantine luster. My first reaction was puzzlement as I could not think of a confirmed Redmond Mine mineral that matched the properties I was seeing. Knowing that the unknown was a secondary mineral in the oxidized zone of a lead-bearing deposit in association with cerussite and pyromorphite, I thought maybe it was the vanadium-bearing mineral vanadinite. When upon further close inspection I noticed that most of the crystals display hexagonal basal terminations without tapering, I became nearly convinced they were indeed vanadinite. However, after further consideration, in part provoked by comments made on the McRocks.com message board, I wondered if that the arsenic-bearing mineral mimetite might be a better fit than vanadinite for at least two reasons: 1) given the geologic environment, it is perhaps more plausible that arsenic instead of vanadium is present in the vein and/or country rock; 2) mimetite [Pb5(AsO4)3Cl] and pyromorphite [Pb5(PO4)3Cl] are end members of a solid solution series so that a relatively small number of mimetite crystals could have formed along with the predominate pyromorphite if a limited amount of arsenic was available during crystallization.

It is also possible that there could have been a second generation of crystallization that produced mimetite or an intermediate member in the pyromorphite-mimetite solid solution series that would be classified as arsenic-bearing pyromorphite. The following are the chemical formulas and empirical statistics for pyromorphite and mimetite from webmineral.com:

Pyromorphite [Pb5(PO4)3Cl]
Phosphorus 6.85% P 15.70% P2O5
Lead 76.38% Pb 82.28% PbO
Chlorine 2.61% Cl 2.61% Cl
- % Cl -0.59 % -O=Cl2
Oxygen 14.15% O

Mimetite [Pb5(AsO4)3Cl]
Arsenic 15.1% As 23.17% As2O5
Lead 76.38% Pb 82.28% PbO
Chlorine 2.61% Cl 2.61% Cl
- % Cl -0.59 % -O=Cl2
Oxygen 14.15 % O

Assuming a second generation crystallization occurred to form the light-orange basal terminated crystals, they could indeed be pyromorphite, but with just enough arsenic (or maybe something else?) in the chemical makeup so the structure differs from that of the grass-green pyromorphite on the same specimen. If the concentration of arsenic is greater than 7.55%, then it would be classified as mimetite. However, throw a little calcium into the mix, as is readily available at the Redmond Mine, and there's at least a third possibility, phospohedyphane, since its physical properties also match.

Phosphohedyphane [Ca2Pb3(PO4)3Cl]
Calcium 6.57% Ca 9.19% CaO
Arsenic 1.68% As 2.58% As2O5
Lead 62.46% Pb 64.87% PbO
Chlorine 3.32 % Cl 3.32% Cl
- % Cl -0.75% -O=Cl2
Oxygen 19.97% O

There's a large number of other possible minerals, depending on the replacement of certain elements in the crystal structure with intermediate this and intermediate that, but I'm afraid my head might explode if I try to delve any deeper into this geologic conundrum. So, based on the geologic environment and the mineral's physical properties, I believe there's a good chance it is either mimetite, arsenic-bearing pyromorphite, or phosphohedyphane, with an honorable mention to the less-likely vanadinite.

Perhaps the only way to once and for all confirm the mineral's identity would be to perform some sort of sophisticated lab test such as x-ray diffractometry, but I'm going with mimetite for the purposes of this report because it best fits a host of physical and geological criteria and no way would I destroy any one of the rare crystals for lab testing, as these may be the first of their kind to ever be found in North Carolina's Blue Ridge Mountains.

I have since inspected a bunch of other similar specimens and found a few more with similar crystals. I took the following digital images to show this new find.

 

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