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Improving Mindat.orgThese are probably not Purpurite

12th Aug 2016 15:06 UTCJeff Weissman Expert

As far as I know, purpurite has not been found in Custer Co., South Dakota, would be interesting if I am wrong about this. Without analysis, these two should be labeled either as heterosite, or at least heterosite-purpurite series.


http://www.mindat.org/photo-764730.html


http://www.mindat.org/photo-5476.html

12th Aug 2016 19:49 UTCFred E. Davis

Hi Jeff. There does appear to be some evidence that purpurite is found in Custer County. In "Pegmatite Investigations 1942-1945 Black Hills, South Dakota" by L. R. Page et al., USGS Professional Paper 247 (1953), they state on page 12: "Purpurite was noted as particularly abundant at the Dyke Lode and Rainbow Number 4 pegmatites." Both of these localities are in Custer County. FWIW, Yale Peabody Museum has two specimens from Hill City, Pennington Co. just north of Custer Co. Of course, this doesn't mean the specimens you mentioned are purpurite, merely that it is a possibility.


Correction: Mindat may have the Dyke Lode located in the wrong county. Page et al. (1953) mention the Dyke Lode in Pennington Co., not Custer Co. shown on Mindat. Rainbow No. 4 is, apparently, in Custer Co. as shown on Mindat.


Update to the correction: Mindat lists a Dyke (Dike) Lode in both Custer and Pennington Counties! That needs to be verified.

12th Aug 2016 21:47 UTCJeff Weissman Expert

Fred - you are correct, but most of this purple stuff was always considered purpurite - did the paper by Page have suitable analysis? Since most of the primary phosphate is triphylite and not lithiophilite, I would expect the alterations to proceed from ferrisicklerite through to heterosite. The chemistry is just not amenable to purpurite. I will check my copy of "Mineralogy of the Black Hills" some time soon and see what Roberts says about this.

12th Aug 2016 22:21 UTCFred E. Davis

I could find no mention in PP-247 of the method used to ID minerals.

12th Aug 2016 23:56 UTCRobert Farrar

Roberts, in Mineralogy of the Black Hills, lists a few locations with purpurite, several with heterosite, and most as heterosite-purpurite series. Those listed only to the series had not been analyzed as of 1964.
 
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