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GeneralArsenopyrite or Lollingite

28th Apr 2017 12:28 UTCMark Redmond

Hi

I have a specimen that was described as Arsenopyrite from the Belstone mine near Sticklepath in Devon, UK. However it has come to my attention that Lollingite also occurs there. The specimen was obtained prior to April 1997 and judging by the tarnished back surface was recovered from the dumps.

How do I tell if it is Arsenopyrite as described or the rarer Lollingite? It has the appearance of a columnar crystalline mass unlike other arsenopyrite specimens that I have. I have attached 2 photos. The specimen is about 10cm long.

Thanks and best regards

Mark Redmond

28th Apr 2017 13:20 UTCChris Stanley Expert

If the mass is almost entirely either arsenopyrite or loellingite, then it is possible to distinguish on the basis of the density since arsenopyrite has a density of about 6.1 and loellingite a density significantly higher at around 7.4.

Otherwise I don't think it's too easy to distinguish them unless well-crystalline

bws

Chris S

28th Apr 2017 13:25 UTCErik Vercammen Expert

Arsenopyrite contains sulfur while löllingite doesn't: should heating in a tube not produce a sulfur condensate for arsenopyrite?

28th Apr 2017 14:03 UTCHarold Moritz 🌟 Expert

Density is one way, this piece seems pure enough and large enough to easily measure. Fred Pough's Field Guide to R&M chemical tests says dissolve in nitric acid. The loellingite will leave a clear yellow solution (or one tinted pink or pale green if Co or Ni are present) whereas arsenopyrite will decompose and leave a spongy sulfur mass.

28th Apr 2017 16:03 UTCThomas Lühr Expert

Carefuly heated, arsenopyrite will produce at first the smell of sulphur(dioxide) and only afterwards the garlic like smell of arsenic.

Loellingite will produce a lot white smoke with garlic smell (more than aresenopyrite) and not produce a sulphuric smell before.

A mini-gas-torch works fine for that test. Take the sample outside the flame to observe the smell/smoke.

Do the test also with (known) arsenopyrite and compaire.

28th Apr 2017 17:20 UTCcascaillou

there is the Hepar test too (attack powdered mineral grain with a moten NaOH granule, add a drop of distilled water: on contact, a black stain appears over a silver coin. This reaction identifies presence of either S, Te or Se)

28th Apr 2017 18:08 UTCFranz Bernhard Expert

Concerning tests for sulphur to distinguish Aspy and Loell: Some loellingites contain a little sulphur in its crystal structure, maybe up to 1 w%. Very sensitive tests for sulphur could be misleading. See also:

http://www.handbookofmineralogy.org/pdfs/lollingite.pdf

Franz Bernhard

28th Apr 2017 21:33 UTCMark Redmond

Thanks everyone. Now I have retired I no longer have access to chemical tests.

28th Apr 2017 23:35 UTCcascaillou

well, anything that isn't already available from your local hardware store or drugstore will be readily available from online chemical suppliers.

Anyway Franz comment is raising a concern.

29th Apr 2017 00:54 UTCBen Grguric Expert

The other complicating factor is it is not unusual to have earlier-formed loellingite overgrown or replaced by later arsenopyrite with the replacement incomplete so both species are present. Becomes very obvious in backscattered mode in the SEM as the loellingite core are brighter. Send me a PM if you want me to check a small chip of your material.

29th Apr 2017 17:48 UTCDana Morong

A specimen that size might work quite well in the S.G. test, using a homemade beam balance (no set of weights necessary). I have done closed tube tests (refers to only one end closed the other open) on pyrite, arsenopyrite, loellingite. It can be quite interesting to see the differences in the sublimates. The hepar reaction will work with trace amounts of sulfur (or Se or Te) but one needs have patience for these trace amounts. I have used it on loellingite that had nearly 1% sulfur impurity, and it gave a reaction. However, it can still be useful to distinguish species, along with other tests. One need not use NaOH in the hepar reaction, however; sodium carbonate will work well enough (and has usually been used). In any test, some experience and running knowns against unknowns helps immensely to understand how it works and interpreting the results. It is also possible that the specimen contains both.

3rd May 2017 14:02 UTCMark Redmond

I have compared it to similar sizes pieces of Arsenopyrite and Safflorite (density 7.4 ).. It seems decidedly less dense than the safflorite so I conclude that it must be Arsenopyrite.

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