Sewardite

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Can you help make this a better article? What good localities have we missed? Can you supply pictures of better specimens than those we show here? Can you give us more and better information about the specimens from these localities? Can you supply better geological or historical information on these localities?


Sewardite orthorhombic
Ca Fe₂⁺³ (AsO₄)₂(OH)₂
Namibia
Tsumeb Mine 31st Level.
Dr. Terry Seward collected the mineral in the 1982. Bright red acicular crystals. It was collected as a single specimen of gangue that has since been split for scientific study. From the original find perhaps only 1 or 2 mg remain out side of the holotype material. Carminite from the mine is sometimes mistaken for sewardite. The mineral was found in a 3cm size vug on the 31st level of the Tsumeb mine. It is intimately intergrown with a very dark green to black botryoidal mas of material that was identified as a member of the tsumcorite group and appeared to be in intimate intergrowth of two distinct phases one of which conformed to the composition of ferrilotharmeyerite and the other which may have been a cuprian, zincian variety of ferrilotharmeyerite. The material occus as platy to compact anhedral to subhedral masses that do not exceed 0.3 mm in maximum size and are randomoly scattered throughout the dark greento black matrix. There were no obvious crystal forms and twinning was not observed megascopically or in X-ray single-crystal studies. The holotype material, consisting of a few small aggregates in two gelatin capsules, is housed in the Systematic Reference Series of the National Mineral Collection at the Geological Survey of Canada, Ottawa, Ontario, under catalogue number 68095. The polished section used for the quantitative electron-microprobe and reflectance studies is preserved in the mineral collection at the Natural History Museum, London, Great Britain, as BM 2001.36.₁
1. Canadian Mineralogist, August 2002, Vol. 40, part 4, p.1191-1198. Sewardite....The Ca-Analogue of Carminite from Tsumeb, Namibia: Description and Crystal Structure, Andrew C. Roberts, Mark A. Cooper, Frank C. Hawthorne, Alan J. Criddle & John A. R. Stirling.
[Rock Currier, 30 December 2008]


Sewardite
Mexico
Durango, Mun. de Mapimí, Mapimi, Ojuela Mine₁.

© Stuart McClure & Uwe Kolitsch

The specimens from Mapimi appear to be substantially better than the ones from Tsumeb because the appear to have free standing crystals. The specimens shown above have been analysed and confirmed by Uwe Kolitsch and the image on the right, is a rosette-shaped sewardite aggregate (PXRD- and EDS-analysed). SEM micrograph and is definitely Sewardite and was provided for us by Uwe Kolitsch. This photo was originally published in U. Kolitsch (2002): Sewardite from the Mina Ojuela, Mexico: a second occurrence. Lapis 27 (12), 43. (in German)
1. Kolitsch, U. (2002): Sewardite from the Mina Ojuela, Mexico: a second occurrence. Lapis 27(12), 43 (in German). 2. Personal communication from Jesse Crawford, 2009.
[Rock Currier, 30 December 2008]


Sewardite
Mexico
Sonora, La Mur, Las Animas mine
Click here for two pictures of definitely identified Sewardite. They don't look like much till you click on them for the full size images.

We need someone to tell us about the Sewardite specimens from this mine.


Sewardite
Mexico
Sonora, Mun. de Benjamin Hill, Benjamin Hill

As further evidence of the rarity of Sewardite and what is often requirred to identify it correctly, read the caption below of the image presented above.

Set of SEM images. Many specimens from Benjamin Hill were investigated in search of sewardite. It was confirmed only in the single specimen, where red carminite crystals are included in brown fibrous arseniosiderite. On these SEM images is visible, that sewardite form only thin outer zones (grey in BSE) over carminite cores (brightest phase on photos) in complex carminite/sewardite crystals. Matrix is grey in BSE arseniosiderite.
Compositions of sewardite zones are (clockwise from the lower right photo, which is my favourite):
(Ca.83Pb.12Sb.03)0.98Fe1.99[(As2.02S.02)2.04O8](OH)2
(Ca.89Pb.09Sb.01)0.99Fe1.99[As2.02O8](OH)2
(Ca0.88Pb.07Sb.02)0.97Fe2.05[(As1.94S.03)1.97O8](OH)2
(Ca.92Pb.06Sb.03)1.01Fe1.99[(As1.93S.06)1.99O8](OH)2

The result is the next: most of unanalysed red crystals of "sewardite" from Benjamin Hill are usual carminite. If you want to have real sewardite from the locality, you should to analyse it.

Specimen and analytical data of Mikhail N. Murashko.



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Rock Currier
Crystals not pistols.



Edited 19 time(s). Last edit at 02/16/2011 08:44PM by Rock Currier.

Hi Rock!
About sewardite look [www.mindat.org] and [www.mindat.org] ;) Not one dozen of specimens was investigated to find these real sewardite rims around carminite. Most part of specimens contained only pure carminite. So I don't believe, that all "sewardite" specimens on market really contains it.

Regards,
Pavel

"I had no reason to doubt that it was sewardite, but have since tested it with thiourea which is very sensitive and specific for lead and have found no lead present. I ran the test in parallel with a known descloizite and got the expected positive results."

The thiourea test is by no means specific, and not even particularly selective. The test is carried out in a dilute HNO₃ solution of the sample in question, by addition of a pinch of solid thiourea. Presence of lead is indicated by precipitation of a complex salt with composition [Pb{S=C(NH₂)₂}₆](NO₃)₂, which crystallizes as colourless needles with a high refractive index.

However, copper (e.g. from descloizite ...) and thallium give precipitates of very similar appearance and composition. Se(VI), Pt(IV), Os(IV), Fe(III), Cr(VI), Mn(VII), U(VI), Sb(III), Sn(II), Hg(I) and Ag interfere: their presence suppresses the formation of the precipitate (secondary reactions with thiourea, etc.).

To properly carry out the test, the samples have to be fully digested in HNO₃. Then, Se, Pt, Os, Fe, Cr, Mn and U, if present, need to be reduced; Hg and Ag need to be precipitated as chlorides; Sb and Sn need to be masked (e.g. tartaric acid). Reaction products, if any are formed, need to be separated. Then, the solution needs to be acidified again (it must not be too dilute after all these steps), and thiourea added. In order not to mistake Pb for Cu or Tl, the precipitate has to be examined under a microscope (this is strongly recommended for any precipitate formed in a test reaction).



Edited 1 time(s). Last edit at 01/14/2009 11:53AM by Peter Haas.

Sewardite itself contain Fe³⁺ according to definition in quontities enough to make the test with thiourea meaningless. Besides that it usually occur together with 2-3 other Fe³⁺ minerals (limonite etc.).

Thanks guys, Ill pass that on to Jessie.

Rock Currier
Crystals not pistols.

Mr.Rock Currier

An authentic and 100% confirmed Sewardite specimen:

[rruff.info]

Thanks for your atention.

Looks very different !

Armand, definitely a worthwhile contribution. I have made changes in the article.

Rock Currier
Crystals not pistols.



Edited 1 time(s). Last edit at 04/02/2009 07:01PM by Rock Currier.

Photos uploaded: [www.mindat.org] + 3 child photos.

Uwe,
Thanks! Its been added to the article. I am back from the Tucson show and should be able to now be able to resume my place at the ores with the rest of the slaves here on the mindat galley.

Rock Currier
Crystals not pistols.