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MineralsGrayite

30th Dec 2012 03:08 UTCDean Allum Expert

Is Grayite = Brockite?

When discussing the Kemp uranothorites recently Pavel wrote:

"draw triangle with ThSiO4, USiO4 and ZrSiO4 on its apexes. Then found the middle points of sides of triangle where contents are 50%. Then join these points and you obtain smaller inverted triangle within previous triangle. All compositions from this inner triangle is the same phase without difference how you name them uranothorite, cyrtolite or thorocoffinite.

But such intermediate compositions are really rare. Most of your uranothorites contains too low contents of U to occur in this central triangle and are within small triangle closer to ThSiO4 apex. So they are only U-bearing or uranoan thorites.

In addition real natural systems has one more apex (Y,HREE)PO4 in the tetrahedron."

Is there also a theoretical triangle for the Th, U, and Zr Phosphates, but in the hexagonal system?

Using Ningyoite for Uranium: (U,Ca,Ce)2(PO4)2·1-2H2O

Using Brockite for Thorium: (Ca,Th,Ce)(PO4)·H2O

Why is there no corresponding hydrated Zirconium Phosphate?

Specific gravity, Lattice constants and XRD lines are the same between Brockite and Grayite {(Th,Pb,Ca)PO4·H2O} which are also similar to Rhabdophane. Grayite seems to be a grandfathered IMA mineral, but there seems to be more analysis done on Brockite. Are there guidelines for the chemical substitution level required for a distinct mineral?

I have a feeling that both types of specimens are always impure. Are both mineral names needed?

30th Dec 2012 13:22 UTCReiner Mielke Expert

Hello Dean,

Since Grayite, Brockite and Rhabdophane are isostructural one should be able to construct a ternary diagram for them. Ningyoite however is Orthorhombic so could not be part of it. It could be that the uranium ion is too big for the rhabdophane structure. Do you know if anyone has done an EDS on the Grayite from Colorado? If so, is the scan available, I would like to get a copy?

31st Dec 2012 17:05 UTCPavel Kartashov Manager

All Grayite, Brockite and Smirnovskite are one the same mineral CaTh4>₂*2H₂O. It belong to rhabdophane group together with Ningyoite.

Grayite is in the same grade "grandfathered" and in reality Questionable/Doubtful as Smirnovskite is. Both vere descredited in article of Doinikova e.a. in beginning of 90th.

Zr analogue of rhabdophane isn't known in nature and technic.

31st Dec 2012 20:11 UTCJim Ferraiolo

Pavel,

Can you send me that article, if possible?

Thanks.

31st Dec 2012 21:41 UTCJim Ferraiolo

Pavel,

Unless the article is a different one from Doinikova's article in Geology of Ore Deposits v49 (2007) : "Uranium Deposits wirh a New phosphate Type of Blacks", I have a copy. thanks.

31st Dec 2012 22:57 UTCPavel Kartashov Manager

As I remember, the article was published in "New data on minerals" FMM edition. In any case, I will ask Olga about her article, when the hollidays will finished.

By the way, in my specimen of smirnovskite (part of holotype of Dolomanova collection) the phosphate is Sr-dominant - (Sr,Ca)Th4>₂*2H₂O.

31st Dec 2012 23:47 UTCDean Allum Expert

Thank you for the thoughtful comments. I hope that Rudy sees this thread and posts his Grayite EDS.

Pavel,

"Zr analogue of rhabdophane isn't known in nature" .....yet

What do you think the precursors of a naturally occurring Zr(HPO4)2∙nH2O would be? Only baddeleyite and kosnarite? Perhaps rhadophane, monazite and xenotime? Might it exist in your eudialyte/fluor-apatite ores from Khibiny?

1st Jan 2013 02:57 UTCJim Ferraiolo

Thanks Pavel.

In the 2007 Geology of Ore Deposits, Olga references three of her articles: one in Geochemistry International 41 (12); one in Zapiski 1993 (3) and one in Geology of Ore Deposits 45(6). Any (or all) and any others would be of interest.

Thanks.

1st Jan 2013 14:47 UTCPavel Kartashov Manager

Dean,

I don't know anything about Zr(HPO4)2∙nH2O - its occurrence, association or way, how it was formed. So - no comments.

But I know only single thing - no one Zr phosphate wasn't observed in Khibina or Lovozero, nor on whole Kola peninsula or on all Russia territory. But we have innumerable localities of zircon of all genetical types - magmatic, metasomatic, hydrothermal, sedimentary, metasomatic and placer accumulations. In many associations zircon present together with apatite, xenotime and other phosphates. But Zr phosphates were never observed.

This tell us, that mutual mobilities of these elements are different. In other words, when Zr⁴⁺ is mobile, in solutions PO₄^3- ion absent, and vice versa. Only in magmatic conditions (in pegmatites) we observe some solubility of xenotime within zircon, and never zircon within xenotime.

So my prognosis of existence in nature of Zr member of rhabdophane group is quite pessimistic.

1st Jan 2013 23:07 UTCLefteris Rantos Expert

What about Tristramite? What's its relation to Brockite, Grayite etc.?

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