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Mineralogical ClassificationElectrum

9th Mar 2006 20:47 UTCJolyon Ralph Founder

I have Electrum in the database listed as Grandfathered.

I'd rather this particular grandfather died. Can we just ignore it and hope it goes away, or can we (hopefully) put an offical "discredited" or "rejected" notice on it?

Does anyone have any references for this?

JOlyon

9th Mar 2006 20:50 UTCJolyon Ralph Founder

Didn't want to wait. He's dead!

Jolyon

12th Mar 2006 02:32 UTCAndrew G. Christy Manager

Hi Jolyon-

Interesting one.

I agree with you completely that this grandparent should be nailed into its coffin.

The best 'reference' I can come up with would be any of the parallel papers on the '50% rule', stipulating that species are defined by the predominant element in a simple one-site solid solution (ie Ernie Nickel's IMA report 'solid solutions in mineral nomenclature', 1992). That *implicitly* defines the species as silver and gold, with no separate valid name needed or wanted for intermediates. Electrum is not needed as a species name, and becomes a varietal name for silver-rich gold. It's unfortunate that Ernie's paper did not include a list of such cases that could have been voted on by the CNMMN and zapped there and then, but there are actually quite a few of them. The sicklerite/ferrisicklerite names for non-purple intermediates in the heterosite-purpurite-triphylite-lithiophilite square are another example.

12th Mar 2006 20:53 UTCJim Ferraiolo

Andy,

Can you give some additional information on your 'sicklerite/ferrisicklerite' statement?

Thanks.

13th Mar 2006 11:18 UTCAndrew G. Christy Manager

Jim F asked:

"Can you give some additional information on your 'sicklerite/ferrisicklerite' statement?"

This is one of those things that is self-evident to us crystal chemists, but maybe not otherwise ;-)

Have a look at the formulae, space groups and unit cell dimensions. All these minerals are basically isostructural with olivine. Examples of ideal end-members are:

forsterite: Mg Mg

monticellite: Ca Mg

glaucochroite: Ca Mn

lithiophilite: Li Mn

triphylite: Li Fe

purpurite: <> Mn with Mn3+, and <> = vacancy

heterosite: <> Fe with Fe3+, and <> = vacancy

Since it is reasonbale to assume that no Mn oxidises from 2+ to 3+ until (virtually) all of the Fe has done so, intermediate compositions between the last four can be written:

(Li)x<>(1-x) (Mn2+)x (Mn3+)(y-x)(Fe3+)(1-y)

for x Li per formula unit, y total Mn per formula unit, y > x,

or

(Li)x<>(1-x) (Mn2+)y (Fe2+)(x-y) (Fe3+)(1-x)

for x Li p.f.u, y total Mn, x < y.

For intermediate values of x, these correspond to the sicklerite-ferrisicklerite composition range, but there is no reason (other than history) for special names in these composition ranges.

14th Mar 2006 01:03 UTCJim Ferraiolo

Thanks, Andy. I had already seen similar general formulas done.

Anyway, Alberti's structure paper on ferrisicklerite does state that 'Ferrisicklerite is therefore an intermediate member of the group natrophilite, lithiophilite, triphylite and heterosite.'

Is there be a reason, other than historical, for keeping sicklerite and ferrisicklerite as species? And while I'm not adverse to losing species (well, a little), a formal discreditation would have to be submitted to the IMA.

And, aren't there accepted 'intermediate members' in other series?

14th Mar 2006 10:02 UTCMarco E. Ciriotti Manager

Hi Jim,

basically all the following species are in the olivine group:

A Buchwaldite NaCa(PO4) orth olivine

G Calcio-olivine Ca2(SiO4) orth olivine

G Chrisoberyl BeAl2O4 orth olivine

G Fayalite Fe2+2(SiO4) orth olivine

Q Ferrisicklerite Lix<>1-xMn2+yFe2+x-yFe3+1-x(PO4) (x Li apfu, y total Mn apfu, x < y)" orth olivine

G Forsterite Mg2(SiO4) orth olivine

G Glaucochroite CaMn2+(SiO4) orth olivine

G Heterosite (Fe3+,Mn3+)(PO4) orth olivine

Q Hyalosiderite (Mg,Fe)2(SiO4) orth olivine

G Kirschsteinite CaFe2+(SiO4) orth olivine

A Liebenbergite (Ni,Mg)2(SiO4) orth olivine

G Lithiophilite Li(Mn2+,Fe2+)(PO4) orth olivine

G Lithiophosphate Li3(PO4) orth olivine

A Maricite NaFe2+(PO4) orth olivine

G Monticellite CaMg(SiO4) orth olivine

A Nalipoite NaLi2(PO4) orth olivine

G Natrophilite NaMn2+(PO4) orth olivine

G Purpurite (Mn3+,Fe3+)(PO4) orth olivine

Q Sicklerite Lix<>1-xMn2+xMn3+y-xFe3+1-y(PO4) (x Li apfu, y total Mn apfu, y > x) " orth olivine

A Simferite Li(Mg,Fe3+,Mn3+)2(PO4)2 orth olivine

G Sinhalite MgAl(BO4) orth olivine

G Tephroite Mn2+2(SiO4) orth olivine

G Triphylite Li(Fe2+,Mn2+)(PO4) orth olivine

so the Alberti words are correct.

In MINERAL MDI Database electrum is with status G (grandfathered), but agree with Andy. I classify electrum as non valid species and sicklerite and ferrisicklerire as questionables.

14th Mar 2006 10:37 UTCMarco E. Ciriotti Manager

The following article is in the forthcoming of European Journal of Mineralogy:

K. LANGER, M.N. TARAN, A.-M. FRANSOLET.- Electronic absorption

spectra of phosphate minerals with olivine-type structure: I. Members

of the triphylite-lithiophilite series, M1<6>LiM2<6> (Fex2+ Mn1-x2+)
.

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