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Identity HelpWhite Erythrite?

28th May 2012 02:01 UTCReiner Mielke Expert

I have some samples that have radiating fibrous pink Erythrite. However on one sample are areas with snow white radiating crystals that grade to pale pink. They all turn conc. HCl bright blue indicative of cobalt. Has anyone ever heard of white erythrite? I know annabergite can be white to pink but it gives a strong green color in conc. HCl?

28th May 2012 08:39 UTCPeter Haas

Colours of Ni2+ and Co2+ are cancelling, but their specific absorptivities (i.e. extinctions per unit concentration) are different (as has to be expected, since this is an intrinsic property of the respective chromophor).

Since Ni2+ has the lower specific absorptivity, there can be white and even pink annabergite (to compensate the colour produced by a given amount of Ni2+, it takes a lesser amount of Co2+). For the same reason however, colourless erxthrite does not exist (it would take more Ni2+ than Co2+, which places the specimen in the compositional range of annabergite).

In aqueous HCl, the situation is a similar one in that the different species (chlorocobaltate and chloronickelate, respectively) have different specific absorptivities. Although their colours will not cancel, the colour of one or the other will be dominant, depending on concentration. In addition, the colour of the cobalt-bearing solution will gradually change from blue to pale pink with decreasing temperature and concentration (this is an equilibrium effect between two different species; you don't have either one or the other, but always both of them in varying proportions). While the pink aquo-complex is a weak chromophor (i.e. its specific absorptivity is relatively low), the blue chloro-complex is a strong chromophor. Thus, temperature and concentration are another two important parameters to consider when conclusions have to be drawn from the colour of the solution. The only logical conclusion however is that this test is not conclusive unless the specimen has a near end-member composition of either erythrite or annabergite.

28th May 2012 09:57 UTCUwe Ludwig

it is not impossible that Pharmakolithe is together with Erythrine on the Specimen.

Uwe Ludwig

28th May 2012 16:31 UTCReiner Mielke Expert

Thank you guys. I think I will get EDS done just to get an idea of what is there before I go further with it. Pharmacolite did come to mind, EDS should tell me if that is possible.

28th May 2012 16:49 UTCLefteris Rantos Expert

Also consider other elements, such as Mg and Zn in the formula of a Vivianite-group mineral (i.e. Erythrite). Erythrite (Co-dominant), Annabergite (Ni-dominant), Hörnesite (Mg-dominant) and Köttigite (Zn-dominant) all form series between each other, and complex compositions and/or zoning may occur in some specimens. Theoretically, you can have an Erythrite with less than 30% Co in the metal position.

EDS should clearly differentiate between a Vivianite-group mineral containing some of the above elements, Pharmacolite (with Ca) and Picropharmacolite (with Ca and Mg).

Lefteris.

22nd Jun 2012 01:19 UTCReiner Mielke Expert

Here is the EDS results for the sample, looks like white Erythrite! Any other possibilities?

22nd Jun 2012 04:34 UTCPeter Haas

There appears to be something overlapping with the smaller cobalt peak. Peak shape is highly suspect.

23rd Jun 2012 03:24 UTCPeter Nancarrow 🌟 Expert

The peak labelled "Cu" is incorrectly annotated.

The energy of Cu Kα is 8.04 KeV, whereas that peak is actually nearer 8.25 KeV, which is about right for NiKβ.

Therefore the peak at about 7.5 KeV labelled "Co", is an overlapping composite of NiKα (7.47 KeV) and Co Kβ (7.65 KeV) and there is no significant Cu in the specimen.

Pete N.

24th Jun 2012 14:23 UTCReiner Mielke Expert

Hello Peter,

Thank you! I didn't do the EDS but will contact the person who did. A high nickel content could account for the color.

24th Jun 2012 16:30 UTCKerry Day

Point conceeded. I was far too quick to call that tiny peak a Copper peak. On closer inspection it is the Ni Kbeta peak. The mineral is still overwhelmingly Co-rich. Kerry Day Kaygeedee Minerals.

25th Jun 2012 01:53 UTCReiner Mielke Expert

Seems there isn't enough Ni to account for the color, anyone have any suggestions as to why the crystals are white?

25th Jun 2012 02:24 UTCAlfredo Petrov Manager

Is it possible for dehydration to cause a whitening of the colour?

26th Jun 2012 01:03 UTCReiner Mielke Expert

Hello Alfredo,

Have never heard of that happening. You find eyrthrite on dumps that has been in the sun for years that is bright pink, besides this was found buried deep in moist rubble and it came out white.

26th Jun 2012 16:52 UTCUwe Kolitsch Manager

Note that the Mg peak completely overlaps with the first As peak.

It appears, however, there is not much Mg.

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