Eric
Yes most of the Cr in the silicates is Cr3+, and this seem to be moving
about (in highly alkaline, reduced fluids?) from the original chromite
into retrograde metamorphic fuchsite, stichtite, etc, but also,
interestingly, there is sometimes Cr3+ minerals associated with the
crocoite also, suggesting crocoite is formed by oxidising, low-T Pb-rich
fluids (in gossanous hydrothermal veins) reacting with Cr3+ rich fluids
and minerals.
There are also some Cr3+ -rich (grossular-uvarovite) garnets,
Cr-vesuvianite, Cr-diopside etc in contact metamorphosed ultramafics in
the area also, suggesting some local Cr3+ mobility over a very wide
temperature range.
regards
Ralph Bottrill
Mineral Resources Tasmania
03 6233 8359
-----Original Message-----
From: Metamorphic Studies Group [mailto:[log in to unmask]]
On Behalf Of Eric J Essene 1
Sent: Tuesday, 23 January 2007 12:14
To: [log in to unmask]
Subject: Re: chromium mobility?
Ralph,
I would expect that the Cr6+ as in crocoite represents formation
from soluble Cr6+ in highly oxidized solutions. Those are not
typical of hard rocks however.
eric
On Jan 22, 2007, at 8:00 PM, Ralph Bottrill wrote:
> Ed
> In Dundas, Tasmania, there seems to be remarkably extensive Cr
> mobility
> from the original chromite (in serpentines) into stichtite (Mg-Cr
> carbonate) veins (+/- Cr-chlorites, magnetites, etc) and fuchsite
> along
> fault contacts, during metamorphism. Then, during weathering, much of
> this Cr was remobilised into the groundwaters and locally deposits in
> quite large masses of crocoite (lead chromate) and other secondary Cr
> (III and IV) minerals.
>
> regards
> Ralph Bottrill
> Mineral Resources Tasmania
> 03 6233 8359
>
> -----Original Message-----
> From: Metamorphic Studies Group [mailto:GEO-
> [log in to unmask]]
> On Behalf Of Eric J Essene 1
> Sent: Tuesday, 23 January 2007 08:15
> To: [log in to unmask]
> Subject: chromium mobility?
>
>
> Ed,
> One sometimes sees violet "kammererite" (chromian chlorite) flakes
> in country rocks near serpentinites, suggesting the mobility
> of Cr during serpentinization over scales of 10's of meters. The
> "fuchsite" (chromian muscovite) that is seen commonly in Archean
> metasediments probably forms primarily from nearby detrital chromite,
> but also indicates some mobility of Cr. eric
>
>
> On Jan 22, 2007, at 2:27 PM, Edward Grew wrote:
>
>> Chromium is normally considered immobile in metamorphic environments.
>> Has Cr mobility been reported in either natural or experimental
>> systems?
>>
>> Thanks!
>>
>> Ed
>>
>>
>
>
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