[Rockhounds] Cookeite

[Andrew Turner]

Cookeite is a lithium mica, although it is part of the chlorite group.  Unfortunately, there are numerous lithium micas that are extremely similar with only small variabilities in the elemental substitutions and solution series.  If I remember, the lithium mica (e.g. lepidolite) phase diagram has something like 17 end members, although I know they've tried to clean this up and better organize this group.

So, EDS will not work (it will show a lithium mica).  XRD may work if it is pure cookeite, although single crystal XRD would be the best.  Microprobe or Raman may work as well.

The real question is whether it is worth going through all of this to positively identify the mineral, which still may not be possible.  Cookeite in itself has very little value especially since it isn't all that uncommon, crystals typically aren't colorful or really unusual, and crystals are almost always micro.  I did find a photo of cookeite from that quarry, and it is associated with quartz (not surprisingly given the site).  In California, you do get cookeite associated with elbaite (tourmaline) from the Himalaya Mine and other pegmatites.  The cookeite adds contrast and associations to the main mineral which does increase the value, but it usually isn't very valuable by itself.

You probably do have cookeite (or a very similar lithium mica) based on the location and associations.  I don't think there would be an issue labeling it as that.  Thank you.

Andrew Turner
Salt Lake City, UT

________________________________
From: Rockhounds <rockhounds-bounces at rockhounds.drizzle.com> on behalf of DON HALTERMAN <donhalterman at comcast.net>
Sent: Sunday, October 4, 2020 4:29 PM
To: Rockhounds at drizzle.com: A mailing list for rock and gem collectors <rockhounds at rockhounds.drizzle.com>
Subject: Re: [Rockhounds] Cookeite

Looks like XRD would be the best bet.  The lithium and (OH) make positive ID by EDS difficult. I don't know what other minerals have similar formulae, but for a solid positive ID I'd get x-ray diffraction patterns coupled with chemistry by wavelength dispersive spectroscopy or perhaps Raman spectroscopy.



> On 10/04/2020 3:02 PM stu at arcrystalmine.com wrote:
>
>
> I have a pint jar full of what I believe to be yellow/gold color cookeite
> micro crystals from Jeffrey Quarry, near North Little Rock, Pulaski County,
> Arkansas.  Does anyone have the capability of testing the crystals to
> confirm it is cookeite and is there any value in the cookeite?
>
>
>
> With Appreciation & Gratitude,
>
> Stuart Schmitt
>
> Clear Creek Crystals
>
> 60 Mary's Eagle Trail
>
> Mt Ida, AR 71957
>
> (870) 867-2443
>
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