Fluorescent potassium feldspar?

The other day I put some samples of Archean gneiss (Compeau Creek Gneiss, northern Michigan) under shortwave UV, and discovered that many of the K-feldspar crystals fluoresced deep red. Does anyone know anything about this? A web search hasn't provided any clues. Thanks!

James

James,

K-feldspar often fluorescences a deep red. Mont Ste. Hilaire comes to mind. At Franklin, NJ, the amazonite variety of microcline fluoresces a gray to blue-gray - quite unique.

James,
Please see here: [www.fluomin.org] . Check microcline and orthoclase - not sure which one occurs in that gneiss.

The red fluorescence of feldspar is, in most cases, caused by trivalent iron replacing aluminium. Fe³⁺ is a 3d⁵ transition element that has the right charge and ionic radius to replace Al³⁺. The fluorescence is, due to the strong crystal field, shifted to the deep red when Fe³⁺ is found in sites that put it in tetrahedral coordination with oxygen... such as in feldspars. Hence the deep red colour, which is usually found peaking around 700 nm. Attached are two examples: orange-yellow fluorescing zicon with fierce red fluorescing albite and agrellite with weak cherry red fluorescing albite.

About the blue gray fluorescing feldspar from Franklin; go see [mkafluorescence.org] .
Most likely due to titanium. An emission spectrum is on my "to do list"

Axel,

Regarding the photograph which you linked to in your last post, where the specimen is purportedly from the Limecrest Quarry, it is absolutely NOT from that location. It is from the Double Rock pegmatite which intruded the Franklin ore body near its southern end, just North of the Buckwheat open cut. Although a slightly grayish green microcline occurs at Limecrest, it is not fluorescent like the Double Rock material. There is NO bright red fluorecent calcite at Limecrest, only light "baby" bluish with persistent phosphorescence (SWUV) and pale pinkish (SWUV) in restricted occurrences (secondary mineralization). Pure white microcline, occurring as "graphic granite" was encountered near the end of the quarry's life and fluoresced the typical dark red (SWUV). The Double Rock pegmatite intruded the Franklin ore body, possibly at great depth (high P&T values), and melted the adjoining ore which recrystallized as coarse-textured material, mixing the pre-existing calcite and willemite into the pegmatite. The microcline here is amazonite from pale green to almost black, with a pegmatitic or gneissic texture (pictured in your linked photo), and carries Pb (galena; amazonite coloration), REE (Allanite-Ce; monazite-Ce), and U + Th (uraninite, thorutite, thorite). I have one specimen where galena in small specks is clearly visible. As far as I know, there is no other locality with the gray fluorescent response of the Double Rock material. There is titanite in this pegmatite as I recall, plus the thorutite. Obviously, this pegmatite is a soup of the intruded pegmatitic components with the ore body components.

There are several localities in Maine and New Hampshire that have feldspar that are a deep red under a short wave light. In New Hampshire Hurricane Mt, North Conway is special as the Zircon found there are a deep yellow color under short wave. With the feldspar they have a rich color contrast.

Check out this microcline from the Fillow Quarry, Redding, Connecticut, USA: [www.mindat.org]

Chester,

I got this one in a batch with some other minerals. I'm not an expert on the Franklin locality so the label may be wrong, I have no way to tell. I'll put your comment in the folder: "serious! to be acted upon soon" and act accordingly.

Thanks for bringing the subject up. I want my fluorescent mineral site to be as credible as possible and such ill described localities are not to be tolerated.

Thank you again
Axel

Axel,

Glad to help. Your website is great. I'll be visiting it often.

Chet Lemanski

Much of the K-spar granite that forms the eastern contact at Rock Candy mine, Grand Forks, BC fluoresces dull to bright red SW. I had no idea this was unusual, thanks for the edifying posts!

Bob

I really don't think it is unusual.....the brightness of the fluorescence seems to vary but all my Texas & New Mexico K-spars fluoresce to one degree or another in shades of red.

Don

I've noticed it in K-spars from Maine and Michigan pegmatites - usually a very dull red. I had assumed it was typical.

Dan,

that may have two causes. To little of the Fe³⁺ activator may cause a dim fluorescence but too much of it will cause concentration quenching.

Based on your discussion I took some of my amazonite from Crystal Peak, Colorado and put it under short wave UV and it glows red.

Bob Hembree

Bob, put it in the deep freezer for some time, or even better: burry it in dry ice for a few minutes. THEN hit it with the SW and see what happens.

Axel,

I've seen some very good presentations for cryofluorescence. The effects are amazing as the ultra-low temperatures restrict the movement of the atomic structures. As I recall, the K-feldspar should then emit a yellow fluorescence.



Edited 1 time(s). Last edit at 05/24/2012 01:45PM by Chester S. Lemanski, Jr..

On my recent collecting trip to Eureka Mine at St Peters Dome, Colorado. I noticed quite a bit of red color under SW for the microcline inside the tunnel. In the darkness of the tunnel the red color was rather intense even with a 4 watt lamp. I found a wonderful display specimen that has two microcline blocks that frame at a 90 degree angle a matrix of white quartz with embedded zircon crystals. I suspect it will be quite impressive under a standard cabinet UV lamp setup with higher lumens. When I get access to a cabinet UV lamp, I'll try and take a photo of the specimen.

Devils Head, Colorado also has some microcline that will show a dull pale red color under SW although I don't think it is as intense as what I found at St Peters Dome.

I haven't heard of cryofluorescence before. It was a bit cooler in the tunnel than it is at home which might explain why the red color doesn't seem as intense with the same lamp at home for the same microcline crystals. Are there any good papers or books that cover cryofluorescence in minerals? I can't seem to find much online regarding minerals, mostly stuff in biology or microscopy.

James,

the subject is touched in several books... Gaft, Gorobets, ... I looked into it myself (on a somewhat more simplistic level). You can see the resuts of my endeavour at: [mkafluorescence.org]
Quite a few activators show temperature quenching and low temps are almost always beneficial to fluorescent emission. Especially REE produce a much clearer spectrum if you cool them to dry ice or liquid N2 temperature.
Some uranium minerals don't even fluoresce in room temp. They do so, however, frantically when you cool them!

Feldspar in the Golden Horn Batholith in Washington Pass, Okanogan Couty, Washington fluoresces red undr short wave.