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Color in feldspars

Posted by Becky Coulson  
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Becky Coulson February 21, 2018 05:00PM
I also have a question about mineral color - specifically in feldspars, which can be cream, yellowish, pink, salmon, red, etc. I was asked - and I am also curious - about the color variety of the feldspars in granites.
Many thanks, Becky
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Sean February 21, 2018 05:32PM
Well, they tend to be the colours that you've just mentioned.

Now for Feldspars that are not on Granites, they tend to have different colours like blue or green (like Amazonite).
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Becky Coulson February 21, 2018 06:28PM
Thank you, Sean, although most feldspar specimens are not green, but white or of the shades I mention. Mindat gives a good explanation for the green in amazonite, but I am specifically asking about the non-amazonite feldspars in granites.
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Ralph Bottrill February 21, 2018 08:28PM
The red and yellow colours are usually attributed to iron, sometimes in solid solution and sometimes as exsolution of microscopic or often submicroscopic inclusion of iron oxides.
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Becky Coulson February 21, 2018 08:40PM
Thank you, Ralph - that is helpful.
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Keith Compton February 21, 2018 10:44PM
Becky

You may want to take a look at the book "Collector's Guide to the Feldspar Group" by Robert Lauf. It is a relatively cheap publication but contains a lot of good basic info on feldspars and provides comments on colour and on the types of feldspars found in igneous, metamorphic and sedimentary rocks - even extra terrestrial.

Keith
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Paul Brandes February 22, 2018 02:23AM
There are several things that can cause colours in feldspars, including metal ion impurities, intervalence charge transfers, radiation, and just physical defects in the crystal structure. The book Keith suggested is great for learning about feldspar and is not too technical.
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Becky Coulson February 22, 2018 08:09AM
Many thanks to you both - and Keith, I have several of the Lauf book series, which are indeed excellent and understandable for interested amateurs like me. I'll order the book on feldspars.
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Nick Gilly February 24, 2018 10:18AM
I wonder what is responsible for the colour in this gemmy feldspar fragment? Vanadium maybe?:




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Wayne Corwin February 24, 2018 12:15PM
Nick
Are you sure thats feldspar?
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Harold Moritz February 24, 2018 01:06PM
I can see the polysynthetic twinning lines characteristic of plagioclase in the second photo, so it does appear to be a feldspar. It would help to know the exact location so that the chemistry of the mineral forming environment that might have an influence on the color can be determined.
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Jolyon & Katya Ralph February 24, 2018 01:54PM
Probably the same coloring agent as amazonite - lead and/or divalent iron.
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Nick Gilly February 24, 2018 02:05PM
Hi guys. Yes, it was sold as oligoclase, but location is uncertain. This is what is written on the label that came with it:

"Gem quality oligoclase
Variety of albite, plagioclase feldspar
African. Either Namibia or Kenya. 6.5 Moh"

Can't be any more specific than that.

If it helps it shows strong pleiochroism too from yellowish green to bluish green, and through the polarising filter of the computer screen a vivid change as it is rotated from a phosphophyllite-like blue-green to a pale yellow/brownish-green.



Edited 2 time(s). Last edit at 02/24/2018 02:09PM by Nick Gilly.
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Owen Lewis February 24, 2018 04:35PM
Nick,
Without examining the piece, I'm as sure as I can be that this (tending to colourless?) specimen shows a play of colour caused by thin film effect light interference, occasioned by polysynthetic twinning in plagioclase feldspar.. Given the fairly clean condition of your specimen, whether it is oligoclase or some other variety of plagioclase can be determined by visual inspection (as Harold said) and a careful SG determination. SG's (together with RI's) are definitive for the six varietal members of the plagioclase series as follows:

- Albite (Al100-90). SG 2.61 - 2.63.
- Oligoclase (Al90-70). SG 2.64 - 2.66.
- Andesine (Al70-50). SG 2.66 - 2.68.
- Labradorite (Al50-30). SG 2.68 - 2.71.
- Bytownite (Al30-10). SG 2.71 - 2.72.
- Anorthite (Al10-0)(An100-90). SG 2.72 - 275.

The varietal splitting of the series into six varieties is quite arbitrary and based of SG and RI alone. The rationale for such sub-division is lost somewhere in the 19th C annals of geology/mineralogy. IMHO it provides nothing useful but is an interesting quirk of the history of science.

Here are a couple of pics showing the same effect in a colourless piece of plagioclase with the SG of 2.68. The piece is in the collection of a friend but the photos and SG determined are mine The first pic is a general description of the specimen and the second is a x60 photomicrograph that attempts to catch the play of colour and show evidence of the lamellar twinning.


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Tony Albini February 24, 2018 04:58PM
Nick,

White grading to facet quality light green oligoclase was found in Haddam, Connecticut. Analyzed by Richard Schooner.
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Nick Gilly February 25, 2018 03:28PM
Thanks Owen & Tony. Yes there is a Newton Ring effect on this piece and the striations from the polysynthetic twinning are clearly visible on the base, so it must be some sort of plagioclase. I'd never seen any feldspar specimen remotely like it before though, apart from the gem quality yellow orthoclase from Itrongay, Madagascar. Green was a new one for me.



Edited 1 time(s). Last edit at 02/25/2018 03:29PM by Nick Gilly.
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Ralph Bottrill February 25, 2018 08:53PM
I have never heard of plumbian plagioclase but maybe; or much Cr or V going into the structure. It’s a rare colour for plagioclase excepting when it has inclusions, which doesn’t seem to be the case here. But see https://www.mindat.org/photo-245036.html.
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Nick Gilly February 25, 2018 09:08PM
Interesting Ralph. That does look very similar in colour to my specimen. Is that crystal in the pic Cr or V-bearing?
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Alfredo Petrov February 25, 2018 09:39PM
Another question to consider: Is that really the color of the feldspar itself, or is it the result of microinclusions of clinochlore?
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Jolyon & Katya Ralph February 25, 2018 11:29PM
Some infomrmation:

https://www.gia.edu/gems-gemology/spring-2008-gem-news-international


Summary: Green feldspar from Tanzania

"EDXRF analysis of the faceted stones detected major amounts of Na, Al, Si, and Ca; minor K, Fe, and Sr; and traces of Mn, Zn, Ga, Ba, and Pb. LA-ICP-MS analysis cor- roborated the presence of these elements, and additionally detected Be, B, Mg, and Ti (Mg must be present in signifi- cant quantities to be detected by EDXRF, and any traces of Ti in the EDXRF results were obscured by the Ba signal)."

So, no Cr and no V.


" An absorption band was centered at about 615 nm in the β direction that had a full width at half maximum (FWHM) of ~4800 cm–1 and an intensity of 0.24 cm–1. These characteristics are sim- ilar to those presented for blue plagioclase by Hofmeister and Rossman (1986), suggesting a similar cause of color related to the presence of traces of Pb and water combined with exposure to radiation. "
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Owen Lewis February 25, 2018 11:37PM
The weak green colour does not look right for either Cr or V to me. More like Fe or as Alfredo suggests microinclusions. Brian Jackson, who wrote the excellent chapter on feldspar in 'Gems' 6th edn., gives green as one of the known colours of oligoclase but does not attribute causes to its several colourations. Rayleigh scattering in orthoclase gives a weak blue (Nassau). Combines with yellow from trace quantity of Fe3 impurity and one ends up with the weak, slightly murky and unattractive green the Nick's piece shows. However, Hofmeister and Rossmann (A.M.70 1985) describe a unique blue-green colour centre in feldspar, for which three precursor conditions are all necessary, namely:
Pb, H2O and irradiation. The irradiation changes Pb2+ to Pb3+, creating the colour center - but only in the presence of an H2O molecule.

So, 'you pays your money and makes your choice'. I don't think that chat can take this further - only some serious quantitative analysis of the particular specimen..

Edit: Crossed with Jolyon's post above.



Edited 2 time(s). Last edit at 02/25/2018 11:45PM by Owen Lewis.
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Ralph Bottrill February 26, 2018 10:03PM
Yes Owen, an analysis is worth 1000 opinions!
I don’t know the cause of the colour in the photo I linked to above.
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Nick Gilly February 26, 2018 10:14PM
Would micro-inclusions cause pleochroism Owen? Here's a couple of photos illustrating the change using the LCD screen as a backlight:




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Jolyon & Katya Ralph February 26, 2018 10:57PM
I would think pleochroism could only be caused by inclusions if they were 100% oriented with the crystal structure of the feldspar - ie unlikely.
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Owen Lewis February 27, 2018 12:52AM
AFAIK, there are no reports of pleochroism in feldspar and the usual sources all report plagioclase as showing none and FWIW I have never seen it.. That said the colour variation you show in your pics does look as would a single pleochroic colour change with a 90 deg axial rotation.

Though it's OK as a cost-free means of introducing folk to pleochroic colour change, I'm really not a fan of using an LCD screen as a means of examining it. Even a home-made London dichroscope, using two small polaroid filters oriented at 90 deg to each other is a big improvement IMO but for those wishing check specimens for pleochroic effect on a frequent basis, getting a calcite dichroscope is the best tool to have. It's worth remembering too that one can often see strong directional (pleochroic) colour change without the assistance of polarised light or polarising filters at all. All good lapidiarists understand this well and cut their stones to maximize full-frontal display of the most wanted colour/colour saturation. Only colourless and isotropic gems are free of this concern.

And no, microscopic inclusions never cause a pleochroic effect. Quite the reverse, Where present in even a strongly pleochroic host crystal, they prevent the expected pleochroic display from the host crystal, the light scattering always associated to some extent with microinclusions destroying any directionally oriented colour-change.This often prevents a mineralogist from seeing pleochroism in a crystal where it is expected to be found. Much less of a problem for gemmologists as they tend to work with gem grade crystals and crystal fragments that (with a few notable exceptions) mostly do no scatter light.



Edited 1 time(s). Last edit at 02/27/2018 10:43AM by Owen Lewis.
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Nick Gilly February 28, 2018 05:57PM
Jolyon, I have managed to look at that write-up on green sodic plagioclase from East Africa. Very interesting. From what I can see, the specimens in the pictures are paler in colour than mine and it also notes "pleochroism—none to very weak (light green and colorless)". The pleochroism in my sample is much more obvious. Even without the assistance of the LCD screen the colour shifts from a yellowish-green to a more bluish-green as the specimen is turned and rotated. So I'm not sure if the cause of the colour in my specimen is the same or different.

I've not seen anything like it since at any shops or the gem shows at Kempton Park.
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Peter Slootweg March 01, 2018 01:36PM
Interesting subject. I have a similar specimen from, I believe, the same find. I bought it in Tanzania from a gem dealer who had a small lot (200 grs approx.). It was was much rarer than the hematite included oligoclase sunstone that is found in great quantities. I do believe they are not from the same location. Locality information on many East African gems is often poor and hardly reliable. I was told my specimen came from Northern Tanzania but it could also be Kenya.



specimen size 50 x 28 x 15 mm

I took for some testing just to be sure. Readings from my refractometer were 1.530 - 1.540 and 1.530 - 1.538 (birefringence 0,01) with the limitation that these readings could only be taken from one cleavage face on the specimen. The SG measured was 2.65 with corresponds with the findings in the G&G article and known properties for oligoclase. Twin lamellae are clearly visible on two sides of the specimen.



The strong pleochroism shown by Nick’s specimens is also shown by my specimen. Not only in front of a LCD screen but also with a calcite dichroscope (see photo above). Pleochroism is noted for Feldspars in different publications. It seems it requires a strong color which is not often seen in gem feldspars in general. The vivid yellow sanidine from Madagascar show it as well but not as strong. The bright green gem feldspar (orthoclase) form Luc Yen in Vietnam shows the same colorless to green or yellowish to blueish pleochroism as Nick’s specimen with the same intensity. This may indicate the same origin of color (Pb, water content and irradiation).

My specimen may not be as gemmy but was different from the other specimens in the lot that it showed a remarkable schiller-effect as seen in the first photo. Strange this was not mentioned in the G&G article. Maybe the smal particles that may have turned my specimen less transparent are responsible for the schiller effect.

The same area also produces blue feldspars. I had seen small fragments offered as gem rough as coming from Kenya. A lage euhedral crystal of this material was found a few years back and was claimed to be found near the city of Korogwe in Northern Tanzania. Being so close to border this specimen may also be from Kenya. The color cause for this specimen is different since it lacks the strong pleochroism as seen in green colors. I have no idea what causes this blue hue. Any suggestions are welcome.



3 colorfull feldspars with gemmy area's. Blue crystal (oliogoclase?) from Tanzania/Kenya (110 x 101 mm), Green crystal (orthoclase) from Vietnam (65 x 40 mm) an yellow crystal (sanidine) from Madagascar (26 x 27 mm). The colors in the photo appear a bit darker than normal.







Edited 1 time(s). Last edit at 03/01/2018 01:38PM by Peter Slootweg.
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Nick Gilly March 01, 2018 03:24PM
Very interesting Peter! There is no schiller effect at all in my specimen. It does look like your specimen could be from the same deposit though. So probably northern Tanzania or southern Kenya rather than Namibia.

I bought mine from a shop called 'Crystals' in Salisbury, UK, several years ago. They have several branches in the SW of England. It had been there for months as I didn't buy it when I first saw it. Definitely something out-of-the-ordinary for a high street gem shop.



Edited 1 time(s). Last edit at 03/01/2018 03:30PM by Nick Gilly.
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Nick Gilly April 16, 2018 11:33AM
An update: I see that these pale-mint green oligoclase feldspars are being sold as cut gems on eBay under the name of 'polar feldspar' or 'Tanzanian sunstone':



From: https://www.ebay.com/itm/2-10ct-very-rare-gem-Mint-Green-Polar-Feldspar-Oligoclase-Tanzania-oval-cut-/152661592739

The latter name is confusing as 'sunstone' is orangey coloured plagioclase with a schiller effect caused by haematite inclusions, and looks nothing like this mint green material.
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Donald B Peck April 16, 2018 04:32PM
Hi Nick,

I agree with you on the "Sunstone". I don't know the source of the green color in the stone that you have shown, but I it could be due to a trace of Fe2+ in an otherwise quite pure feldspar. In any case, there is no accounting for the names that gem merchants apply to their wares (I know, I exaggerate).

Don
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Nick Gilly April 16, 2018 05:56PM
Actually, I took some better photos of my specimen earlier and will attach them, as Flickr is still saying no. The originals were taken with an older camera about 7 years ago. See below:



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Nick Gilly April 16, 2018 05:57PM



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Nick Gilly April 16, 2018 05:57PM


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