Color in feldspars
21st Feb 2018 17:00 UTCBecky Coulson 🌟 Expert
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.
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.
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.
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.
