Albite-Anorthite Series, Biotite : K(Fe2+/Mg)2(Al/Fe3+/Mg)([Si/Al]Si2O10)(OH/F)2

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minID: D61-7W8

Albite-Anorthite Series, Biotite : K(Fe2+/Mg)2(Al/Fe3+/Mg)([Si/Al]Si2O10)(OH/F)2

This image is copyrighted. Unauthorized reproduction prohibited.
Field of View: 10 cm

The sample is 14 cm x 8 cm. I received it as a gift in 1969.

The pegmatite pocket in a coarse-grained Microcline-Biotite granite contains 1 cm sized Plagioclase crystals. The blue "Peristerite" schiller, characteristic of Plagioclase, is visible on the crystal to the left. In the correct light almost all of the crystals in the pocket have that schiller.

This photo has been shown 324 times
Photo added:30th Nov 2011
Dimensions:2272x1704px (3.87 megapixels)
Date/Time of Photo:30th Nov -0001 00:00:00
Camera:NIKON E4300
Exposure time:1/15s
Focal Length:8mm
Focal Length (35mm film equivalent):38mm
ISO speed:100
View Richard Gunter's Photos View Albite-Anorthite Series Gallery

Discuss this Photo

PhotosMicrocline - Highway 62 north roadcut, Monteagle Township, Hastings Co., Ontario, Canada

14th Jun 2016 23:24 UTCRichard Gunter Expert

The crystals in this cavity appear to be Microcline rather than Albite. The Mindat description says that Peristerite is restricted to Albite and that the same texture in Microcline/Orthoclase is Moonstone. As the schiller's designation is only a convention could Peristerite include the blue schiller in Microcline? The remainder of the sample is a normal Microcline-Biotite pegmatite so there is no reason to suspect these crystals are Albite. The schiller is restricted to the euhedral crystals in the cavity; the remaining feldspar on the matrix does not have schiller.

15th Jun 2016 01:25 UTCPhil M. Belley Expert

It is plagioclase.

15th Jun 2016 15:22 UTCRichard Gunter Expert

Hi Phil

How do you know? Has this material been analyzed?

15th Jun 2016 15:53 UTCPhil M. Belley Expert

Diagnostic peristerite schiller

15th Jun 2016 16:13 UTCRichard Gunter Expert

Hi Phil:

I agree that the schiller is present but does that make it plagioclase? Unfortunately Sabina (1986) missed this locality but there are many other Microcline-bearing pegmatites in the area. I wonder if some schiller-bearing microcline is missed by calling all of the schiller-bearing feldspar plagioclase. I have found that plagioclase and biotite do not normally occur together; generally plagioclase occurs with clinopyroxenes. Microcline and biotite are constant companions in these primitive sweat-pegmatites, often as their only constituents.

15th Jun 2016 16:33 UTCPhil M. Belley Expert

Yes, it makes it plagioclase. I have seen plag and biotite occur together numerous times.

15th Jun 2016 17:01 UTCRichard Gunter Expert

Hi Phil:

I will change it.

15th Jun 2016 17:12 UTCAndrew Debnam

Is this locale not a good example of the two together (the source of the CN Rock pile in Bancroft)


15th Jun 2016 18:12 UTCRichard Gunter Expert

Hi Andrew:

Yes the rock dump is a good example. The Albite posted in the detailed article's photographs are what I would normally consider plagioclase. It has a tabular shape and abundant parallel striations. Depending on the chemistry of the Albite-Anorthite series the blue schiller may develop due to the light refraction on the internal compositional planes. When the twin striations are not present the identity of the feldspar is less certain. Microcline can develop a similar schiller but it is less common.
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