Major and trace element zoning in sanidine is known, sanidine is common in volcanic rocks (see Ref. below). Zoning occurs when equilibrium has failed to keep up with falling liquidus composition during crystallization. Alkali feldspars is not only potash feldspar but also include albite (Na-end member) of plagioclase series. If the crystal is more sodic towards the rim, it is said normally zoned; if it is more calcic, it is reversely zoned. If the composition fluctuates back and forth, with uncertain causes, the crystal has oscillatory zoning.

you may look at the following reference:

Ginibre, C., Wörner, G., and Kronz, A. (2004) Structure and dynamics of the Laacher See magma chamber (Eifel, Germany) from major and trace element zoning in sanidine: a cathodoluminescence and electron microprobe study. Journal of Petrology: 45: 2197-2224.

agree with dawood

Hello Ahmed Morad:

Any handy way is to use electron probe to analyze several phenocrysts with optical zoning to see if there is any chemical zoning from core to rim of the feldspar crystal. You may determine they are potassic or sodic in composition.

Regrads,

Xue-Ming Yang

You can try by using cathodoluminescence microscope to see whether chemical zoning is exist, and electron microprobe analysis to determine the chemical composition. Exsolution in some alkali feldspar has been documented.

This paper (and together with other paper suggested above) should clarify your question.

Regards,

Andy

Hello Ahmed Morad

I have been using cathodoluminescence microscopy many times, especially in quartz. But in feldspars case it is also a very good idea.

You can also look at the following references:

Maki et al. 2016 (Cathodoluminescence characterization of feldspar minerals from granite-syenite rocks in Iwagijima Island, Ehime Prefecture, Japan), Naturalistae, 20.

Scholonek and Augustsson 2016 (Can cathodoluminescence of feldspar be used as provenance indicator), Sedimentary Geology, 336.

Best regards

Gosia (Malgorzata) Szczepaniak

Dear Ahmed Morad: surely also alkali feldspar can show zoning patterns. Just look at the accepted phase diagram for the system Albite-F.K. and notice that it is composed of two merging "loops", similar to the loops observed in the phase diagrams of the systems Albite-Anorthite and Forsterite-Fayalite (the loop in the upper FK-side of the diagram is distorted with the appearance of leucite, making this system at least ternary, with a peritectic piercing point). Therefore, under non equilibrium crystalisation, quite usual in volcanic environments, it is possible to have zoning in Na-sadinide or K-anorthoclase with respect to their major elements: Na and K. Curiously both loops merge into an azeotropic point at Or33, so normal zoning in K-rich feldspar goes from K-rich nucleous to Na-rich rims Ab67; instead normal zoning in Na-rich anorthoclase goes from Na-rich nucleous to K-rich rims Or33! FK in volcanic rocks is never microcline or orthoclase, it can be only a high T polymorph of F.K. like sanidine (often containing Na), or anorthoclase (sometimes even monalbite). In southern Venezuela I found an alkali quartz-trachyte breccia containing large FK phenocrysts, but the rock is so old (Mesoproterozoic) that the former sanidine has had exsolution and inversion, so it is now a microperthitic microcline! Surely no perthite is possible in young volcanic rocks, but in quite old rocks, like the aforementioned, it is possible, with the passage of almost 1.5 Ga..., to have exsolution at a microscopic scale, and even to have a polymorphic inversion to microcline. The former felsic vitreous groundmass of this alkali quartz-trachyte has been recrystallized to a maze of microgranophyric intergrowths. And some green, pleochroic, and zoned aegirine phenocrysts still survive!

With regards, Sebastian.

It is certainly less common than for plagioclase, as it easier to even out by diffusion, since no Al-Si exchange is involved. A colleague once told me that the zoning you see in alkali-feldspar is often associated with changes in barium content.

Hi,

zonation of K-feldspars could be also the result of later post-magmatic processes, eg. albitization. The hydrothermal processes can be recurent and correlated with magma pulses. Use the electron microprobe and cathodoluminescence

Both plagioclase and K-feldspar can display oscillatory zoning but it is easier to recognise in plag under a conventional polarising microscope. I agree with the comments of other contributors. See Xia Wen et al. 2015. Subterranean origin of accreted lapilli in cone-sheets of the Houshihushan sub-volcanic ring complex, Shanhaiguan, China, Journal of Earth Science, V.26, 661-668 for examples.

Sure, but first of all what kind of zonation are you talking about? How did you notice it?

We found oscillatory zoning displayed in plagioclase by variable extinction in multiple twins. There was slight variation from more An-rich cores to Ab-rich rims (normal zoning) but the variation in ratios in the multiple rim zones was almost as great. In K-feldspar (in our case microcline) it was sometimes displayed by variable alteration, mentioned by Katarzyna Machowiak

I would like to thank all of you dear professors and what if there's no avaliable facility to ensure as cathode. Or MICROPROBE, is there a way using polarizing microscope? :)

If you want to distinguish between alkali-feldspar and plagioclase, one could try relief: K-feldspar will always have negative relief, plagioclase not. Also, K-feldspar is biaxially negative, whereas plagioclase generally biaxially positive.

We need slight modification to statement of Dr Marlina Elburg. The alkali feldspar of the plagioclase series, i. e., albite, with An 0-10), also displays negative relief, except in certain orientations where r is more than Canada balsam when its anorthite content is more than 4.

Regards

You can use the staining of feldspar in thin sections. There is much information on the internet about it, as the example I send you.

Cheers

thanks Dr A.Cepedal :)

I recommend Marlina's answer

Best Regards

Dear Ahmed: definetely you need to make a transect across your alkali feldspar crystals with an electron or ion microprobe. This way you will know if the zoning is due to Ba, or if it is real zoning in the Sanidine-Monalbite system. Regards, Sebastián.

I recommend Marlina's answer

Studies have shown that Alkali feldspar phenocrysts are compositionally and microtexturally zoned.

you can go through the following references for further details.

1. Compositional and microtextural zoning in alkali feldspars from the Shap granite and its geochemical implications

M. R. LEE I. PARSONS

Journal of the Geological Society (1997) 154 (2): 183-188.

2. Perthite microstructure in magmatic alkali feldspar with oscillatory zoning; Weinsberg Granite, Upper Austria

• Rainer Abart📷 Elena Petrishcheva Stefan Käßner Ralf Milke
• Mineralogy and Petrology December 2009, 97:251

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