Feldspars

K - Feldspars Alkali feldspars Alkali feldspars

sanidine-anorthoclase (K,Na)AlSi3O8 B C D

These alkali feldspars are stable at high temperatures and occur in felsic volcanic rocks and some contact metamorphic rocks.  The name sanidine refers to potassic compositions (K/(K+Na) > 0.37), which are monoclinic.  More sodic compositions are termed anorthoclase which crystallize with monoclinic symmetry but invert to triclinic upon cooling, which results in polysynthetic twinning.  Both are colourless in plane light (A and C), and have maximum interference colours of first order white (B, D).  Sanidine commonly forms simple Carlsbad twins (B).  The polysynthetic twinning in anorthoclase (Albite and Pericline Laws) is superficially similar to that of microcline, but the twins are much finer in anorthoclase (D), and these two feldspars do not form in the same environment, as anorthoclase is a volcanic product and microcline occurs in slowly cooled plutonic rocks.  A and B are from a welded tuff from White River Canyon, Nevada, and C and D are from a rhyolite from an unknown location.  All views are 2.2 mm across.  A and C ppl, B and D x-nicols.

Plane-Polarized Light Low relief Clear Cleavage often visible (A) Usually numerous inclusions, giving it a much dustier look than quartz. The inclusions are due to microscopic alteration (B) Crossed Polarizers Gray to white interference colors, slightly lower than quartz Alteration products often have high interference colors (A) Often has a distinctive crosshatch or "tartan" twinning pattern.(B)

Phase transitions in K-feldspar, KAlSi3O8 Fully Al,Si ordered K-feldspar is called microcline. Microcline has characteristic cross-hatched twinning, seen in a polarizing microscope : This characteristic microstructure is due to the existence of both albite and pericline twinning in the crystal which has transformed from the high temperature disordered monoclinic structure.

Microcline Low relief Inclusions distinctive crosshatch or "tartan" twinning pattern = albite and pericline twinning

Twin domains in microcline are spindly and somewhat wispy. This is in contrast to the straight and generally continuous twin domains in plagioclase.

Carlsbad twins of K-feldspar as phenocrysts in vulcanic rock

Myrmekite Quartz “worms” intergrown with microcline

The alkali feldspar phase diagram Na-feldspar + K-feldspar “Perthite” Al,Si ordering solvus The disordered solid solution can only exist at high temperatures. Below the solvus the solid solution breaks down to 2 phases - one Na-rich, the other K-rich. This exsolution process results in a 2-phase intergrowth, called perthite

Cross-hatched twinning Perthite microstructure - an intergrowth of Na-feldspar in K-feldspar Antiperthite: K-feldspar in Na-Feldspar Cross-hatched twinning in K-feldspar Na-feldspar white

Refractive indices of Albite > Refractive indices of K-feldspar Perthite: Albite lamellae in K-feldspar Becke lines move into lamellae Antiperthite: K-feldspar lamellae in Albite Becke lines move into matrix Refractive indices of Albite > Refractive indices of K-feldspar

Plagioclase Inclusion and corrosions Tiger-stripe twinning

Plane-Polarized Light Low relief Clear Cleavage often visible (A) Usually numerous inclusions, giving it a much dustier look than quartz. The inclusions are due to microscopic alteration (B) Crossed Polarizers Gray to white interference colors, slightly lower than quartz Tiger-stripe twinning; (not all plagioclase shows twinning) Euhedral crystals are common and frequently zoned (A). Corroded crystals (B) are also common due to alteration or reaction with magma or other minerals Alteration products often have high interference colors (C).

Twinning in plagioclase feldspars Albite twinning is caused by the presence of pseudo-mirror symmetry parallel to (010): Twin plane is // (010) Components are related by reflection in (010) Polysynthetic twinning

Michel-Levy Method

Michel-Levy method Determination of anorthite component in plagioclase via extinction angle Select a grain in which the (010) plane is vertical. Indications: All twins have the same interference colour between crossed polarizers when placed to the N-S and in the 45° position B. The composition planes between the twin lamellae are crisp and sharp. C. The extinction angles measured below should differ by no more than 4 °

Michel-Levy Method

Michel-Levy Method

Zoning of plagioclase crystals

Zoning albite, carlsbad, and pericline twins

Alkali feldspars Oligioclase Andesine Labradorite Bytowinite

Plagioclase feldspars : NaAlSi3O8 (albite) – CaAl2Si2O8 (anorthite) At high temperatures there is complete solid solution, involving the coupled substitution: Na+ + Si4+  Ca2+ + Al3+ NaAlSi3O8 albite CaAl2Si2O8 anorthite 1100 1200 1300 1400 1500 1553 Temperature oC 20 40 80 60 melt melt + plagioclase plagioclase solid solution Note: In albite the Al:Si ratio is 1:3. In anorthite it is 2:2

Zoning Variations in Plagioclase Normal zoning: anorthite-rich core to albite-rich rims Reverse zoning: albite-rich core to anorthite-rich Oscillatory zoning: repeated alternations between albite-rich and anorthite-rich zones