Feldspar Group

Feldspar Ternary Diagram. Image Credit: SkyFall Meteorites

Feldspars are the most abundant rock-forming silicate minerals in Earth’s crust. Feldspars are also common in almost all stony meteorites including lunar and martian meteorites. The two most common types of feldspar are “plagioclase feldspar” that forms as a solid solution between albite and anorthite, and “alkali or K-rich feldspar” that forms as a solid solution between albite and orthoclase. K-feldspars commonly exhibit pink to reddish hues, while plagioclase feldspars tend to be white or gray.

Feldspar has the following general formula: A2+xB+1-xAl1+xSi3-xO8
– ‘A’ cations are typically Ca2+, or less commonly Ba2+
– ‘B’ cations are typically K+, Na+, or  less commonly Rb+ or Cs+

Ba-rich feldspar is also considered an alkali feldspar and forms as the result of a substitution between potassium and barium in the mineral structure. It does occur in some meteorites. Ba-rich feldspars are subdivided based upon their BaAl2Si2O8 abundance:
– Celsian feldspar has > 90 mol% 
– Hyalophane feldspar has < 30 mol%

 

Feldspar compositions are often expressed in terms of their molecular percentages of anorthite (An), albite (Ab), and orthoclase (Or). The results may be easily plotted on the feldspar ternary diagram (left). Depending on temperature and other environmental factors, alkali (Na – K) feldspars contain < 5 – 10 mol% An, whereas, plagioclase (Ca – Na) feldspars contain < 5 – 10 mol% Or. Intermediate compositions are sometimes given specific names, although this is falling out of common usage. If there is enough K to stabilize an alkali feldspar (Ab-Or solid solution) then the possibility exists to have both an alkali feldspar and a plagioclase feldspar in the same rock as seen in terrestrial felsic rocks like granodiorite and granite. Since the moon is highly depleted in K, but rich in Ca, based on the miscibility gap there is no ability to form an alkali feldspar.

Ordering in alkali feldspars

The polymorphs of alkali feldspar are determined based upon order-disorder rather than structural differences. Whether an alkali feldspar within a melt retains its disordered structure or transforms depends largely upon the cooling rate. The K-rich (Or>37) feldspars have three common forms including:
– Sanidine occurs in volcanic (extrusive) high-temperature lavas and forms by quenching (rapid cooling).
– Orthoclase occurs in intrusive plutonic rocks and forms by moderate cooling.
– Microcline is found in deep-seated plutonic rocks and pegmatites and forms by slow cooling.

The K-poor (Or<10) plagioclase feldspar, albite, also shows disordered and ordered forms depending upon initial temperature and cooling, while the Ca-feldspar, anorthite, is perfectly ordered at room temperature.

Feldspar structure

The structure of feldspar can an be considered a “stuffed” version of silica group structures, consisting of an infinite network of (SiO4)4- and (AlO4)5- tetrahedra. Alkali metal and earth cations are housed in available voids to maintain charge balance. K+ or Na+ (or rarely Rb+) must fill the voids when a single Al tetrahedron substitutes for a Si tetrahedron; Ca2+ (or rarely Sr2+ or Ba2+) must be added when two Al tetrahedra substitute for Si tetrahedra. Its structure consists of crankshaft-like zigzag paired chains running parallel to the a axis. Each chain is linked to adjacent chains. The cations fit into cavities between crankshafts. The two prominent cleavages observed in feldspar occur along the vertical plane between chains (001) and along the mirror planes between layers (010).


Some or all content above used with permission from J. H. Wittke.


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