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Feldspar Group

Feldspar structureMost abundant rock-forming silicate minerals in Earth’s crust. Feldspars are common in almost all stony meteorite and lunar and martian rocks. 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).

K-feldspars commonly exhibit pink to reddish hues, while plagioclase feldspars tend to be white or gray.

 

 

 

Feldspar Crankshafts
Structure colored to highlight “crankshafts”. Image Source: Unknown

There are several polymorphs of alkali (Na, K) feldspar, based upon order-disorder rather than structural differences. The high-temperature polymorph, sanidine, has Al3+ and Si4+ randomly distributed between the tetrahedral sites with K+ in large interstices bonded to nine nearest oxygens. The lower temperature polymorph, microcline, has Al3+ and Si4+ completely ordered in the tetrahedral sites. The intermediate polymorph, orthoclase, has an intermediate degree of ordering. This is displayed in the figure below, which shows the concentration of Al3+ in the four tetrahedral sites.

 

 

 

 

 

 

Ordering in alkali feldspars

Whether high-temperature K-feldspar retains disordered structure or transforms depends largely upon the cooling rate. Microcline is found in deep-seated plutonic rocks and pegmatites (slow cooling), orthoclase in intrusives formed at intermediate temperatures, and sanidine in extrusive high-temperature lavas (rapid cooling). The Na-alkali feldspar, albite, also shows disordered and ordered forms depending upon temperature and cooling; whereas, the Ca-feldspar, anorthite, is perfectly ordered at room temperature.

 

The general chemical formula for feldspar is A2+xB+1-xAl1+xSi3-xO8. A cations are Ca2+ and Ba2+; B cations are K+, Na+, Rb+, and Cs+. All these cations are large. Al and Si occur in variable amounts to maintain charge balance by the coupled substitution of B+ + Si4+ ↔ A2+ + Al3+. 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 a ternary diagram (below). 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. Ba-rich and Sr-rich feldspars are are rare except in alkaline terrestrial rocks. Ba-rich feldspars are subdivided based upon Ba abundances: celsian has >90 mol% of the BaAl2Si2O8 component; hyalophane feldspars have <30 mol%.

Content above used with permission from J. H. Wittke.