Most abundant rock-forming The most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the minerals in Earth’s [caption id="attachment_46127" align="alignleft" width="250"] Image Source: Pearson Prentice Hall, Inc.[/caption] Outermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of Read More. Feldspars are common in almost all Meteorite composed of silicate minerals, but that may have up to 25% Ni-Fe metal by weight. Stony meteorites are extremely heterogeneous as a group, ranging from samples of primordial matter that have remained more or less unchanged for the last 4.56 Ga (chondrites) to highly evolved younger rocks from differentiated and lunar and martian rocks. The structure of An alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars. Read More can an be considered a “stuffed” version of Mminerals formed exclusively from silica. There are ten known silica polymorphs, two of which are synthetic. Five of the naturall polymorphs are related by reconstructive transformations and can exist metastably: stishovite, coesite, quartz, tridymite, and cristobalite. Conditions to form coesite and stishovite are attained only during meteoroid impacts where there structures, consisting of an infinite network of (SiO4)4- and (AlO4)5- tetrahedra. Large univalent (1+) metal elements from Group 1 (leftmost column) on the periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium or cesium (Cs) and francium (Fr). Note that hydrogen is not included within the Alkali Metals. The alkali metals are silvery colored, soft, low density metals, which Read More 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 Common rock-forming series of feldspar: (Na1-x,Cax)(Alx,Si1-x)Si2O8, where x = 0 to 1. The Ca-rich end-member is called anorthite; the Na-rich is albite. feldspars tend to be white or gray.
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 Measure of the average energy of random motion of the constituents (e.g., molecules, atoms, photons) of a system. 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.
Whether high-temperature K-feldspar retains disordered structure or transforms depends largely upon the cooling rate. Microcline is found in deep-seated Igneous rocks that form from the cooling of magma in the interior of a planet or asteroid. rocks and pegmatites (slow cooling), orthoclase in intrusives formed at intermediate temperatures, and sanidine in Refers to igneous rocks erupted on a planetary body's surface. Read More 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, Rare compositional variety of plagioclase and the calcium endmember of the plagioclase feldspar mineral series with the formula CaAl2Si2O8. Anorthite is found in mafic igneous rocks such as anorthosite. Anorthite is rare on the Earth but abundant on the Moon. Read More, 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 Substitution in which the charges of substituting ions are not same and charge balance is achieved by a second substitution on a different crystallographic site. The most common example in the solid solution series of the plagioclase feldspars: anorthite, CaAl2Si2O8, to albite, NaAlSi3O8. Here, there are two substitutions taking place: Read More 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.