and feldspathoids. its entry explain to alone fails neighbors of Mg one might expect there to be an Mg-. The larger collectively among Earth's most abundant minerals, so abundant elements, and and feldspathoids are feldspar or feldspathoid. Mg is one of Earth's most otherwise only able to enter tectosilicates where H able to enter an anhydrous tectosilicate when Al stable configuration. Ca potential engender sufficiently tetrahedral cations, wherea mu feldspar or feldspathoid could not exist because of the the horizontal axis) lessens boundary is that increasing Al nature (in gray). The apparent explanation of that analogous compositions that feldspar and feldspathoid compositions (in black) from dashed diagonal line separates the field of known compositions of feldspars and feldspathoids, in that the The plot at right suggests a possible control on the repulsion that Mg tectosilicate. Instead, it appears that the cation-cation mi form feldspars, and its smaller neighbor, Li One result of a buffers the repulsion of the cations. substitutes for Si the vertical axis). According to this logic, a Mg cations of increasing ionic po in tetrahedral sites and thus The limits of interstitial cations Feldspars and feldspathoids VIII: Na Rb K Cs Li nerals consisting of Al tual repulsion between the interstitial cation and the + + + + + Be Ca Mg Ba Sr 2+ 2+ 2+ 2+ 2+ Sc Al B 2+ 4+ 3+ in the periodic table, Na 3+ in half the tetrahedral sites but 3+ ll this is that there is no Mg would cause precludes the absence of an Mg Si C Ti 3+ 2+ 4+ 4+ 4+ and Si falls somewhere in the middle, s cations of lesser ionic the density of positive charge allows inclusion of interstitial have not been found in tential or charge density (on 3+ little repulsion to allow a substtiution for Si 4+ tetrahedra, so size LBR SFMGFeldspars14 1/2010 into feldspars + + 2+ and Ca , enters two -bearng 2+ 2+ -bearing 3+ or Be 4+

2 2+ Railsback's O 2+ (on

, 2+

So

Ionic potential (charge÷radius) of 1+ or 2+ cation me 0.0 0.5 1.0 1.5 2.0 2.5 Fundamentals of Mineralogy and Geochemistry and Mineralogy of Fundamentals 0.0 VI VII IX IX VIII VIII IX VI VIII IV X Na K Rb Ba Mg Ba Na Li Na Mg Ca + Quartz and its polymorphs + + + 2+ 2+ + 2+ + 2+ 2+ VI Overall ionic potential (charge ÷ radius) in Al-Si tetrahedral sites tetrahedral in Al-Si ÷ radius) (charge potential ionic Overall Li . . 8.0 8.5 9.5

Non-minerals +

goosecreekite, and goosecreekite, hydr tschernichite are tschernichite 0.1 1978) Y Virgilite, an Li-bearing anhydrous tectosilicate (French et al., et (French tectosilicate anhydrous Li-bearing an Virgilite, Petalite, an Li-bearing anhydrous silicate in which Al and Si and Al which in silicate anhydrous Li-bearing an Petalite, are in tetrahedral coordination, is not shown here because because here shown not is coordination, tetrahedral in are

Mine not ugawaralite, ugawaralite, it is considered a phyllosilicate rather than a tectosilicate tectosilicate a than rather a phyllosilicate is considered it ous analogs. ous well constrained and because it is a very rare mineral. rare a very it is because and constrained well is not shown here both because its chemical formula is is formula its chemical because both here shown is not Feldspathoids Feldspars Non-mineralfeldspars hypothetical Non-mineraltectosilicates hypothetical rals 2001). Nickel, and Strunz 1997; al, et (Gaines K-feldspars IX VI-VII VI KAlSi LiAlSi 0.2 9.0 CaAl NaAlSi & wairikite Laumontite are hydrous analogs. are hydrous (R 3 O 3 b,K)AlSi Rubicline O 2 A 8 Si 8 l/Al+Si 3 6 O O 8 16 3 O (K 8 CaAl 0.3 ,Ba)(AlSi) KAlSi Leucite 2 Si

Sc are hydrous analogs. are hydrous 4 O olecite and cowlesite and cowlesite olecite

2 Pl O 12

(2001) agioclase feldspars 4 6 O Ga CaAl from Deer 8 in . Ionic radii are fr are radii . Ionic eset al. (1997) and Strunz and Nickel 0.4 ( 2 VIII VIII Si VI Data regarding coordination are Nepheline Na 3 , Ho MgAl MgAl ( O VI 10 IX Na wie, andZussman (1966), K 2 2 2 )AlSiO Banalsite Si Si X om S BaAl Ba)Al 2 2 KAlSiO CaAl O O Kalsilite Anor 8 8 hannon (1972). 0.5 4 2 Si 4 2 Si Si thite 2 4 O 4 2 O O 8 16 8