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A white to colorless triclinic mineral of the alkali feldspar group that contains a higher proportion of sodium than potassium and is a common constituent in the matrices of slightly alkaline volcanic rocks.

[Greek an-, not; see a-1 + orthos, straight + klasis, a breaking (from its having two planes of cleavage intersecting at a slightly oblique angle); see orthoclase.]
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References in periodicals archive ?
Similarly, the weakest signals observed approximately at 25[degrees] (possible anorthoclase, A), 45[degrees] (cristobalite, Crs) and 58[degrees] (unidentified), respectively, mark the difference between this sample and the previous ones: It can be seen in the diffractogram of the center of Figure 2.
Crystals range from K-rich, to Na-rich, and Carich varieties (n = 1974: orthoclase, anorthoclase, and labradorite) indicating that multiple crystal populations are present within these batholiths.
In addition, anorthoclase (Kal[Si.sub.3][O.sub.8]) and brinrobertsite [[(Na, K, Ca).sub.x][(Al, Fe, Mg).sub.4][(Si, Al).sub.8][O.sub.20][(OH).sub.4]*3.54([H.sub.2]O) (x = 0.35)] recombination at high temperature may generate albitecalcium [NaCaAl [(SiAl).sub.2][O.sub.8]].
After the analyses, the soil minerals found were: quartz, calcite, sanidine, albite, nontronite, muscovite, montmorillonite, hematite, faujasite, dolomite, microcline, anorthite, anorthoclase, clinochlore and epidote.
On the other hand, feldspar from the central facies is classified as plagioclase, with compositions ranging from labradorite to anorthoclase ([An.sub.64-14] [Ab.sub.34-73] [Or.sub.01-12]; Table 1; Fig.
Crystals are small (<3 mm maximum diameter) and include rounded embayed quartz, subhedral K-feldspar (anorthoclase, based on the presence of fine-scale cross-hatch twinning), and subhedral plagioclase set in a locally spherulitic groundmass of cryptocrystalline quartz and feldspar.
This confirmed that the material consists of felspar (disordered anorthoclase), alpha quartz, kaolinite and illite.
Anil points out that "Webster's 2nd defines pilandite as 'a leucocratic porphyry with phenocrysts, a groundmass of anorthoclase, and a little amphibole" and adds "Well at least I know what 'a', 'with', 'of', 'and', and 'little' mean." I checked the definition and it concludes with "Cf.
Anorthoclase is a constituent of phonolite, restricted to groundmass (Birmingham, 1987).
Flow-aligned phenocrysts of feldspar (mainly anorthoclase; Fyffe and Barr 1986; 0.1-0.5 mm in length) and quartz (0.3-1 mm) are enclosed in a quartzofeldspathic groundmass with rare green clinopyroxene (hedenbergite; Fyffe and Barr 1986).
Essential minerals are phenocrysts and glomerocrysts of anorthoclase, sanidine up to 1,5 cm and anorthoclase-mantled plagioclase; accessory minerals are red biotite, and hastingsite while trace minerals are augite, zircon, sphene and magnetite.