Alunite

Abstract

Alunite KAl3(SO4)2(OH)6 c 2001-2005 Mineral Data Publishing, version 1 Crystal Data: Hexagonal. Point Group: 3 2/m or 3m. Crystals typically pseudocubic {0112} or tabular {0001} with flat vicinal rhombohedra, to 1 cm; fibrous to columnar, porcelaneous, commonly granular to dense massive. Physical Properties: Cleavage: On {0001}, perfect. Fracture: Conchoidal. Tenacity: Brittle. Hardness = 3.5–4 D(meas.) = 2.6–2.9 D(calc.) = 2.82 Strongly pyroelectric. Optical Properties: Transparent to translucent. Color: Colorless if pure; may be white, pale shades of gray, yellow, red, to reddish brown. Streak: White. Luster: Vitreous, somewhat pearly on {0001}. Optical Class: Uniaxial (+). ! = 1.572  = 1.592 Cell Data: Space Group: R3m or R3m (synthetic). a = 6.981(1) c = 17.331(4) Z = 3 X-ray Powder Pattern: Marysvale, Utah, USA. 2.89 (100), 2.99 (94), 2.293 (76), 1.926 (66), 4.96 (50), 1.503 (34), 5.77 (26) Chemistry: (1) (2) SO3 38.34 38.66 P2O5 0.58 SiO2 0.22 Al2O3 37.18 36.92 Na2O 0.33 K2O 10.46 11.37 H2O 12.99 13.05 Total 100.10 100.00 (1) Marysvale, Utah, USA. (2) KAl3(SO4)2(OH)6. Polymorphism & Series: Forms a series with natroalunite. Mineral Group: Alunite group. Occurrence: Formed between 15 C and 400 C by the action of sulfate, which may be generated from pyrite or solfataric action, on aluminous rocks, commonly accompanied by kaolinitization and silicification. Association: Kaolinite, halloysite, diaspore, pyrite, gypsum, quartz.