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Orthoclase (Photo) [abbr. Or ]

Year of discovery: 1823 (World), 1875 (Japan)

Locality list: 575 localities of orthoclase

• KAlSi₃O₈
• (K,Na)AlSi₃O₈
• [K⁺][Al³⁺Si₃O₈]^-
• Si-Al is intermediately ordered. Formed between 300-650 degree.

• Class: Alkaline Feldspar subgroup - Feldspar group
• Class: Tecto-aluminosilicate
• Insoluble in: HCl
• Magnetism: Diamagnetic
• Radioactive: Very weak
• Substitution pseudomorph: Replaced by Turquoise.

• Polymorph: Microcline (Triclinic, Formed below 300 degree, Ordered Si-Al by annealing)
• Polymorph: Adularia (Monoclinic, Formed between 300-650 degree, Intermediately ordered Si-Al, Rhombic)
• Polymorph: Sanidine (Monoclinic, Formed over 650 degree, disordered Si-Al)

• K => Rb: Rubicline
• K => (Na,K): Anorthoclase (Series, Stable above 700 degree)
• K => Na: Albite (Series), Kumdykolite, Lingunite

• KSi => CaAl: Anorthite, Dmisteinbergite, Svyatoslavite
• KSi => BaAl: Celsian (Series), Paracelsian
• KSi => SrAl: Slawsonite

• Color: Colorless, White, Pale green
• Streak: White
• Luster: Vitreous
• Diaphaneity: Transparent-Translucent

• Habit: Thick tabular

• Cleavage: Perfect on {001} (1 plane), Good on {010}, Parting on {100}, {110}, and {201} (4 planes)
• Hardness: 6
• Density: 2.6

• System: Monoclinic
• Group: 2/m, C2/m
• Lattice: a=8.6, b=13.0, c=7.3, β=116.1

Etymology: Ortho (Straight) -klasis (crack) in Greek