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Halloysite-7Å (Photo)

Year of discovery: 1826 (Angleur, Belgium), 1935 (Japan)

Locality list: 396 localities of halloysite-7A

• Al₂Si₂O₅(OH)₄
• [Al³⁺₂][(OH)₄|Si₂O₅]^6-

• Class: Kaolinite subgroup < Serpentine-Kaolin group
• Class: Phyllosilicate - Hydroxide
• Soluble in: HCl
• Polytypes are not recognized as independent mineral species, but clay minerals are exceptions.

• Polymorph: Dickite (Monoclinic), Kaolinite (Triclinic), Nacrite (Monoclinic)

• [] => 2H₂O: Halloysite-10Å

• Color: White, Pale brown
• Streak: White
• Luster: Earthy
• Diaphaneity: Translucent-Opaque

• Habit: Earthy

• Cleavage: No
• Hardness: 1-2
• Density: 2.6

• System: Monoclinic
• Group: m, Cc
• Lattice: a=5.1, b=8.9, c=14.9, β=101.9

Synonym: Metahalloysite, Dehydrated halloysite