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Montmorillonite (Photo) [abbr. Mnt ]

Year of discovery: 1847 (Montmorillon, France), 1930 (Japan)

Locality list: 972 localities of montmorillonite

• Na_0.33(Al,Mg)₂Si₄O₁₀(OH)₂,nH₂O
• (Na,Ca)_0.33(Al,Mg)₂Si₄O₁₀(OH)₂,nH₂O
• [Na⁺_0.33][(Al³⁺,Mg²⁺)₂][(OH)₂|(Si₂O₅)₂]^6-,nH₂O

• Class: Dioctahedral < Smectite group < Mica supergroup
• Class: Phyllosilicate - Hydroxide - Hydrate
• Spacing parallel to the basal plane changes by dehydration.

• Al => Fe: Nontronite
• Al => Mg_1.5: Stevensite

• NaAl => Mg₂: Vermiculite

• Na_0.03(Al,Mg)₂ => Al₂: Beidellite (Series)

• Color: White, Gray, Pale yellow
• Streak: White
• Luster: Earthy
• Diaphaneity: Translucent

• Habit: Earthy

• Cleavage: Perfect on {001} (1 plane)
• Hardness: 1-2
• Density: 2.4

• System: Monoclinic
• Group: 2/m, C2/m
• Lattice: a=5.2, b=8.9, c=10.0, β=99.5

Synonym: Bentonite (Ore name)