Smectite group

(montmorillonite, nontronite, beidellite, hectorite, saponite, sauconite)

Formula: di-octahedral (0.5Ca,Na)_0.7(Al,Mg,Fe)₄[(Si,Al)₈O₂₀](OH)₄.nH₂O

                tri-octahedral (0.5Ca,Na)_0.7(Al,Mg,Fe)₆[(Si,Al)₈O₂₀](OH)₄.nH₂O

Reflected/transmitted colour: colourless, yellow, green, brown

Pleochroism: -

Optical isotropy/anisotropy: biaxial

Optical sign: -

Refractive indices: α = 1.48-1.61, β = 1.50-1.64, γ = 1.50-1.64

Birefringence δ: 0.01-0.04

2V_α angle: mostly small

Dispersion: -

Optical orientation: α ~ z, β = y, γ = ~ x

Optical axial plane: (010)