Formula: Ca2Al2O.AlOH(Si2O7)(SiO4)
Reflected/transmitted colour: colourless (thulite pink-yellow)
Pleochroism: thulite α pale pink (rose), dark pink, β nearly colourless, bright pink, γ pale yellow, yellow
Optical isotropy/anisotropy: biaxial
Optical sign: +
Refractive indices: α = 1.685-1.705, β = 1.688-1.710, γ = 1.697-1.725
Birefringence δ: 0.003-0.008
2Vγ angle: 0-69°
Dispersion: r > v
Optical orientation: α = x, β = y, γ = z (ferrian zoisite α = y, β = x, γ = z)
Optical axial plane: (010) [ferrian zoisite (100)]
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