Formula: (Ca, Sr)Mn (Al, Fe)4 [H((Si,P)O4] (PO4)3(OH)4
Reflected/transmitted colour: colourless
Pleochroism:
Optical isotropy/anisotropy: biaxial
Optical sign: +
Refractive indices: a = 1.650-1.655, b = 1.654-1.664, g = 1.661-1.675
Birefringence δ: 0.01-0.02
2V angle: 75-84°
Dispersion: r >> v, strong
Optical orientation: b = b, g = c, a Ù a = 24°
Optical axial plane:
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