Abstract
The usually quoted indices of refraction for thomsonite are the values of Des Cloizeaux1 and Lacroix,2 namely, α = 1.498, β = 1.503, γ = 1.525, which were probably determined indirectly from measurements of optic axial angles and birefringences of oriented sections. In 1912, Scheit3 described thomsonite crystals containing a nuclear crystal of natrolite, from Jakuben in the Bohemian Mittelgebirge, which gave the values, upon immersion, for yellow light: α = 1.521, β = 1.523, γ = 1.534; γ − α = .013, 2V = 49°.
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