Optical microscopic examination of orthorhombic Na2SO4(III) in relation to its transformation to Na2SO4(I) hexagonal*

Optic orientation of orthorhombic Na 2 SO 4 (III) was determined as follows: X = c , Y = a and Z = b . The optic axial angle 2 V c was measured to be ≈ 80°. The crystallographic phase transition of III into hexagonal I is discussed in the light of the optical data as well as our earlier single crystal X-ray work.