A New Calcium Oxalate Monohydrate Produced by Thermal Dehydration of Weddellite

A high temperature‐Guinier photograph of weddellite power showed at 118 °C a transition to a so far unknown phase which transformed to β‐Ca(COO)2 at 135 °C and during cooling down to whewellite at 103 °C. X‐ray diffraction studies on single crystals revealed the new phase to have an orthorhombic symmetry with the lattice constant a = 12.088 Å, b = 10.112 Å, c = 14.634 Å, α = β = γ = 90°. There are very simple orientation relations to weddellite: a1 (wed) ∥ a (new phase), a2 (wed) ∥ b (new phase), c (wed) ∥ c (new phase). DTA/TG studies showed that the new phase is a calcium oxalate monohydrate.