Dollaseite-(Ce) (magnesium orthite redefined); structure refinement and implications for F + M²⁺ substitutions in epidote-group minerals

The crystal structure of dollaseite-(Ce) [a = 8.934(18), b = 5.721(7), c = 10.176(22) Å, β = 114.31(12)°, space group P2₁/m] has been refined, giving rise to the ideal formula CaREE³⁺Mg₂AlSi₃O₁₁(OH)F with Z = 2. Formerly known as “magnesium orthite,” it has been renamed dollaseite-(Ce) in honor of Dr. Wayne Dollase of UCLA. Dollaseite-(Ce) represents the probable limit of substitution of octahedrally coordinated M²⁺ cations in members of the epidote group. Mg is ordered over two equipoints, Ca and the REEs are each ordered over two different sites, and F and OH are each ordered over two different sites. The heretofore unrecognized substitution of Mg + F in dollaseite-(Ce) for M³⁺ + O of allanite suggests that such substitutions may occur commonly in other members of the epidote group.