Single-crystal X-ray diffraction data have been obtained for crystals of the F analog of superhydrous phase B, Mg 10 Si 3 O 14 F 4 . Twinned crystals were synthesized using a splitsphere anvil apparatus (USSA-2000) at pressures between 17.8 and 22.3 GPa and temperatures between 1450 and 1600 degrees C. Orthorhombic (space group Pnnm) unit-cell parameters are a = 5.050(3), b = 13.969(2), and c = 8.640(3) Aa. The substitution of F for H results in minor crystal chemical changes. Notably, average Mg-F distances (1.95 Aa) are shorter than the corresponding average of Mg-OH distances (1.98 Aa). These differences are reflected in shortening of unit-cell axes a and c in superfluorous B by 0.6 and 0.8%, respectively, relative to superhydrous B, while the b axis is unchanged. The close similarities between superhydrous and superfluorous B phases suggest that F will usually substitute for OH- in mantle phases.