A new occurrence of rhönite is reported from a mafic alkaline sill in west Texas. Chemical analysis and pleochroism closely match those of Rhon, Germany, but refractive indices, birefringence and 2V are greater than those previously recorded. X-ray powder data were obtained and a strong similarity to the aenigmatite pattern is observed. Unit cell refinement was made, using adjusted aenigmatite cell parameters and indexing. From the unit cell volume obtained (747.79 Á ± 0.23), chemical analysis, and measured specific gravity (3.64±.04) it is calculated that the number of oxygens per unit cell is 40, and that the idealized formula is analogous to that of aenigmatite with the substitution of Ca, Al, and Mg for Na, Si, and Fe2+ respectively. The formula for the Big Bend rhönite is calculated to be (Ca3.45Na0.38K0.01) (Fe2+3.60Fe3+1.91Mg4.28Mn0.06Al0.15Ti2.84) (Si6.68Al5.32) (O39.4OH0.6). No evidence was obtained concerning possible solid solution between rhönite and aenigmatite.

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