New electron microprobe analyses of rhoenite from an exceptionally oxidized alkali gabbro from Kauai, Hawaii are presented which greatly expand the range in composition known for this mineral. The Kauai rhoenites contain more abundant Mg, IVFe3+, VIFe3+, and Na, and less abundant Fe2+, VIIICa, and Ti than any previously reported terrestrial example. The abundances of Si and IVAl are among the lowest and highest, respectively, yet reported. These analyses are compared to all other available rhoenite analyses and a representative set of aenigmatite analyses. The twelve-dimensional rhoenite composition space is defined with aenigmatite as an additive component and a set of eight linearly independent simple- and coupled-ionic exchange components.

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