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Reppiaite occurs in manganese ores at the Gambatesa Mine, near Reppia village (Val Graveglia, Italy), mainly associated with a Mn-sheet silicate and Mn – Ca-carbonates. It was found in small amounts of very minute tabular crystals, (100) flattened, grown in fracture. Reppiaite, with ideal formula Mn 5 (OH) 4 (VO 4 ) 2 , crystallizes in  C 2/ m  space group with  a  = 9.604(2) Å,  b  = 9.558(2) Å,  c  = 5.393(1) Å and  β  = 98.45(1)°. The strongest lines in the X-ray powder pattern are  d  13[unk]  = 2.68 Å and  d  200  = 4.76 Å. It is orange-red in colour, transparent, biaxial negative with 2 V large and slightly pleochroic. Microprobe analyses reveal a partial As substitution for V, leading to the empirical formula Mn 5.01 (OH) 3.98 [(V 1.65 -As 0.35 )O 8.02 ]. The crystal structure of reppiaite is constructed of a quasi cubic close-packing of oxygens and hydroxyl groups, whose layers are stacked parallel to (100). The structure, refined to  R  = 0.038, consists of Mn octahedral layers bound together by isolated (V, As) tetrahedra and hydrogen bonds. Among the minerals, whose chemical formula has the same atomic proportions of reppiaite, only cornubite shows very similar structural features.

Reppiaite, Mn 5 (OH) 4 (VO 4 ) 2 , a new mineral from Val Graveglia (Northern Apennines, Italy)