Hongheite

Chemical Properties

Chemical Formula

Ca19Fe2+Al4(Fe3+,Mg)8(☐4)B[Si2O7]4[(SiO4)10]O(OH,O)9 The formula is a partial simplification of the full structural formula. The large (VII-IX)-coordinated (X4)2(X3)8(X2)8(X1) sites are here combined (e.g. Ca19) and are typically filled with Ca, although other large cations such as the REE may be present. The square-pyramidal Y1 site can host a variety of M2+ and M3+ ions and is the basis for the distinction of several species. The VI-coordinated Y2 site typically is filled with Al, whereas the also VI-coordinated Y3 site may contain Al, Mg, and other cations of similar charge and size. The tetrahedral T1 site is typically vacant but may contain B (less commonly Al); the trigonal T2 site is also typically vacant but may also contain B. Some of the (SiO4) may be replaced by (H4O4), akin to the Si4+ ↔︎ 4H+ hydrogarnet substitution. Among the oxygen that are not part of the silica tetrahedra, there are eight "O11" that typically occur as OH, two "O10" that are typically O & OH or OH & OH (the latter arrangement notably when Y1 is an M2+ cation). There may also be up to three "O12" that in most vesuvianite-group minerals are absent (and are not included here), but may be present particularly when T1 is occupied. Note that because the Y3 site occupancy of "end-member" hongheite has not been explicitly defined, and that based on other vesuvianite-group minerals it could be likely considered as either (Fe3+4Mg4) or (Fe3+6Mg2) (with charge-balancing concomitant occupancy of the "O11" position as [OH]9 or [(OH)7O2], respectively), the more generic occupancies denoted by (Fe3+,Mg)8 and (OH,O)9 have been maintained here, pending further data.

Elements

Al, B, Ca, Fe, H, Mg, O, Si