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