First terrestrial occurrence of the complex phosphate chladniite: crystal-structure refinement by synchrotron through-the-substrate microdiffraction

Chladniite, a complex phosphate species belonging to the fillowite group so far only found in meteorites, was identified in a pegmatite from Córdoba (Argentina) as inclusions in beusite. Applying synchrotron through-the-substrate microdiffraction technique to polished thin sections prepared on very thin glass-substrates (0.1 mm), diffraction data with nearly single-crystal quality were directly measured at selected chladniite microvolumes of the section. Chladniite is trigonal (space group $R3¯$⁠), with a = 15.0133(3) Å, c = 42.887(2) Å, V = 8371.6(5) ų. The subsequent crystal-structure refinement confirms that the accuracy attainable by using 0.1-mm-thick glass-substrates is comparable to that of routine single-crystal measurements [R₁ = 3.34% for 1617 unique reflections]. The refined crystal structure of pegmatitic chladniite differs from the meteoritic one in the cation ordering (Mg, Fe + Mn and Ca) at the various metal sites, with Mn being dominant at the M(1) position.