Potassic-jeanlouisite from Leucite Hill, Wyoming, USA, ideally K(NaCa)(Mg4Ti)Si8O22O2: the first species of oxo amphibole in the sodium-calcium subgroup

Potassic-jeanlouisite, ideally K(NaCa)(Mg4Ti)Si8O22O2, is the first characterised species of oxo amphibole related to the sodium-calcium group, and derives from potassic richterite via the coupled exchange (Mg-1OH-2-)-Mg-C-O-W (Ti1+WO22-)-Ti-C-O-4. The mineral and the mineral name were approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification, IMA2018-050. Potassic-jeanlouisite was found in a specimen of leucite which is found in the lava layers, collected in the active gravel quarry on Zirkle Mesa, Leucite Hills, Wyoming, USA. It occurs as pale yellow to colourless acicular crystals in small vugs. The empirical formula derived from electron microprobe analysis and single-crystal structure refinement is: (A)(K0.84Na0.16)(Sigma 1.00)( B) (Ca0.93Na1.02Mg0.04Mn0.012+)(Sigma 2.)(C)(00)(Mg3.85Fe0.162+Ni0.01Fe0.333+V0.013+Ti0.65)(Sigma 5.01)(T)(Si-7.76 Al0.09Ti0.15)(Sigma 8.00)O-22(W)[O1.53F0.47](Sigma)(2.00). The holotype crystal is biaxial (-), with alpha = 1.674(2), beta = 1.688(2), gamma = 1.698(2), 2V(meas.)= 79(1)degrees and 2V(calc). = 79.8 degrees. The unit-cell parameters are a = 9.9372(10), b = 18.010(2), c = 5.2808(5) angstrom, beta = 104.955(2)degrees, V = 913.1(2) angstrom(3), Z = 2 and space group C2/m. The strongest eight reflections in the powder X-ray pattern [d values (in angstrom) (I) (hkl)] are: 2.703 (100) (151); 3.380 (87) (131); 2.541 (80) ((2) over bar 02); 3.151 (70) (310); 3.284 (68) (240); 8.472 (59) (110); 2.587 (52) (061); 2.945 (50) (221,(1) over bar 51).