Refinement of the crystal structure and revision of the chemical formula of olgite: (Ba,Sr) (Na,Sr,REE)_2 Na [PO_4]_2

摘要

The crystal structure of olgite, ideally Ba (Na,Sr), Na [PO_4]_2, hexagonal, sample (I), a 5.5606(2), c 7.0450(6) A, V 188.65(5) A~3, and sample (III), a 5.5492(2), c 7.0286(5) A, V 187.44(4) A~3, Z = 1, D (calc.) = 3.904 (I) and 3.930 (III) g.cm~(-3), were refined in space group P3ml to R_1 indices of 2.73 and 2.58 percent, respectively, based on 236 and 233 observed [F_o > 4 sigma F] unique reflections measured with MoK_(alpha) X-radiation. The two samples differ only slightly in Ba and Sr content: (Ba_(0.76)Sr_(0.20)K_(0.04))_(sum 1.00) (Na_(1.15)Sr_(0.68)La_(0.10)Ca_(0.04)Ce_(0.02))_(sum 2.00) (Na_(0.97)Mn_(0.03))_(sum 1.00) [PO_4]_2 for sample (I) and (Ba_(0.88)Sr_(0.12))_(sum 1.00) (Na_(1.24)Sr_(0.66)Ce_(0.06)La_(0.04))_(sum 2.00) (Na_(0.96)Ca_(0.04))_(sum 1.00) [PO_4]_2 for sample (III), on the basis of 8 atoms of oxygen pfu. In the crystal structure, there is one tetrahedrally coordinated P site with = 1.526 and 1.522 A. There are three M sites: the [12]-coordinated M(l) site is occupied primarily by (with subordinate Sr and minor K), with = 2.978 and 2.969 A, the [10]-coordinated M(2) site is dominated by Na (with subordinate Sr and REE~(3+)), with = 2.806 and 2.801 A, and the octahedrally coordinated M(3) site is occupied by Na [with very minor Mn or Ca], with = 2.290 A. In the crystal structure of olgite, M polyhedra and P tetrahedra link together through common vertices, edges and faces to form a super-densely-packed structure related to glaserite, K_3 Na (SO_4)_2. Refinement of the olgite structure indicates that its general chemical formula should be written as (Ba,Sr) (Na,Sr,REE)_2 Na [PO_4]_2, and its end-member formula is Ba (NaSr) Na [PO_4]_2.