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This paper explores the crystal chemical features of the bulk and the outermost (001) surface layers of two trioctahedral Li-rich mica-1M (space group C2) polytypes, i.e., a polylithionite (MLG-114) from Li-mica granitic pegmatite at St. Austell (SW England) and a Fe 2+ -rich polylithionite (Ch-140) from a rhyolite at Profitis Ilias, Chios Island, Greece. Structural formulas are  [xii] (K 0.952 Na 0.019 Rb 0.019 )  [vi] (Al 1.034  Li 1.459 Fe 2+  0.389 Fe 3+  0.046 Mn 0.038 Mg 0.002 Zn 0.002 Ti 0.001 )  [iv] (Al 3+  0.477  Si 3.523 )O 10.081 (F 1.735 OH 0.184 ) and  [xii] (K 0.992 Na 0.014 )  [vi] (Al 0.980 Li 1.028 Fe 2+  0.787 Fe 3+  0.022 Mn 0.059 Mg 0.052 Zn 0.010 Ti 0.024 )  [iv] (Al 3+  0.857 Si 3.143 ) O 10.095  (F 1.617 OH 0.288 ) for MLG-114 and Ch-140, respectively. Each mineral is characterized by a high F content in the anion site and has tetrahedral and octahedral compositions related to the exchange vector  [vi] Li -1  [iv] Si -1  [vi] Fe 2+  [iv] Al. Unit-cell dimensions are a = 5.251(1), b = 9.066(2), c = 10.087(2) Å; β = 100.694(5)° for polylithionite MLG- 114 and a = 5.282(1), b = 9.121(3), c = 10.080(3) Å; β = 100.764(5)° for Ch-140. Crystal structure refinements (agreement factors are R = 3.58% and 3.75% for MLG-114 and Ch-140, respectively) demonstrate that the  [vi] Li -1   [vi] Fe 2+  [iv] Si -1  [iv] Al exchange vector produces a decrease in the lateral dimensions of the tetrahedral and octahedral sheets. The decrease in basal oxygen distances results from the effect of the strain caused by the orientation of opposing tetrahedral sheets within a 2:1 layer. The decrease reduces the strain so that the basal oxygen plane can remain nearly planar. Changes in these dimensions via distortions of the tetrahedral basal-oxygen ring (α = 3.3° and 4.1° for MLG-114 and Ch-140, respectively) are limited. Octahedral M1 and M3 sites are similar in size and much larger than M2 and the mean electron count is M3 &lt; M1 &lt; M2 in MLG-114 and M1 ≈ M2 &lt; M3 in Ch-140. Al preferentially occupies the M2 site, whereas Fe and Li are nearly disordered between M1 and M3 sites with a slight preference of Fe for the M1 site in MLG-114 and for M3 site in Ch-140.

Crystal chemistry and surface configurations of two polylithionite-1M crystals