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

Abstract

This paper explores the crystal chemical features of the bulk and the outermost (001) surface layers of two trioctahedral Li-rich mica-1 M (space group C 2) 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 0. 2+ 389 Fe 0. 3+ 046 Mn 0.038 Mg 0.002 Zn 0.002 Ti 0.001 ) [iv] (Al 0. 3+ 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 0. 2+ 787 Fe 0. 3+ 022 Mn 0.059 Mg 0.052 Zn 0.010 Ti 0.024 ) [iv] (Al 0. 3+ 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) A; β = 100.694(5)° for polylithionite MLG-114 and a = 5.282(1), b = 9.121(3), c = 10.080(3) A; 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 Element concentrations on the (001) surface, obtained through X-ray photoelectron spectroscopy (XPS) high-resolution spectra for Si 2p , Al 2p , Fe 2p , K 2p , Li 1s , and F 1s core levels, indicate that a greater amount of lithium and a smaller amount of potassium characterize the surface with respect to the bulk. The decrease in K content, commonly observed in micas, is related to its location on the cleavage surface, because the cation must be distributed equally between the two (001) surfaces generated upon cleavage. The increase in Li content on or near the (001) cleavage surface suggests a preference for cleavage near lithium-enriched regions. The surface structure of the polylithionite crystals suggests that Al, Li, and Fe cations maintain coordination features at the surface similar to the bulk. Silicon, however, which is generally in fourfold coordination, shows also a small number of [1]-fold coordinated components at a binding energy of 99.85 eV.