Muscovite dehydroxylation; high-temperature studies

Abstract

Ansl:n.Lcr Unit-cell dimensions of muscovite-2M, from the Diamond mine, South Dakota, were determined to 1000'C, single-crystal X-ray structural refinements were made at 20'C and 300'C, and additional structural refinements at 20, 525. and 650'C were made on similar material from Panasqueira, Portugal. Single-crystal data showed linear expansivity of the metric dimensions to about 850C followed by a phase change to muscovite dehydroxylate with increases in the b and c parameters and a decrease in a near 850'C. The mean atomic distances ofthe K polyhedron increased more rapidly than those for the other polyhedra as temperature increased. The result shows that (l) the K-O(2) bond is longer than KO(l) and K-O(3) at each temperature and (2) the K4(2) bond distance [2.867(3) A at 20'Cl expands with temperature more rapidly [with a mean thermal-expansion coefficient (MTEC) of 5.41 than the other two interlayer bonds tK-O(l): 2.832(3) A at 20'C, MTEC : 4.7; K-O(3): 2.844(3) A at 20'C, MTEC : 4.51. It is concluded that the proton position weakens (lengthens) the K4(2) bond relative to K-O(l) and K-O(3) and that a preferred path for dehydroxylation results. Misfit between the tetrahedral and octahedral sheets is not a factor in thermal decomposition, since the tetrahedral rotation angle is about 8.5 at 950c. Thermal analysis (rce, orc) indicated a very broad dehydration peak, interpreted as two ovedapping and poorly resolved dehydration peaks at apparently 550 and 750'C on the basis ofcomparison to pyrophyllite. This result suggests that dehydroxylation is not a homogeneous process. A model for dehydroxylation of muscovite was developed from Pauling bond-strength summation calculations. The strength of the AI-OH bond is greatly affected by the coordination number of neighboring polyhedra. When neighboring polyhedra are in octahedral coordination, the hydroxyl group is lost at lower temperatures than when neighboring polyhedra are in fivefold coordination (after partial dehydroxylation). This efect results from the distribution ofoversaturated apical oxygens that affect also the proton position and the AI-OH bond strength. The model is applicable to pyrophyllite and similar dioctahedral 2: I layer silicates and is supported by previously reported infrared, Nun, and rnna data.