R2：結晶構造・結晶化学・物性・結晶成長・応用鉱物

16:30 〜 16:45

[R2-04] 天然及び合成プレナイト(Ca₂Al₂Si₃O₁₀(OH)₂) におけるSi-Alの秩序性の多核NMR及び第一原理計算による解明

*薛 献宇¹、神崎 正美¹、Jonathan Stebbins² (1. 岡山大・惑星物質研、2. Stanford Univ.)

キーワード：

プレナイト、Si-Alの秩序性、核磁気共鳴測定、第一原理計算

Prehnite (Ca₂Al₂Si₃O₁₀(OH)₂) is a common mineral in low-grade metamorphic rocks. It has a unique structure consisting of two types of tetrahedral sites, T1 (occupied by equal Si and Al) with 4 neighboring tetrahedra (i.e., Q⁴), and T2 (occupied by only Si) linked with 2 neighboring T1 tetrahedra (i.e., Q²) and two AlO₄(OH)₂octahedra. Although previous XRD and NMR studies (Stebbins, J.F. 1992. MRS Bull., 27, 45) suggested strict Si/Al ordering in the Q⁴ sites, discrepancies have been noted between experimentally determined phase boundaries (Liou, JG 1971 Am Mineral 56, 507) and those calculated thermodynamically (Rose and Bird 1987 J Petrol 28, 1193), which may indicate Si-Al disorder. We performed comprehensive 1D and 2D ¹H, ²⁹Si and ²⁷Al NMR measurements on four prehnite samples (two natural samples, and two synthetic samples from Liou 1971), as well as first-principles calculations to clarify the issue. Our high-quality ²⁹Si NMR results clearly revealed the presence of two extra peaks due to Si(Q²) with 2Si0Al and 0Si2Al next-nearest neighbors (NNN), respectively, in addition to those with 1Si1Al NNN, as are expected for a fully ordered structure, for all samples. For the two synthetic samples, significantly greater proportions of extra Q² sites were detected, and an extra peak of Si(Q⁴) with 3Si1Al NNN, in addition to Q⁴ with 2Si2Al NNN as are expected for strict alternating Si/Al distribution within the Q⁴ chains, were also observed, suggesting the presence of Al-O-Al and Si-O-Si linkages in the Q⁴ chains. The occurrence of extra Q²sites, but not extra Q⁴ sites for the two natural samples could be a result of constrained Si-Al disorder in Q⁴ sites (under the constraint of alternating Si/Al within a given chain), or Si/Al disorder associated with domain boundaries. The greater abundances of extra Q² sites and an extra Q⁴ site associated with Si-Al short-range disorder within the Q⁴ chains for the synthetic samples indicate greater structural disorder and higher configurational entropy than natural samples, likely due to failure to attain equilibrium in the synthesis experiments. These results may explain the previously reported discrepancies in phase boundaries. The observed ¹H, ²⁹Si and ²⁷Al NMR results were well reproduced by first-principles calculations.