13:45 〜 15:15
[SMP27-P02] Si-Alの配置と水素結合距離の相関: Ussingite(Na2AlSi3O8OH)からの洞察
キーワード：Si-Alの配置、水素結合、ussingite、含水鉱物
Hydrogen bonding is an important factor governing physical properties of minerals. Many hydrous silicate minerals exhibit Si-Al disorder. However, a general understanding about how Si-Al disorder affects hydrogen bonding is lacking. Here I present study on ussingite (Na2AlSi3O8OH) that clearly revealed a strong correlation between Si-Al disorder and hydrogen-bonding distance, which could be a universal phenomenon.
Ussingite (Na2AlSi3O8OH) has a partially interrupted framework structure (Rossi et al. 1974; Williams & Weller 2012). There are four tetrahedral (T) sites: T1 & T2 are Q4 (i.e., having 4 next nearest neighbor (NNN) T sites), and T3 & T4 are Q3 (with 3 NNN T sites). Their NNN environments (in brackets) are: T1(1T2,1T3,2T4), T2(1T1,2T3,1T4), T3 (1T1,2T2), T4(2T1,1T2). Hydrogen is in T4-O8-H…O2-T3, with a short hydrogen-bonding distance. In the ordered structure, T1 is occupied by Al, and the other T sites by Si. Previous studies gave contradictory conclusions as to whether Al-Si disorder is present. I was able to unambiguously clarify the issue via combined NMR measurements and first-principles calculation.
Powdered samples of ussingite from two localities (one Lovozero alkali Massif, Russia, another uncertain, possibly Greenland) were examined by NMR using Bruker 9.4 T Avance NEO spectrometer. First-principles calculations were performed with the GIPAW method using the Quantum-ESPRESSO package (v.7.0). Both ordered structure and models for Si-Al disorder in ussingite were studied. For the latter, a 2x2x2 supercell (256 atoms) with one pair of Al-Si exchanged between a T1 site and each of its NNN (1T2, 1T3, 2T4), as well as between two remote T1 and T2 sites were examined. NMR calculations were performed on optimized crystal structure.
The 1H-29Si CPMAS NMR spectra of both ussingite samples revealed three main peaks near -84.2, -87.9 and -96.5 ppm, which correspond, respectively, to Q3(1Si2Al), Q3(2Si1Al) and Q4(3Si1Al), as expected for T4, T3 and T2 site in the ordered structure. However, all three peaks show splitting and/or shoulder, and additional weak peaks near -80.8 and -91.6 ppm, attributable to Q3(3Al) and Q3(3Si), respectively, were also revealed, indicating Si-Al disorder. 1H MAS NMR revealed a main peak near 13.8 ppm (with a shoulder), and a group of weaker, partially resolved peaks near 15~16 ppm, and another weaker peak near 11 ppm for both samples. 2D 1H DQ-1Q MAS and 1H-29Si HETCOR NMR revealed that all these 29Si and 1H peaks belong to the same structure, and the 1H peaks near 15~16 and 11 ppm are correlated with the extra 29Si peaks/shoulders related to Si-Al disorder.
Interpretation of the NMR data was further facilitated by first-principles calculation. The calculation revealed that Si-Al exchange over T1-O-T2 is energetically most favorable and thus the most plausible model. The calculated 29Si and 1H chemical shifts for this model well reproduced all the main features in the experimental 1D and 2D NMR data, shedding light on their origin. It was revealed that Si-Al exchange over T1-O-T2 alters NNN of six adjacent T3/T4 sites, which are the origin of the observed extra 29Si NMR peaks/shoulders. The 1H NMR peaks near 15~16 and 11 ppm are due to OH sites that are bonded or hydrogen-bonded to these affected T3/T4 sites: the former due to 4 OH between two Si of the same NNN (Q3(1Si2Al) or Q3(2Si1Al)); the latter due to 1 OH in Q3(3Al)-OH…O-Q3(3Si). The relative intensities of both the 1H NMR and 1H-29Si CPMAS NMR spectra are consistent with ~3% Al-Si disorder over T1-O-T2.
The calculation revealed that 1H chemical shift is correlated with hydrogen bonding distance, which is in turn correlated with Si-O bond distance. This suggests that hydrogen bonding distance and Si/Al disorder are correlated via bond valence transmission. To my knowledge, this is the first study that has clearly revealed such a relationship. Work is in progress to extend to other minerals to gain a deeper understanding of the phenomenon.
Ussingite (Na2AlSi3O8OH) has a partially interrupted framework structure (Rossi et al. 1974; Williams & Weller 2012). There are four tetrahedral (T) sites: T1 & T2 are Q4 (i.e., having 4 next nearest neighbor (NNN) T sites), and T3 & T4 are Q3 (with 3 NNN T sites). Their NNN environments (in brackets) are: T1(1T2,1T3,2T4), T2(1T1,2T3,1T4), T3 (1T1,2T2), T4(2T1,1T2). Hydrogen is in T4-O8-H…O2-T3, with a short hydrogen-bonding distance. In the ordered structure, T1 is occupied by Al, and the other T sites by Si. Previous studies gave contradictory conclusions as to whether Al-Si disorder is present. I was able to unambiguously clarify the issue via combined NMR measurements and first-principles calculation.
Powdered samples of ussingite from two localities (one Lovozero alkali Massif, Russia, another uncertain, possibly Greenland) were examined by NMR using Bruker 9.4 T Avance NEO spectrometer. First-principles calculations were performed with the GIPAW method using the Quantum-ESPRESSO package (v.7.0). Both ordered structure and models for Si-Al disorder in ussingite were studied. For the latter, a 2x2x2 supercell (256 atoms) with one pair of Al-Si exchanged between a T1 site and each of its NNN (1T2, 1T3, 2T4), as well as between two remote T1 and T2 sites were examined. NMR calculations were performed on optimized crystal structure.
The 1H-29Si CPMAS NMR spectra of both ussingite samples revealed three main peaks near -84.2, -87.9 and -96.5 ppm, which correspond, respectively, to Q3(1Si2Al), Q3(2Si1Al) and Q4(3Si1Al), as expected for T4, T3 and T2 site in the ordered structure. However, all three peaks show splitting and/or shoulder, and additional weak peaks near -80.8 and -91.6 ppm, attributable to Q3(3Al) and Q3(3Si), respectively, were also revealed, indicating Si-Al disorder. 1H MAS NMR revealed a main peak near 13.8 ppm (with a shoulder), and a group of weaker, partially resolved peaks near 15~16 ppm, and another weaker peak near 11 ppm for both samples. 2D 1H DQ-1Q MAS and 1H-29Si HETCOR NMR revealed that all these 29Si and 1H peaks belong to the same structure, and the 1H peaks near 15~16 and 11 ppm are correlated with the extra 29Si peaks/shoulders related to Si-Al disorder.
Interpretation of the NMR data was further facilitated by first-principles calculation. The calculation revealed that Si-Al exchange over T1-O-T2 is energetically most favorable and thus the most plausible model. The calculated 29Si and 1H chemical shifts for this model well reproduced all the main features in the experimental 1D and 2D NMR data, shedding light on their origin. It was revealed that Si-Al exchange over T1-O-T2 alters NNN of six adjacent T3/T4 sites, which are the origin of the observed extra 29Si NMR peaks/shoulders. The 1H NMR peaks near 15~16 and 11 ppm are due to OH sites that are bonded or hydrogen-bonded to these affected T3/T4 sites: the former due to 4 OH between two Si of the same NNN (Q3(1Si2Al) or Q3(2Si1Al)); the latter due to 1 OH in Q3(3Al)-OH…O-Q3(3Si). The relative intensities of both the 1H NMR and 1H-29Si CPMAS NMR spectra are consistent with ~3% Al-Si disorder over T1-O-T2.
The calculation revealed that 1H chemical shift is correlated with hydrogen bonding distance, which is in turn correlated with Si-O bond distance. This suggests that hydrogen bonding distance and Si/Al disorder are correlated via bond valence transmission. To my knowledge, this is the first study that has clearly revealed such a relationship. Work is in progress to extend to other minerals to gain a deeper understanding of the phenomenon.