Nature of Si-Al disorder in natural and synthetic prehnite (Ca2Al2Si3O10(OH)2) clarified by multi-nuclear NMR and first-principles calculation

Abstract Prehnite (Ca2Al2Si3O10(OH)2) is a common mineral in low-grade (prehnite-pumpellyite facies) metamorphic rocks and hydrothermally altered igneous rocks. It has a unique crystal structure containing two types of tetrahedral sites, T1 (occupied by only Si) linked with two neighboring T2 tetrahedra and two AlO4(OH)2 octahedra, and T2 (occupied by equal Si and Al) linked to two other T2 tetrahedra in a helical chain and two T1 tetrahedra. Although most X-ray diffraction studies refined the crystal structure in space group P2cm or P2/n with an ordered alternating Si-Al distribution in the T2 chains, discrepancies have been noted between experimentally determined phase boundaries and those calculated thermodynamically, which have been discussed in terms of Si-Al disorder. We performed one- and two- dimensional 1H, 29Si and 27Al NMR measurements on natural and synthetic prehnite samples used in previous thermodynamic and experimental studies, and first-principles calculations to clarify the issue. Our 29Si NMR results revealed an extra peak near −103.1 ppm for Si in the T2 sites with one less Al next-nearest neighbors (NNN) (i.e., 3Si,1Al) than the main peak near −98.8 ppm for T2 sites (2Si, 2Al NNN), indicative of some local Si-Al disorder within the silicate chains violating the Al-avoidance principle for the synthetic prehnite, but not the natural prehnite samples. This suggests the synthetic samples may not have reached an equilibrium state, possibly due to slow kinetics at the relatively low experimental temperatures (400 and 545 °C). The data also revealed a pair of extra 29Si NMR peaks near −80.5 and −88.5 ppm attributable to Si in T1 sites with 2Al and 2Si NNN, respectively, in addition to a main peak near −84.6 ppm attributable to Si in T1 sites with 1Al and 1Si NNN as expected in an ordered structure, for all the samples. For the natural prehnite samples, these extra peaks may be explained by the presence of a different type of Si-Al disorder, i.e., Si-Al exchange in the T2 chains under the constraint of strict alternating Si-Al (chain disorder). Such a chain disorder was found by first-principles calculation to cause significantly less enthalpy increase than local Si-Al disorder that violates the Al-avoidance. It could be a common phenomenon for natural silicate minerals, but is rarely detectable for other minerals. Its detection for prehnite was rendered possible because this mineral has an unusual structure in which the local environments of the T1 sites are sensitive to chain disorder of the T2 chains. Our first-principles calculations reproduced the observed 1H, 29Si and 27Al NMR results well, and also revealed that the two ordered structures of prehnite with space groups P2cm and P2/n only differ slightly in enthalpy and NMR parameters. This is consistent with the previously reported coexistence of domains of both space groups in natural prehnite from various localities. The existence of two modifications with the same topology and ordered Si-Al distribution, differing only in how some of the equivalent Si and Al sites are distributed, is unique among silicate minerals.