Pfaffenbergite & “phase 430”, new minerals crystallizing in nanorocks

The study of nanorocks (crystallized inclusions of anatectic melts) has been delivering in the last 15 years many intriguing novel insights into crustal anatectic processes at depth. From a mineralogical standpoint, the systematic use of micro-Raman Spectroscopy (MRS) on nanorocks has revealed that the mineral phases crystallizing from the melt in the inclusions are distinctive, for example, the feldspar polymorphs (kokchetavite, kumdykolite, svyatoslavite and dmisteinbergite) and the SiO₂ polymorphs cristobalite and tridymite (Wannhoff et al., 2022). In addition, MRS data collected on samples from numerous localities worldwide have revealed the presence in nanorocks of two novel crystalline phases, which are the primary focus of this study. Initially identified for their unique micro-Raman spectra, their crystal structures have been solved ab-initio and refined through three-dimensional electron diffraction (3DED) data, collected with a TEM.

The first novel phase is pfaffenbergite (Ferrero et al., 2024), KNa₃(Al₄Si₁₂)O₃₂, which has the composition of an unmixed binary K-Na feldspar, hence it can be regarded as a feldspar polymorph. Its micro-Raman spectrum is characterized by a very strong vibrational mode at 412 cm^-1 (hence its informal name of “phase 412”,  e.g. Borghini et al., 2024), two secondary peaks at 105 cm^-1 and 832 cm^-1 and two weaker peaks at 130 cm^-1 and 470 cm^-1. 3DED revealed that pfaffenbergite is a hexagonal mineral, crystallizing in space group P6/mcc. This mineral can be classified as a sheet silicate and it is isostructural with kokchetavite (KAlSi₃O₈) and wodegongjieite (KCa₃(Al₇Si₉)O₃₂). The latter is a mineral recently found as crystallization product of melt in inclusions within corundum from chromitites in the Luobusa ophiolite (Tibet, China; Mugnaioli et al., 2022), and subsequently with pfaffenbergite in garnet from the Saxony Granulite Complex.

The second novel mineral has not yet been approved by the IMA CNMNC and it is being called provisionally “phase 430” from its main micro-Raman vibrational mode. Three secondary peaks are present at 186 cm^-1, 264 cm^-1 and 292 cm^-1, along with two weaker peaks at 485 cm^-1 and 823 cm^-1. Our results show that this phase has the ideal formula KK₂Na₃(Al₆Si₃₆)O₈₄ and crystallizes in the P6/mcc space group. The arrangement of tetrahedral sites (Si,Al) in “phase 430”  resembles that of frameworks in feldspathoids, but with a topology not yet reported as far as we know.

We interpret pfaffenbergite, wodegongjieite and “phase 430” as metastable phases crystallizing rapidly in a silicate melt enclosed in a small pore under non-equilibrium conditions. The increasing number of recent findings of metastable phases suggests that these minerals are more common than expected. It is even possible that they may represent rock-forming minerals in natural rocks which experienced rapid cooling/rapid crystallization, for instance lavas and ignimbrites, along with experimental products involving silicate melts.

References

Borghini et al. (2024) Eur J Mineral 36, 279–300

Ferrero S et al (2024) IMA 2023-105, in CNMNC Newsletter 78, Eur J Mineral 36

Mugnaioli E et al (2022) Min Mag 1–13

Wannhoff I et al (2022) Am Min 107, 2315–2319