TY - JOUR

T1 - Crystal structure of nyerereite: A possible messenger from the deep Earth

AU - Zucchini, Azzurra

AU - Gavryushkin, Pavel N.

AU - Golovin, Alexander V.

AU - Bolotina, Nadezhda B.

AU - Stabile, Paola

AU - Carroll, Michael R.

AU - Comodi, Paola

AU - Frondini, Francesco

AU - Morgavi, Daniele

AU - Perugini, Diego

AU - Arzilli, Fabio

AU - Cherin, Marco

AU - Kazimoto, Emmanuel

AU - Kokh, Konstantin

AU - Kuznetsov, Artem

AU - Medrish, Inna V.

N1 - Funding Information: The experiments were performed thanks to the financial program of the Department of Physics and Geology of the University of Perugia (“Fondo ricerca di Base 2018”; Principal Investigator AZ) and MIUR (project no. PRIN2017-2017LMN-LAW “Connect4Carbon”). P.N.G., A.V.G., K.K., and A.K. were supported by the state assignment project of Sobolev Institute of Geology and Mineralogy SB RAS. M.C. thanks all the members of the Italian School of Paleoanthropology and of the Tanzania Human Origins Research (THOR) project (www.thorproject. it). M.R.C. acknowledges laboratory support from PRIN2017-2017J277S9 and PRIN2020-202037YPCZ. Publisher Copyright: © 2022 by Mineralogical Society of America.

PY - 2022/11/1

Y1 - 2022/11/1

N2 - Carbonates in the system Na2CO3-CaCO3 are nowadays suggested as having a wide stability field at conditions of the mantle transition zone. Our structural analysis of nyerereite, which has limited stability fields at ambient conditions, and its similarities with already known carbonates that are stable at high-pressure conditions, allowed us to propose that nyerereite likely undergoes phase transitions at both high-pressure and high-temperature conditions. This supports the hypothesis that nyerereite takes part in carbon transportation from the mantle/deep crust toward the surface, with important implications for the deep carbon cycle associated with carbonatites. K-free nyerereite [Na2Ca(CO3)2] was synthesized both at hydrothermal conditions and from the melt. The structure of nyerereite was refined as a three-component twinned structure in the centrosymmetric Pbca space group with ratios of the three twinning components 0.221(3):0.287(3):0.492(3). Twinning at micro- and nano-level can introduce some minor structural deformations that influence the likely occurrence of the inversion center as one of the symmetry elements in the nyerereite structure. Based on the automated topological algorithms, we show that nyerereite has a unique crystal structure, not having analogs among the known structures, except for the structure with a similar composition K2Ca(CO3)2 fairchildite. A comparison between the centrosymmetric Pbca nyerereite structure and that of aragonite (CaCO3, Pmcn space group) reveals two main scenarios for the high-pressure form of Na2Ca(CO3)2: (1) polysomatic relations as the interlayering of the high-pressure polymorph Na2CO3 and CaCO3-aragonite, and (2) high-pressure structure with ninefold-coordinated Na and Ca sites resembling that of aragonite. Our discussion heightens the interest in the high-pressure behavior of the nyerereite structure and strengthens the hypothesis about the possibility for nyerereite to be stable at high-pressure/high-temperature conditions.

AB - Carbonates in the system Na2CO3-CaCO3 are nowadays suggested as having a wide stability field at conditions of the mantle transition zone. Our structural analysis of nyerereite, which has limited stability fields at ambient conditions, and its similarities with already known carbonates that are stable at high-pressure conditions, allowed us to propose that nyerereite likely undergoes phase transitions at both high-pressure and high-temperature conditions. This supports the hypothesis that nyerereite takes part in carbon transportation from the mantle/deep crust toward the surface, with important implications for the deep carbon cycle associated with carbonatites. K-free nyerereite [Na2Ca(CO3)2] was synthesized both at hydrothermal conditions and from the melt. The structure of nyerereite was refined as a three-component twinned structure in the centrosymmetric Pbca space group with ratios of the three twinning components 0.221(3):0.287(3):0.492(3). Twinning at micro- and nano-level can introduce some minor structural deformations that influence the likely occurrence of the inversion center as one of the symmetry elements in the nyerereite structure. Based on the automated topological algorithms, we show that nyerereite has a unique crystal structure, not having analogs among the known structures, except for the structure with a similar composition K2Ca(CO3)2 fairchildite. A comparison between the centrosymmetric Pbca nyerereite structure and that of aragonite (CaCO3, Pmcn space group) reveals two main scenarios for the high-pressure form of Na2Ca(CO3)2: (1) polysomatic relations as the interlayering of the high-pressure polymorph Na2CO3 and CaCO3-aragonite, and (2) high-pressure structure with ninefold-coordinated Na and Ca sites resembling that of aragonite. Our discussion heightens the interest in the high-pressure behavior of the nyerereite structure and strengthens the hypothesis about the possibility for nyerereite to be stable at high-pressure/high-temperature conditions.

KW - alkali-carbonates

KW - hydrothermal synthesis

KW - melt crystallization

KW - Nyerereite

KW - Raman spectroscopy

KW - single-crystal X-ray diffraction

UR - http://www.scopus.com/inward/record.url?scp=85132724115&partnerID=8YFLogxK

U2 - 10.2138/am-2022-8106

DO - 10.2138/am-2022-8106

M3 - Article

AN - SCOPUS:85132724115

VL - 107

SP - 2054

EP - 2064

JO - American Mineralogist

JF - American Mineralogist

SN - 0003-004X

IS - 11

ER -