Lithiomarsturite, LiCa2Mn2Si5O14(OH)

Lithiomarsturite, ideally LiCa2Mn2Si5O14(OH), is a member of the pectolite–pyroxene series of pyroxenoids (hydropyroxenoids) and belongs to the rhodonite group. A previous structure determination of this mineral based on triclinic symmetry in space group P by Peacor et al. [Am. Mineral. (1990), 75, 409–414] converged with R = 0.18 without reporting any information on atomic coordinates and displacement parameters. The current study redetermines its structure from a natural specimen from the type locality (Foote mine, North Carolina) based on single-crystal X-ray diffraction data. The crystal structure of lithiomarsturite is characterized by ribbons of edge-sharing CaO6 and two types of MnO6 octahedra as well as chains of corner-sharing SiO4 tetrahedra, both extending along [110]. The octahedral ribbons are interconnected by the rather irregular CaO8 and LiO6 polyhedra through sharing corners and edges, forming layers parallel to (1), which are linked together by the silicate chains. Whereas the coordination environments of the Mn and Li cations can be compared to those of the corresponding cations in nambulite, the bonding situations of the Ca cations are more similar to those in babingtonite. In contrast to the hydrogen-bonding scheme in babingtonite, which has one O atom as the hydrogen-bond donor and a second O atom as the hydrogen-bond acceptor, our study shows that the situation is reversed in lithiomarsturite for the same two O atoms, as a consequence of the differences in the bonding environments around O atoms in the two minerals.

Lithiomarsturite, ideally LiCa 2 Mn 2 Si 5 O 14 (OH), is a member of the pectolite-pyroxene series of pyroxenoids (hydropyroxenoids) and belongs to the rhodonite group. A previous structure determination of this mineral based on triclinic symmetry in space group P1 by Peacor et al. [Am. Mineral. (1990), 75, 409-414] converged with R = 0.18 without reporting any information on atomic coordinates and displacement parameters. The current study redetermines its structure from a natural specimen from the type locality (Foote mine, North Carolina) based on single-crystal X-ray diffraction data. The crystal structure of lithiomarsturite is characterized by ribbons of edge-sharing CaO 6 and two types of MnO 6 octahedra as well as chains of corner-sharing SiO 4 tetrahedra, both extending along [110]. The octahedral ribbons are interconnected by the rather irregular CaO 8 and LiO 6 polyhedra through sharing corners and edges, forming layers parallel to (111), which are linked together by the silicate chains. Whereas the coordination environments of the Mn and Li cations can be compared to those of the corresponding cations in nambulite, the bonding situations of the Ca cations are more similar to those in babingtonite. In contrast to the hydrogen-bonding scheme in babingtonite, which has one O atom as the hydrogen-bond donor and a second O atom as the hydrogen-bond acceptor, our study shows that the situation is reversed in lithiomarsturite for the same two O atoms, as a consequence of the differences in the bonding environments around O atoms in the two minerals.
Lithiomarsturite is isotypic with nambulite. Its structure is characterized by ribbons of edge-sharing Ca1O 6 , Mn1O 6 and Mn2O 6 octahedra and chains of corner-sharing SiO 4 tetrahedra, both extending along [110] (Fig. 1). The octahedral ribbons are interconnected by the Ca2O 8 and Li1O 6 polyhedra through sharing corners and edges to form layers parallel to (1 1 1), which are linked together by the silicate chains. Whereas the Ca1, Mn1, and Mn2 octahedra are fairly regular, the  Narita et al., 1975). This difference can be ascribed mostly to the replacement of the significant amount of Li (43%) by Na in nambulite examined by Narita et al. (1975).
The two Ca polyhedra in lithiomarsturite are better compared to those in babingtonite (Tagai et al., 1990;Armbruster, 2000). The Ca1O 6 octahedra in lithiomarsturite, on the one hand, are much less distorted than those in babingtonite in terms of both the octahedral angle variance (OAV) and quadratic elongation (OQE) (Robinson et al., 1971), which are 146 and 1.040, respectively, for the former, and 311 and 1.092 for the latter. The greater distortion of the Ca1 octahedra in babingtonite is understandable, because they share edges with the Fe1O 6 and Fe2O 6 octahedra in the octahedral ribbons, supplementary materials sup-2 which are primarily occupied by cations with different sizes and charges (Fe 2+ and Fe 3+ , respectively). In contrast, both Mn1O 6 and Mn2O 6 octahedra that share edges with the Ca1O 6 octahedra in lithiomarsturite ( Fig. 1)  Very intriguingly, both previous neutron and X-ray diffraction studies (Tagai et al., 1990;Armbruster, 2000) have demonstrated that the hydrogen bonding in babingtonite is between O1 and O11, with the former as the H-donor and the latter as the H-acceptor. However, our study shows the opposite case for lithiomarsturite, in which O11 is the H-donor and O1 the H-acceptor. This difference is the direct result of the change in the bonding environments around O1 and O11 in the two minerals. In babingtonite, both O1 and O11 are bonded to two non-hydrogen cations, with O1 to Si1 and Ca1, and O11 to Si4 and Fe2, but in lithiomarsturite, O1 is bonded to three non-hydrogen cations (Si1, Ca1, and Li1), and O11 to two (Si4 and Mn2). Note that Fe2 in babingtonite is chiefly trivalent Fe 3+ , whereas Mn2 in lithiomarsturite is essentially divalent Mn 2+ . As a consequence of this coupled effect, the more underbonded O1 (relative to O11) in babingtonite becomes less underbonded in lithiomarsturite. Because the more underbonded O atom will be more tightly bonded to the H atom to better satisfy its bond-valence requirement, we see the change from O1 being the H-donor in babingtonite to O11 in lithiomarsturite. Accordingly, it is most likely that other members of the rhodonite group that contain Li or Na as an essential component, such as nambulite, natronambulite, and marsturite, may all behave as lithiomarsturite in terms of the hydrogen bonding scheme, with O11 being the H-donor and O1 the H-acceptor. For some specific chemical compositions, nevertheless, it may also be possible that the H atom is situated halfway between O1 and O11 or hops between the two positions.

Experimental
The lithiomarsturite sample used in this study is from the type locality: the Foote mine, Kings Mountains, North Carolina, USA, and is in the collection of the RRUFF project (deposition No. R100094; http://rruff.info). The chemical composition analyzed by Peacor et al. (1990) was adopted for the structure refinement. To keep consistent with the unit-cell settings for other minerals in the rhodonite group, such as rhodonite, babingtonite, and nambulite, we have adopted a unit-cell setting that differs from the one given by Peacor et al. (1990). The matrix for the transformation from the unit-cell setting of Peacor et al. (1990) to ours is [-1 0 0 / 1 1 0 / 0 0 -1]. The labeling scheme of the atoms in lithiomarsturite is similar to that for numbulite (Narita et al., 1975).

Refinement
The H atom was located from difference Fourier syntheses and its position refined freely with a fixed isotropic displacement (U iso = 0.04). During the structure refinements, Fe was treated as Mn, because of their similar X-ray scattering powers. The final refinement assumed an ideal chemistry for lithiomarsturite, as the overall effects of the trace amount of Mg on the final structure results are negligible. The highest residual peak in the difference Fourier maps was located at ( Fig. 1. Crystal structure of lithiomarsturite. The green tetrahedra represent SiO 4 groups. Large gray, medium yellow, and small bright blue sphares represent Ca2, Li1, H1 atoms, respectively. Both ribbons of edge-sharing octahedra and chains of vertex-sharing tetrahedra run parallel to [110]. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (17)