The crystal structure of manganoan kilchoanite, Ca_2.33Mn_0.67Si₂O₇: a site-preference rule for the substitution of Mn for Ca

Abstract

The crystal structure of a Mn-bearin~ kilchoanite, Ca2.33Mn0.67Si2OT, orthorhombic, I2cm, a = 11.356(2), b = 5.007(1), c = 21.817(1)A, Z = 8, was refined using single-crystal X-ray data to an R value of 3.1% for 1725 unique reflections. It is isostructural with kilchoanite, Ca3Si20 > and shows a partial disorder of Mn and Ca over the four octahedral sites. On a basis of occupancy refinement, the M3 site ((M3-O} = 2.307 ~) contains 0.508Ca and 0.492Mn, and the M4 site ((M4-O) = 2.346 ~) contains 0.822Ca and 0.178Mn, whereas the M1 and M2 sites (<M1-O} = 2.538 ~, (M2-O) = 2.380 A) are occupied only by Ca. Substitution of Mn cations for Ca can be interpreted as taking place on the following sites in preferential order: (1) the smaller Ca site; (2) the less distorted Ca site; (3) the Ca site showing a loss in neutrality of formal electrostatic valence. A statistical survey of Si-O-Si angles observed in silicates containing [Si207] groups confirms that these angles become smaller with an increase in the coordination number of the bridging oxygen. Oversaturation of electrostatic bondstrength to the bridging oxygen in some groups of the crystal structures with two tetrahedra sharing a vertex (pyro-anion, T20~-), which is caused by increase in the coordination number of the bridging oxygen and/or by T cations with larger charge than 4+, results in longer T-Obr bonds length and in smaller T-Ob~-T angles.K E Y w 0 R D S: kilchoanite, manganese, crystal structure, site preference.

Pavlovskyite Ca8(SiO4)2(Si3O10): A new mineral of altered silicate-carbonate xenoliths from the two Russian type localities, Birkhin massif, Baikal Lake area and Upper Chegem caldera, North Caucasus

“…The corresponding site to M1 in olivine is Ca2 in kilchoanite, whereas M2 in olivine corresponds to Ca3 in kilchoanite. Within the kilchoanite structure the preference is opposite; Ca3 has the strongest Mn 2+ selectivity and there is no Mn 2+ at Ca2 (Kimata 1989). It seems that the different Mn 2+ order in kilchoanite compared to glaucochroite is due to strain reduction in the more complex kilchoanite structure.…”

“…A surprising find is the distribution of Mn 2+ in synthetic Mn-bearing kilchoanite (Kimata 1989). In olivine-group glaucochroite CaMn(SiO 4 ), Mn 2+ prefers the more symmetric M1 site, whereas Ca occupies M2 (Lager and Meagher 1978).…”

“…β 1.649(2) Cell dimensions (this study) Refractive indices(Agrell and Gay 1961) optical orientation (?) in analogy to pavlovskyite synthetic Ca 4.66 Mn 1.33 (SiO 4 )(Si 3 O 11 )(Kimata 1989) a = 11.356(2) b = 5.007(1) c = 21.817(1) Note: Questionable optic orientations are indicated by "? ".…”

Crystal structures of Ca5Ge3O11 and wollastonite-type GaGeO3

“…2). taining about 25% Si only), and no simple structural relationship could be established with other silicates of neighbouring compositions such as the rankinite and kilchoanite polymorphs of Ca^SiiOv (Saburi, Kusachi, Henmi, Kawahara, Henmi and Kawada, 1976;Taylor, 1971;Kimata, 1989). Two types of coordination are found for the Ca sites of the Ca5Ge30n structure: the Ca2, Ca21 and Ca3 atoms occupy distorted octahedral sites forming a three-dimensional framework via comer-sharing with the Ge04 tetrahedra (cf.…”

Calcium silicate hydrates investigated by solid‐state high resolution 1H and 29Si nuclear magnetic resonance

“…We are then able to predict isotropic chemical shifts for the 29 Si-NMR between − 75 and − 92 ppm if this phase is part of a mixture [25]. [32]. The simulated structure (by CaRIne) shows calcium and SiO 4 tetrahedrons layers (Fig.…”