Pyrosmalite-(Fe), Fe₈Si₆O₁₅(OH,Cl)₁₀

Pyrosmalite-(Fe), ideally Fe^II₈Si₆O₁₅(OH,Cl)₁₀ [refined composition in this study: Fe₈Si₆O₁₅(OH_0.814Cl_0.186)₁₀·0.45H₂O, octa­iron(II) hexa­silicate deca­(chloride/hydroxide) 0.45-hydrate], is a phyllosilicate mineral and a member of the pyrosmalite series (Fe,Mn)₈Si₆O₁₅(OH,Cl)₁₀, which includes pyrosmalite-(Mn), as well as friedelite and mcgillite, two polytypes of pyrosmalite-(Mn). This study presents the first structure determination of pyrosmalite-(Fe) based on single-crystal X-ray diffraction data from a natural sample from Burguillos del Cerro, Badajos, Spain. Pyrosmalite-(Fe) is isotypic with pyrosmalite-(Mn) and its structure is characterized by a stacking of brucite-type layers of FeO₆-octa­hedra alternating with sheets of SiO₄ tetra­hedra along [001]. These sheets consist of 12-, six- and four-membered rings of tetra­hedra in a 1:2:3 ratio. In contrast to previous studies on pyrosmalite-(Mn), which all assumed that Cl and one of the four OH-groups occupy the same site, our data on pyrosmalite-(Fe) revealed a split-site structure model with Cl and OH occupying distinct sites. Furthermore, our study appears to suggest the presence of disordered structural water in pyrosmalite-(Fe), consistent with infrared spectroscopic data measured from the same sample. Weak hydrogen bonding between the ordered OH-groups that are part of the brucite-type layers and the terminal silicate O atoms is present.

Lotharmeyerite, Ca(Zn,Mn)₂(AsO₄)₂(H₂O,OH)₂

Lotharmeyerite, calcium bis­(zinc/manganese) bis­(arsenate) bis­(hydroxide/hydrate), Ca(Zn,Mn³⁺)₂(AsO₄)₂(H₂O,OH)₂, is a member of the natrochalcite group of minerals, which are characterized by the general formula AM₂(XO₄)₂(H₂O,OH)₂, where A may be occupied by Pb²⁺, Ca²⁺, Na⁺, and Bi³⁺, M by Fe³⁺, Mn³⁺, Cu²⁺, Zn²⁺, Co²⁺, Ni²⁺, Al³⁺, and Mg²⁺, and X by P^V, As^V, V^V, and S^VI. The minerals in the group display either monoclinic or triclinic symmetry, depending on the ordering of chemical components in the M site. Based on single-crystal X-ray diffraction data of a sample from the type locality, Mapimi, Durango, Mexico, this study presents the first structure determination of lotharmeyerite. Lotharmeyerite is isostructural with natrochalcite and tsumcorite. The structure is composed of rutile-type chains of edge-shared MO₆ octa­hedra (site symmetry ) extending along [010], which are inter­connected by XO₄ tetra­hedra (site symmetry 2) and hydrogen bonds to form [M₂(XO₄)₂(OH,H₂O)₂] sheets parallel to (001). These sheets are linked by the larger A cations (site symmetry 2/m), as well as by hydrogen bonds. Bond-valence sums for the M cation, calculated with the parameters for Mn³⁺ and Mn²⁺ are 2.72 and 2.94 v.u., respectively, consistent with the occupation of the M site by Mn³⁺. Two distinct hydrogen bonds are present, one with O...O = 2.610 (4) Å and the other O...O = 2.595 (3) Å. One of the H-atom positions is disordered over two sites with 50% occupancy, in agreement with observations for other natrochalcite-type minerals, such as natrochalcite and tsumcorite.

Redetermination of kovdorskite, Mg₂PO₄(OH)·3H₂O

The crystal structure of kovdorskite, ideally Mg₂PO₄(OH)·3H₂O (dimagnesium phosphate hydroxide trihydrate), was reported previously with isotropic displacement paramaters only and without H-atom positions [Ovchinnikov et al. (1980). Dokl. Akad. Nauk SSSR. 255, 351–354]. In this study, the kovdorskite structure is redetermined based on single-crystal X-ray diffraction data from a sample from the type locality, the Kovdor massif, Kola Peninsula, Russia, with anisotropic displacement parameters for all non-H atoms, with all H-atom located and with higher precision. Moreover, inconsistencies of the previously published structural data with respect to reported and calculated X-ray powder patterns are also discussed. The structure of kovdorskite contains a set of four edge-sharing MgO₆ octa­hedra inter­connected by PO₄ tetra­hedra and O—H...O hydrogen bonds, forming columns and channels parallel to [001]. The hydrogen-bonding system in kovdorskite is formed through the water mol­ecules, with the OH⁻ ions contributing little, if any, to the system, as indicated by the long H...A distances (>2.50 Å) to the nearest O atoms. The hydrogen-bond lengths determined from the structure refinement agree well with Raman spectroscopic data.

High-pressure synthetic (Na_0.97Mg_0.03)(Mg_0.43Fe_0.17³⁺Si_0.40)Si₂O₆, with six-coordinated silicon, isostructural with P2/n omphacite

The title compound, (sodium magnesium) [magnesium iron(III) silicon] disilicate, (Na_0.97Mg_0.03)(Mg_0.43Fe_0.17³⁺Si_0.40)Si₂O₆, is isotypic with ordered P2/n omphacite. Its structure is characterized by single chains of corner-sharing SiO₄ tetra­hedra, extending along the c axis, which are crosslinked by bands of edge-sharing octa­hedra (site symmetry 2), statistically occupied by (Mg²⁺ + Fe³⁺ + Si⁴⁺). Between the bands built up of the octahedra are two non-equivalent highly distorted six-coordinated sites (site symmetry 2), statistically occupied by (Na + Mg). In contrast to omphacites, the great differences in size and charge between Mg²⁺ and Si⁴⁺ result in complete, rather than partial, ordering of Mg and Si into two distinct octa­hedral sites, whereas Fe³⁺ is disordered between the two sites. The octa­hedron filled by (Mg + Fe) is larger and markedly more distorted than that occupied by (Si + Fe). The average (Mg + Fe)-O and (^VISi + Fe)-O bond lengths are 2.075 and 1.850 Å, respectively.

Lithio­tantite, ideally LiTa₃O₈

Lithio­tantite (lithium tritantalum octa­oxide) and lithio­wodg­inite are natural dimorphs of LiTa₃O₈, corresponding to the laboratory-synthesized L-LiTa₃O₈ (low-temperature form) and M-LiTa₃O₈ (inter­mediate-temperature form) phases, respectively. Based on single-crystal X-ray diffraction data, this study presents the first structure determination of lithio­tantite from a new locality, the Murundu mine, Jenipapo District, Itinga, Minas Gerais, Brazil. Lithio­tantite is isotypic with LiNb₃O₈ and its structure is composed of a slightly distorted hexa­gonal close-packed array of O atoms stacked in the [-101] direction, with the metal atoms occupying half of the octa­hedral sites. There are four symmetrically non-equivalent cation sites, with three of them occupied mainly by (Ta⁵⁺ + Nb⁵⁺) and one by Li⁺. The four distinct octa­hedra share edges, forming two types of zigzag chains (A and B) extending along the b axis. The A chains are built exclusively of (Ta,Nb)O₆ octa­hedra (M1 and M2), whereas the B chains consist of alternating (Ta,Nb)O₆ and LiO₆ octa­hedra (M3 and M4, respectively). The average M1-O, M2-O, M3-O and M4-O bond lengths are 2.011, 2.004, 1.984, and 2.188 Å, respectively. Among the four octa­hedra, M3 is the least distorted and M4 the most. The refined Ta contents at the M1, M2 and M3 sites are 0.641 (2), 0.665 (2), and 0.874 (2), respectively, indicating a strong preference of Ta⁵⁺ for M3 in the B chain. The refined composition of the crystal investigated is Li_0.96Mn_0.03Na_0.01Nb_0.82Ta_2.18O₈.

Nioboaeschynite-(Ce), Ce(NbTi)O₆

Nioboaeschynite-(Ce), ideally Ce(NbTi)O₆ [cerium(III) niobium(V) titanium(IV) hexa­oxide; refined formula of the natural sample is Ca_0.25Ce_0.79(Nb_1.14Ti_0.86)O₆], belongs to the aeschynite mineral group which is characterized by the general formula AB₂(O,OH)₆, where eight-coordinated A is a rare earth element, Ca, Th or Fe, and six-coordinated B is Ti, Nb, Ta or W. The general structural feature of nioboaeschynite-(Ce) resembles that of the other members of the aeschynite group. It is characterized by edge-sharing dimers of [(Nb,Ti)O₆] octa­hedra which share corners to form a three-dimensional framework, with the A sites located in channels parallel to the b axis. The average A—O and B—O bond lengths in nioboaeschynite-(Ce) are 2.471 and 1.993 Å, respectively. Moreover, another eight-coordinated site, designated as the C site, is also located in the channels and is partially occupied by A-type cations. Additionally, the refinement revealed a splitting of the A site, with Ca displaced slightly from Ce (0.266 Å apart), presumably resulting from the crystal-chemical differences between the Ce³⁺ and Ca²⁺ cations.

Redetermination of junitoite, CaZn₂Si₂O₇·H₂O

The crystal structure of the mineral junitoite, ideally CaZn₂Si₂O₇·H₂O (calcium dizinc disilicate monohydrate), was first determined by Hamilton & Finney [Mineral. Mag. (1985), 49, 91-95] based on the space group Ama2, yielding a reliability factor R of 0.10, with isotropic displacement parameters for all non-H atoms. The present study reports a structure redetermination of junitoite using single-crystal X-ray diffraction data from a natural sample, demonstrating that the space group of this mineral is actually Aea2, which can be attained simply by shifting the origin. Topologically, the structure models in the space groups Aea2 and Ama2 are analogous, consisting of chains of corner-sharing ZnO₄ tetra­hedra parallel to the b axis, cross-linked by Si₂O₇ tetra­hedral dimers (the site symmetry of the bridging O atom is ..2) along a and c, forming a three-dimensional framework. The Ca²⁺ cations (site symmetry ..2) are situated in cavities of the framework and are bonded to five O atoms and one H₂O mol­ecule (site symmetry ..2) in a distorted octa­hedral coordination environment. However, some bond lengths, especially for the SiO₄ tetra­hedron, are noticeably different between the two structure models. Hydrogen bonding in junitoite is found between the water mol­ecule and a framework O atom.

Robertsite, Ca₂Mn^III₃O₂(PO₄)₃·3H₂O

Robertsite, ideally Ca₂Mn₃O₂(PO₄)₃·3H₂O [calcium manganese(III) tris­(orthophosphate) trihydrate], can be associated with the arseniosiderite structural group characterized by the general formula Ca₂A₃O₂(TO₄)₃·nH₂O, with A = Fe, Mn; T = As, P; and n = 2 or 3. In this study, single-crystal X-ray diffraction data were used to determine the robertsite structure from a twinned crystal from the type locality, the Tip Top mine, Custer County, South Dakota, USA, and to refine anisotropic displacement parameters for all atoms. The general structural feature of robertsite resembles that of the other two members of the arseniosiderite group, the structures of which have previously been reported. It is characterized by sheets of [MnO₆] octa­hedra in the form of nine-membered pseudo-trigonal rings. Located at the center of each nine-membered ring is a PO₄ tetra­hedron, and the other eight PO₄ tetra­hedra sandwich the Mn-oxide sheets. The six different Ca²⁺ ions are seven-coordinated in form of distorted penta­gonal bipyramids, [CaO₅(H₂O)₂], if Ca-O distances less than 2.85 Å are considered. Along with hydrogen bonding involving the water mol­ecules, they hold the manganese-phosphate sheets together. All nine [MnO₆] octa­hedra are distorted by the Jahn-Teller effect.

Redetermination of clinobaryl­ite, BaBe₂Si₂O₇

Clinobaryl­ite, ideally BaBe₂Si₂O₇ (chemical name barium diberyllium disilicate), is a sorosilicate mineral and dimorphic with baryl­ite. It belongs to a group of compounds characterized by the general formula BaM²⁺₂Si₂O₇, with M²⁺ = Be, Mg, Fe, Mn, Zn, Co, or Cu, among which the Be-, Fe-, and Cu-members have been found in nature. The crystal structure of clinobaryl­ite has been re-examined in this study based on single-crystal X-ray diffraction data collected from a natural sample from the type locality (Khibiny Massif, Kola Peninsula, Russia). The structure of clinobaryl­ite can be considered as a framework of BeO₄ and SiO₄ tetra­hedra, with one of the O atoms coordinated to two Be and one Si, one coordinated to two Si, and two O atoms coordinated to one Si and one Be atom. The BeO₄ tetra­hedra share corners, forming chains parallel to the c axis, which are inter­linked by the Si₂O₇ units oriented parallel to the a axis. The Ba²⁺ cations (site symmetry m..) are in the framework channels and are coordinated by eleven O atoms in form of an irregular polyhedron. The Si-O_br (bridging O atom, at site symmetry m..) bond length, the Si-O_nbr (non-bridging O atoms) bond lengths, and the Si-O-Si angle within the Si₂O₇ unit are in marked contrast to the corresponding values determined in the previous study [Krivovichev et al. (2004). N. Jb. Miner. Mh. pp. 373-384].