K. A. Rozenberg

Crystal structure of surkhobite: New mineral from the family of titanosilicate micas

The crystal structure of surkhobite, a new mineral from Tajikistan, was determined by methods of X-ray diffraction analysis (a = 10.723 Å, b = 13.826 Å, c = 20.791 Å, β = 95°, sp. gr. C2, R = 0.047). The structure is a derivative of the bafertisite structure but differs from it and other representatives of the titanosilicate micas: three-layered slabs are connected to form isolated channels orderly filled with Ca, Na, Ba, and K atoms.

Crystal structure of a new representative of the cancrinite group with a 12-layer stacking sequence of tetrahedral rings

The crystal structure of a 12-layer tounkite-like mineral of the cancrinite group was determined for the first time by single-crystal X-ray diffraction analysis (the unit-cell parameters are a = 12.757 Å, c = 32.211 Å). The structure was refined in the space group P3 to R = 0.035 using 3834 reflections with | F| > 2σ(F). Si and Al atoms occupy tetrahedral framework positions in an ordered fashion. The average distances in the tetrahedra are 〈Si-O〉 = 1.611 Å and 〈Al-O〉 = 1.723 Å. The stacking sequence of the layers is described as CACACBCBCACB, where A, B, and C are six-membered rings arranged around the [2/3 1/3 z], [1/3 2/3 z], and [0 0 z] axes, respectively. In the structure of the mineral, the columns along the [0 0 z] axis are composed of cancrinite cages. The columns along the [2/3 1/3 z] and [1/3 2/3 z] axes contain alternating cancrinite, bystrite, and liottite cages.

Decationized and hydrated eudialytes. Oxonium problem

A collection of six decationized eudialytes from alkaline massifs of various regions was investigated by the single-crystal method. These samples differ from typical eudialytes by their low contents of alkaline and alkaline-earth cations and the high degree of hydration. The process of eudialyte decationization is accompanied by hydration, and hydrous species are incorporated into the mineral structure predominantly as H 3 O-groups. Those oxonium groups act as monovalent cations, replacing alkaline and alkaline-earth cations and compensating the positive charge deficiency. In all N -positions, H 3 O-groups replace large cations either partially or completely. Another specific feature of these samples is their low symmetry R 3, which is rare compared to R 3 m in eudialytes.

Crystal structure of golyshevite

The crystal structure of golyshevite, a new calcium-and carbon-rich representative of the eudialyte group, was established by single-crystal X-ray diffraction analysis (sp. gr. R3m, a = 14.231(3) Å, c = 29.984(8) Å, R = 0.062, 1643 reflections with F > 3σ(F)). The idealized formula of golyshevite is (Na10Ca3)Ca6Zr3Fe2SiNb[Si3O9]2[Si9O27]2(OH)3(CO3) · H2O. This mineral is characterized by the presence of calcium atoms both in the octahedral positions of six-membered rings and in extraframework positions, where calcium prevails. CO3 groups are present as the major additional anions. Carbon atoms randomly occupy two positions on the threefold z axis at a distance of 0.75 Å from each other and are coordinated by oxygen atoms arranged around the z axis.

Structural characteristics of a new cation-deficient representative of the labuntsovite group

The crystal structure of a new representative of the labuntsovite group from the Khibiny massif (the Kola Peninsula) has been refined. The unit-cell parameters are a = 14.298(7) Å, b = 13.816(7) Å, c = 7.792(3) Å, β = 116.85(5)°, V = 1373.3 Å3, sp. gr. C2/m, Raniso = 0.047, 1084 reflections with F > 4σ (F). The mineral differs from lemmleinite-Ba and other members of the group by the predominance of the vacancies in two key cationic positions—C position is occupied by Ba cations (37%) and D position is occupied mainly by Mn cations (47%).

Acentric niobium-rich analogue of Labuntsovite-Fe: Crystal structure and microtwinning

A new mineral of the labuntsovite group from Vuoriyarvi (North Karelia) was studied by the method of X-ray diffraction analysis. The unit-cell parameters are a = 14.450(6) Å, b = 13.910(6) Å, c = 7.836(4) Å, β = 117.42(1)°, V = 1398.1 Å3, sp. gr. Cm. The symmetry, unit-cell parameters, splitting of the A position, and ordered distribution of the cations over the B and C positions of the new mineral makes it similar to the zinc-rich analogue of labuntsovite. Unlike the zinc-rich analogue, the D position in the new representative of the labuntsovite group is dominated by Fe atoms when the position occupancy is lower. In addition, the new representative is characterized by high occupancies of the A and B positions and the ordered distribution of the Ti and Nb atoms over two octahedral positions. In both minerals, the A position is split into two positions, with one of the positions in both minerals being occupied by Na and the other position being occupied by K in the new mineral and Sr in the Zn-analogue.

Crystal structures of two new low-symmetry calcium-deficient analogs of eudialyte

The crystal structures of two new low-symmetry (sp. gr. R3) representatives of the eudialyte group from Mont Saint-Hilaire (Quebec, Canada) and the Lovozero massif (Kola Peninsula, Russia) were studied by single-crystal X-ray diffraction analysis and refined to R = 0.068 and 0.054 using 2899 reflections with F > 5σ(F) and 2927 reflections with F > 3σ(F), respectively. The idealized formulas of these representatives are Na13(Ca3Mn3)Zr3(Fe, Mn)3(□)(Si)[Si3O9]2[Si9O27]2(O, OH, Cl)3 · 2H2O and Na15(Ca3Mn3)Zr3(Fe, Zr)3(Si)(Si) · [Si3O9]2[Si9O27]2O2(OH, F, Cl)2 · 2H2O. Both minerals are analogs of oneillite and are characterized by a low Ca content. The distinguishing features of the mineral from Quebec are that the M(4) site is essentially vacant (>50%) and Ca atoms occupy one independent site in the six-membered ring, whereas another site is occupied by Mn along with a small impurity of Na. In the mineral from the Lovozero massif, both the M(3) and M(4) sites are occupied predominantly by silicon, while Ca atoms are distributed between both octahedral sites of the six-membered ring, one of these sites being occupied predominantly by Mn.