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Improvement of the crystal structural model of tinaksite
No full textTinaksite from ultralkaline agpaitic Murun massif (Russia) has been
investigated. The name reflects its composition: titanium (Ti), sodium
(Na), potassium (K) and silicon (Si). Its structure was reported for the
first time in 1971 by Petrunina [1]. To the best of author's knowledge,
tinaksite is the only silicate based on {hB, 21 }[3Si6O17(SiO2)] hybrid
anion [2]. The structural model of Tinaksite proposed by Petrunina [1]
was validated and improved by von Bissert [3] in 1980, who observed a
more regular tetrahedra conformation. The tinaksite investigated by von
Bissert [3] is triclinic with chemical composition: NaK2Ca2Ti[3Si7O19]OH. The
compound here investigated has chemistry more complex, ((Na0.855K1.993Ca
2.008Ti0.792Mg0.083Fe3+0.096Mn0.100Zr0.027Sr0.013Zr0.027Zn0.008Ni0.007Cr0.005)[3Si7O19]OH),
compared with Petrunina's [1] and von Bissert's [3] ones. It has been
refined in P1, using the following lattice parameters: a = 7.0565(1) Å, b =
10.3750(1) Å, c = 12.1885(2) Å, = 92.802(1)°, = 90.763(1)°, =
99.241(1)°.
In this work, the cation partition derived from SCXRD and EPMA data, is
presented. The structure has been anisotropically refined up to an R(F) =
0.025 (for 5238 observed reflections with Fo > (Fo)). Tinaksite exhibits
straight channels parallel to [001] direction, formed by double chains of
silicate. The channels are stretched approximately along [110] direction,
with maximum dimension 10.972(2) Å.
The highest peak in the Fourier difference map is related to some
structural disorder inside the Ti octahedron. This disorder likely can be
ascribed to the entrance of the large K substituting the six-fold
coordinated Na atom. As a matter of fact, the replacing of Na for K
changes the coordination number of the polyhedron from 6 to 7, which
seems to influence the neighbouring Ti atom position.
Another point concerns the K1 and K2 atoms located inside the channels.
According to von Bissert [3], the higher K1 thermal parameter with
respect to the K2 one indicates the site filled by K1 is disorder. The structural refinement shows that K1 atom is located over at least three
different positions. A charge distribution analysis, performed by means of
the Chardi-It software [4], has returned an ECON number [5] of 11 and 9
for the K1 and K2, respectively. The bond distances for the three K atoms,
related to K1, show these atoms are not always bonded to the same
oxygens. Indeed, K1 can move inside a cage formed by 14 oxygen atoms
with a volume of about 90 Å3. This volume results to be much larger even
than the one of the ideal twelve-fold coordinated K atom (~18 Å3)
Spectroscopy and crystal chemical properties of NaCa2[Si4O10]F natural agrellite with tubular structure
No full textAgrellite is a rare inosilicate, having a crystal structure characterized by SiO4-tetrahedral tubes located between continuous wall layers formed by edge-sharing Ca-polyhedra. A detailed crystal chemical and physical study of agrellite specimens is carried out by means of electron probe microanalysis, Fourier transform infrared spectroscopy, electron-paramagnetic resonance, and single crystal X-ray diffraction. Additionally, the electronic structures of agrellite was calculated. Luminescence due to 5d-4f transition in Ce3+ ions is observed in both investigated samples. EPR analysis points out the Mn2+ replaces Ca2+ ion in Ca(1A) and Ca(2B) positions, coordinated by two F sites
Reappraisal of the crystal structure of Tinaksite from Murun massif (Russia): cation partition andstructural disorder
No full textTinaksite is a widespread mineral in the rocks of charoite
complex of the Murun massif (Russia). The structure of
tinaksite was reported for the first time in 1971 by Petranunia
[1]. Until now, it is the only one silicate known based on a
hybrid anion. According to Libeau [2], the latter can be
described briefly by {hB, 21
}[3Si6O17(SiO2)], which represents
the joint of an unbranched dreier single chain with a loopbranched
dreier single chain. In 1980 von Bissert [3] confirmed
the general structure architecture of tinaksite proposed by
Petranunia [1], but found a more regular tetrahedral
conformation. According to von Bissert [3], tinaksite is triclinic
with the following chemical composition: NaK2Ca2Ti
[3Si7O19]OH. The compound here investigated has been solved
in P1, a=7.0565(1) Å, b=10.3750(1) Å, c=12.1885(2) Å,
92.802(1)°, , but differently
from those studied by Petranunia [1] and von Bissert [3], it
shows a more complex chemistry ((Na0.855K1.993Ca2.008Ti0.792
Mg0.082Fe3+
0.142Mn0.100Ba0.006Sr0.006Zr0.007Zn0.004Ni0.003Cr0.002)
[3Si7 O19]OH) and a more interesting structural details.
In this work, the cation partition, derived by means of
SCXRD and EMPA measurements, is presented. The structure
has been anisotropically refined to a R(F) = 0.025 (for 5238
observed reflections with Fo > 3(Fo)). The highest peak in the
Fourier difference map seems due to some structural disorder
inside the Ti octahedron. This disorder likely is related to the
entrance of the large K in place of six-fold coordinated Na
atom. The replacing of Na by K seems to change the
coordination number of the polyhedron from 6 to 7, so
perturbing the neighbouring Ti atom position.
Another point of interest concerns the K1 and K2 potassium
atom located inside the system of one-dimensional channels of
the structure, formed by double chains of silicate. According to
von Bissert [3], the higher K1 thermal parameters with respect
to the K2 one indicates the disorder on K1 site. The structural
refinement shows that in the first case K occupies at least three
different positions. A charge distribution analysis, performed by
means of the Chardi-It software [4], has returned an ECON
number [5] of 11 and 9 for the K1 and K2 site respectively. The
bond distances for the three K atoms concerning K1 show these
atoms are not always bonded to the same oxygens. In fact, K
can move inside a cage formed by 14 oxygen atoms with a
volume of about 90 Å3. This volume results to be much larger
than of one assigned to K atom
Structural disorder in tokkoite from Murun massif (Russia)
No full textTokkiote is a new mineral discovered in 1986 by Lazebnik et al. [1] on Murun massif (Russia). It
belongs to group of alkali-calcium silicates containing hydroxyl and fluorine anions. The SCXRD
investigation on this mineral dates back to 1989 [2]. Tokkoite is triclinic with general formula
K2a4(Si7O18)(OH,F)2
[2] and is a Ca-bearing compound isostructural to tinaksite [3]; both minerals
contain the rare [Si7O18(OH)]9- silicate radical.
In this work a re-appraisal of tokkoite structure was undertaken and a cation partition, obtained by
means of SCXRD and EPMA measurements, is derived.
The structure has been refined in space group P1 (R1 = 4.45%), using the following lattice
parameters: a = 10.4222(5) Å, b = 12.5023(6) Å, c = 7.1146(3) Å, = 89.904(2)°, = 99.714(2)°,
= 92.979(2)°.
The structure consists of layers of calcium octahedra interconnected along (100), between which
the silicate-oxygen chains are located. K atoms are located inside the curved shape channels
formed by silicon chains.
Structural refinement showed that Si-tetrahedra are slightly distorted, but more regular if
compared to [2]. Ca(1), Ca(3) and Ca(4) are octahedrally coordinated and appear more distorted
than [2], with = 2.199, = 2.368, = 2.371, while in Ca(2)
polyhedron Ca-ion seems to be bond to seven oxygen with = 2.860 Å.
The highest peak (3.56 e/Å3) in the Fourier difference map is related to structural disorder at the
Ca(1) octahedron, which could be due to a changing of the coordination number from 6 to 7 of
Ca(2) polyhedron, which shares two edges with Ca(1) octahedron.
According to [2], one of two potassium atoms in the unit cell is weakly linked and evidences some
positional disorder not discussed by [2]. Structural refinement shows that K1 atom is distributed
among at least three different positions having occupancy 0.658, 0.211 and 0.122, respectively.
Similar behaviors were been observed also in tinaksite structure [4]
Crystal structure and crystal chemistry of a rare Mn-Fe-Mg orthopyroxene from Baikal, Eastern Siberia
No full textRefinement of the Crystal Structure of Vlasovite from Burpala Massif (Russia)
No full textThe structural model and chemical composition of vlasovite from the Burpala massif (Russia) have been determined using X-ray diffraction methods and electron probe microanalysis. The crystal structure has been refined in the sp. gr. Cc to R = 1.4% (R w = 1.8%) with the following unit-cell parameters: a = 11.0396(3) Å, b = 10.1042(2) Å, c = 8.5696(2) Å, β = 100.307(1)°, V = 940.48(4) Å 3 , and Z = 4
Armstrongite from Khan Bogdo (Mongolia): Crystal structure determination and implications for zeolite-like cation exchange properties
No full textThe results of a combined electron probe microanalysis, single-crystal X‐ray diffraction, and Fourier transform infrared study of a crystal of armstrongite from Khan Bogdo deposit (Gobi, Mongolia) are reported. Major element analysis provided (wt%): CaO 9.2(1), ZrO2 20.9(2), and SiO2 62.5(2). Significant concentrations of REE (0.45 wt%) were also detected. From single-crystal structural refinement, armstrongite resulted monoclinic [space group C2/m, a = 14.0178(7), b = 14.1289(6), c = 7.8366(3) Å, b = 109.436(3)°, V = 1463.6(1) Å3, Z = 4] and twinned with two individuals rotated around a twin twofold axis parallel to [100]. The analyzed crystal was refined up to R = 3.3% (Rw = 2.9%). The structural refinement showed that the investigated armstrongite has only two water groups per formula unit consistent with the infrared analysis. Indeed, the occurrence in the infrared spectrum of the armstrongite (here reported for the first time) of two bending vibration bands at about 1640 and 1610 cm–1 testifies to the presence of two water groups environments. The results of this integrated approach converged to the following empirical formula (based on Si = 6 atoms per formula unit): (Ca0.96Ce0.01Yb0.01)Zr0.99Si6O14.97·2.02H2O. Finally, the studied mineral shows a framework density (FD = 21.86) lying in the range of zeolites and microporous heterosilicates with tetrahedral-octahedral frameworks. The determined crystal chemical features are relevant for the possible employment of this mineral or of its synthetic analogs for technological applications
Structure and modeling of disorder in miserite from the Murun (Russia) and Dara-i-Pioz (Tajikistan) massifs
No full textThe structure, structural disorder and chemistry of miserite from the charoite-bearing rocks of the Murun massif (Russia) and from alkaline-syenite pegmatitic rocks of the Dara-i-Pioz massif (Tajikistan) were investigated employing a combination of electron microprobe, single crystal diffraction and micro-Fourier transform infrared spectroscopy analysis. Chemical analysis of the sample investigated by X-ray diffraction evidenced that Dara-i-Pioz miserite has a greater REE concentration than Murun miserite (~0.22 vs. 0.05 apfu, respectively) and also contains Y (0.14 apfu), which is absent in Murun miserite. The occurrence of a band at about 1,600 cm−1 testified to the presence of H2O in miserite at hand. Structural analyses yielded average cell parameters of a = 10.092, b = 16.016, c = 7.356 Å, α = 96.60°, β = 111.27° and γ = 76.34°. Anisotropic structural refinement in space group P-1 converged at similar values for the analyzed samples (R ~3.4, R w ~3.8 %). An interesting feature shown by both the miserite specimen is the presence, revealed by difference Fourier analysis, of a disordered part of the structure. This turned out to be due to the flipping of the tetrahedra belonging to the isolated [Si2O7]6− diorthogroups, one of the two radicals (the other is [Si12O30]12−) characterizing the miserite structure. The sixfold and seven-vertex Ca polyhedra linked to the inverted diorthogroups show variation in coordination number with respect to those of the ordered structure
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