# Taseqite

Taseqite is a rare mineral of the eudialyte group, with chemical formula Na$_{12}$Sr$_3$Ca$_6$Fe$_3$Zr$_3$NbSiO(Si$_9$O$_{27}$)$_2$(Si$_3$O$_9$)$_2$(O,OH,H$_2$O)$_3$Cl$_2$. The formula given is derived from the original one and shows a separate silicon at the M4 site, basing on the nomenclature of the eudialyte group. Taseqite, khomyakovite and manganokhomyakovite are three group representatives with species-defining strontium, although many other members display strontium diadochy. Both strontium ($^{N4}$Sr) and niobium ($^{M3}$Nb) are essential in the crystal structure of taseqite. When compared to khomyakovite, taseqite differs in niobium- and chlorine-dominance.

## Occurrence and association

Taseqite's type locality is the Taseq slope located in the Ilimaussaq complex, Greenland – hence its name. At the type locality taseqite occurs in albitite veins, together with aegirine, analcime, catapleiite, ferrobustamite, hemimorphite, pectolite (silicates); ancylite-(La), calcite, dolomite, strontianite (carbonates); fluorapatite, and sphalerite. Taseqite was found also in Odichincha massif in association with nepheline, alkaline feldspar, aegerine and lamprophyllite.

## Notes on chemistry

Admixtures in taseqite include potassium and manganese, with traces of yttrium, cerium, hafnium, tantalum, and tin.

## Raman spectra

The Raman spectra of taseqite have features characteristic of other representatives of the eudialyte group. The most complex structure is observed in the range of 100–1200 cm$^{−1}$. Pronounced peaks are observed at 127 cm$^{−1}$ (this peak is also present in the spectra of eudialyte and golyshevite) and 190 cm$^{−1}$; bands at close (but somewhat higher) frequencies were observed in the Raman spectra of eudialyte, manganoeudialyte, golyshevite, ferrokentbrooksite, and aqualite (at 205–207 cm$^{−1}$) and in the spectra of georgbarsanovite and raslakite (at 213–217 cm$^{−1}$). Thus, it is reasonable to suggest that the bands at 127 and 190 cm$^{−1}$ are due to Na–O and Sr–O stretching vibrations, respectively. A superposition of bands of different widths is observed for taseqite in the range of 250–350 cm$^{−1}$; the intensities of these bands depend strongly on the orientation of the plane of polarization. The band at 270 cm$^{−1}$ is comparable with that of the band at 272 cm$^{−1}$, which manifests itself as a peak in the spectrum of aqualite and as a shoulder in the georgbarsanovite spectrum; however, it is absent in the spectra of the other members of the eudialyte group. Other strong bands are observed at 285 and 310–326 cm$^{−1}$ (the latter group can be put into correspondence with the strong peak observed in the golyshevite and georgbarsanovite spectra). All these bands, having a preferred polarization along the c axis, are most likely due to the out-of-plane bending vibrations of silicon‒oxygen rings. The weaker band at 387 cm$^{−1}$ coincides with wide peaks in the oneillite and eudialyte spectra, it is observed as a weak peak in the georgbarsanovite spectrum.
In the range of 530–590 cm$^{−1}$, there is a strong band of complex shape, peaking at 560 cm$^{−1}$ and having shoulders at 527 and 540 cm$^{−1}$. The band at 560 cm$^{−1}$ was interpreted as a manifestation of the vibrations of (SiO$_3$)$_n$ rings, although the vibrations of Zr–O and Fe–O bonds can also be involved.
The characteristic peak at 605 cm$^{−1}$ is comparable with the maximum at 612 cm$^{−1}$ in the Raman spectrum of golyshevite. The wide peak in the vicinity of 700 cm$^{−1}$ can be compared with that observed at 700–710 cm$^{−1}$ in the spectra of almost all EGMs, except for aqualite. The absorption peak at 740 cm$^{−1}$ is typical of many minerals, including oneillite and eudialyte; it is shifted in the spectra of golyshevite (747 cm$^{−1}$) and georgbarsanovite (751 cm$^{−1}$). The similar band in the IR spectra of EGMs is due to the bending vibrations of silicon‒oxygen rings, in which electric dipole moment oscillates mainly along the c axis [1]. This is confirmed by the preferred polarization of the Raman band at 740 cm$^{−1}$ in the direction perpendicular to the c axis.
The frequency range 900–1150 cm$^{−1}$, corresponds to the Si–O stretching vibrations. The Raman spectrum of taseqite contains a complex band at 930 cm$^{−1}$ with a shoulder at 900 cm$^{−1}$ in this range. The bands in the ranges of 1000–1030 and 1070–1130 cm$^{−1}$, which are due to the vibrations of silicon‒oxygen rings, are assigned to the stretching vibrations of apical Si–O bonds and Si–O–Si bridges, respectively. The region of O–H stretching vibrations contains a weak peak of complex shape at 3632 cm$^{−1}$, with shoulders at 3660 and 3670 cm$^{−1}$, and a wide band in the range of 3400–3550 cm$^{−1}$, which is due to the water molecules forming relatively strong hydrogen bonds.