Silicate Nanotubules in the Crystal Structure of K6(Na4Ca)(Y8Ca3Mn)Si28O68(OH)2(CO3)8F2·9H2O, a Mineral Phase from the Khibiny Alkaline Massif (Kola Peninsula, Russia), and the Problem of Ashcroftine-(Y)

The Lovozero and Khibiny alkaline massifs (Kola Peninsula, Russian Arctic) are the prominent sources of REE minerals, with the Lovozero loparite deposit being the only currently active REE mine in Russia. A new ashcroftine-related mineral phase KA with the idealized chemical formula K6 (Na4Ca) (Y8Ca3Mn) Si28O68 (OH) 2 (CO3) 8F2·9H2O was found in the Khibiny alkaline massif. Its empirical formula determined by electron microprobe analysis is Na4. 14K6. 11Ca3. 89Mn0. 59Y6. 10Ce0. 08 Gd0. 32Tb0. 15Dy0. 78Ho0. 19Er0. 35Tm0. 15Yb0. 12Lu0. 06Si28C8O93. 02F2. 08·9H2O. The crystal structure was determined and refined by means of single-crystal X-ray diffraction analysis. The KA phase is tetragonal, I4/mmm, a = 24. 1661 (3), c = 17. 5914 (4) Å, V = 10, 273. 4 (3) Å3. The crystal structure contains two Y sites. The Y1 site is 8-coordinated and hosts more heavy REEs, whereas the Y2 site is predominantly 7-coordinated and accumulates lighter REEs and Mn. The crystal structure is based upon the Si28X70 nanotubes (X = O, OH) elongated along the c-axis and composed of corner-sharing SiX4 tetrahedra. The external diameter of the tubules is equal to ~19. 54 Å, i. e. , slightly less than 2 nm. The silicate nanotubes are running parallel to the c-axis and centered along the (00z) and (½½z) directions. The tubules are linked by walls of YOn polyhedra that also involve triangular CO3 groups. The K+, Na+, and Ca2+ cations, as well as H2O molecules, are located either inside or outside the tubules. The crystal-chemical formula of the KA phase can be written as K6. 14Na4. 30Ca0. 81Y5. 88Ca3. 12Dy0. 88Mn2+0. 60Gd0. 32 Ho0. 24Er0. 24Tb0. 16Tm0. 16Er0. 12Yb0. 12Ce0. 08Lu0. 08 (Mn3+0. 09) Si28O68. 36 (OH) 1. 65 (CO3) 8F2·8. 97H2O, which agrees well with the idealized formula. According to the information-based complexity analysis, the KA phase has a very complex structure and belongs to less than 3. 5% of the very complex minerals known today. The presence of silicate tubules is the key reason for the exceptional structural complexity of the phase. It is impossible to establish exact relations between the KA phase and ashcroftine- (Y) on the basis of the currently available data, since the last chemical analysis of the latter mineral was done in 1924. Therefore, the mineralogical identity of ashcroftine- (Y) is currently an unresolved problem. The silicate tubule in the KA phase is topologically related to the Linde zeolite A (the LTA zeolite framework) and can be produced from the latter by a series of topological operations. The KA phase forms a homological row with caysichite- (Y) and miyawakiite- (Y), along which the Si content is increasing, and silicate chains in caysichite- (Y) transform into silicate tubules in miyawakiite- (Y) and into silicate nanotubules in the KA phase. Indeed, the M: Si: C ratio (where M = Y, REEs, Ca, Mn, Fe) changes from 1: 1: 0. 75 for caysichite- (Y) through 0. 75: 1: 0. 5 for miyawakiite- (Y) to 0. 43: 1: 0. 29 for ashcroftine- (Y) (and KA). The increasing role of silica along the row results in the formation of zeolite-derived porous one-dimensional units. The KA phase possesses two important crystal chemical properties that distinguish it from other minerals known to date: it hosts a variety of REEs and is based upon nanoscale zeolite-like silicate units. The KA phase, ashcroftine- (Y), caysichite- (Y), and miyawakiite- (Y) have never been prepared under laboratory conditions. The mineralogical occurrence of the KA phase in the Khibiny massif points out to its secondary origin, i. e. , its formation under relatively soft, low-temperature hydrothermal conditions. Thus, the discovery of the KA phase in nature may provide important hints toward its synthesis in the laboratory by means of a soft-chemistry approach.