Vol 10, No 2 (2024)

A unique unusually Mn-rich concentrically zoned polychrome crystal from the cavity of granite pegmatite of the Krutaya vein within the Malkhan pegmatite field (Transbaikalia) is studied. It consists of six valid and two potentially new mineral species of the tourmaline supergroup. The paper provides the detailed physical, chemical and crystallographic characteristics of minerals. A Mn-richest (up to 9.6 wt. % MnO) dark brown core of the crystal is composed of fluor-tsilaisite, princivalleite and a Mn²⁺-F-analog of foitite. A greenish yellow intermediate zone consists of Mn-rich fluor-elbaite and darrellhenryite and is followed by a Mn-poor pink zone composed of fluor-rossmanite, rossmanite and their O^2--analog. A yellowish green peripheral zone consists of late-generation Mn-bearing fluor-elbaite. The Mn content decreases, while the Li + Al content increases from the center of the dark brown zone to the margin of the crystal approximately to the middle of the pink zone, where the evolution of the tourmaline composition is inverted and this trend becomes opposite. The crystal structure of fluor-tsilaisite is refined on a single crystal extracted from this specimen, R₁ = 2.4 %. The mineral is trigonal, space group R3m, a = 15.9595(1), c = 7.14164(7) Å, V = 1575.31(3) ų, Z = 3.

Regular distribution of several trace elements in spodumene crystals of the Pashki pegmatite Li deposits in Afghanistan (Nuristan province) is analyzed using secondary ion mass spectrometry (SIMS). Twenty-seven local analyses of 19 elements were carried out in spodumene samples of different generations taken from pegmatites during field works of 2023. We studied an early generation spodumene crystal from blocky spodumene-quartz-albite pegmatite and two late generation spodumene crystals including an euhedral crystal from a quartz core of pegmatite and a crystal extracted from a mineralized cavity together with quartz crystal, albite, and muscovite. It is found that the Fe, B, and Be content is significantly higher in the growth pyramid of the pinacoid face (010) than in the growth pyramid of the prism face (021). From early to late growth zones, the Be, B, Fe, and Mn content increases and the K, Rb and Ti content decreases. The heterogeneity of trace element composition of spodumene crystals thus mostly depends on their sectoriality, which is caused by different crystal chemical conditions of the incorporation of isomorphic trace elements into the mineral structure on faces parallel and perpendicular to the direction of pyroxene chains of Si–O tetrahedra along axis [001] rather than by their zonation. From early to late generations of spodumene, the content of most analyzed trace elements significantly decreases corresponding to the ideas about the change in the crystallization fractionation of elements during the evolution of the pegmatite process transiting from the closed to open physicochemical system.

The study of minerals with modern electron microscopes requires the solution of anatomical problems of the crystals. To obtain consistent spectroscopic data from different instruments, it is necessary to point the microprobe directly to the same element of the crystal anatomy. This is difficult to do unknown details of an anatomical pattern. The article provides the examples of complex arrangement of elements of crystal anatomy of different minerals to accumulate ontogenetic information for the formulation of the postulates.