Mn-tourmaline from island of Elba (Italy): Crystal chemistry

The crystal chemistry of seven crystal fragments taken from differing regions of the same colorless to yellow-greenish tourmaline macro-crystal from pegmatite pockets in aplite veins (island of Elba, Italy) was studied with a multi-disciplinary (SREF, XRDT) and multi-analytical approach (EMPA, SIMS). EMPA and XRDT studies showed relationships between color and chemical zoning and crystal- growth evolution, indicating which fragments could be considered representative of the chemical evolution of the genetic micro-environment in which the crystal developed. Results showed that the colorless fragment is an elbaite while the yellow-greenish crystal fragments are Mn 2+ -rich (up to 1.34 apfu) and belong to the alkali group and fluor subgroup. They are characterized by dehydroxylation and alkali-defect type substitutions that cooperate in reducing Li and increasing Mn contents. The Y site is populated by Al, Li, and Mn 2+ , and the Z site by Al and Mn 2+ (up to 0.10 apfu). In contrast with data in the literature, Mn 2+ populates both octahedral sites according to the order-disorder reaction: Y Mn + Z Al ↔ Y Al + Z Mn. As Mn 2+ content increases, progressive disorder takes place. This disorder is quantitatively lower than that of the ZMg in dravite, due to the low structural tolerance of the small Z cavity in the incorporation of larger cations by the Z R 2+ → Z Al substitution. Relationships of direct proportionality between lattice parameters and both and are observed. The expansion of both octahedra, as well as of lattice parameters, increases linearly as a function of Y Mn 2+ and Z Mn 2+ . The latter has greater weight in dictating unit-cell variations, due to the degree of size mismatch between Z Mn 2+ → Z Al and Y Mn 2+ → Y Li substitutions, and the way in which the Z octahedra are articulated in the structure.