Links between chemistry and structure in titanites and tourmalines

Part I. Stabilization of the A2/ a dimorph of titanite by creation of antiphase boundaries as a result of incorporation of rare earth elements. The atomic arrangement of a natural rare-earth-rich titanite and two synthetic rare-earth-doped titanites have been refined in space group A2/a, and the atomic arrangement of an undoped P2₁/ a synthetic titanite was also refined for comparison. Previous work has shown that titanite possesses a domain structure, with domains formed of like-displaced Ti atoms in the [001] octahedral chains. P2 ₁/a titanite results when the crystal is formed of a single domain, but as Ti-reversal sites occur in the octahedral chain the apparent A2/a structure results from the average of antiphase domains. Antiphase boundaries occur at O1, which is alternately overbonded or underbonded at the boundaries, depending on the displacement of the neighboring Ti atoms. Type 2 antiphase boundaries exist where two Ti atoms are displaced away from the intervening O1 atom, and are energetically unfavorable because of underbonding of that O1 atom. However, substitution of a trivalent rare earth element in the adjacent Ca2+ site relieves that underbonding, favoring the creation of type 2 antiphase boundaries and stabilization of the A2/ a dimorph. The results of high-precision crystal structure analyses demonstrate that rare earth substituents for Ca stabilize the A2/ a dimorph at lower substitution levels than required for octahedral substitutions.; Part II. Linking chemical variation to structural constraints in the schorl-dravite series of the tourmaline family. Nine tourmalines for which chemical characterizations of major elements are available have been examined by single-crystal x-ray structure refinement. The single crystals were then analyzed individually for major elements (electron microprobe analysis), Fe3+/Fe2+ (synchrotron micro-x-ray absorption near-edge spectroscopy), B and Li (secondary ion mass spectrometry); bulk H determination (uranium extraction) was also undertaken.; Despite recent chemically based claims, structure analysis confirms that no detectable B exists in tetrahedral coordination in these samples. Analysis of cation ordering between the Y and Z octahedral sites suggests that the occurrence of a Fe2+ atom at a Y octahedron site must be locally associated with the absence of Mg at both of the neighboring Z sites, as substitutions of Fe2+ at Y and Mg at Z require antithetic shifts of the O6 anion. Part II is split into two chapters. Chapter 1 presents the refinements and interpretation as published in American Mineralogist; Chapter 2 presents an alternative interpretation that yields a greater correlation between data and structure conclusions.