Coupled substitution of H and minor elements in rutile and the implications of high OH contents in Nb- and Cr-rich rutile from the upper mantle



S. S. Papadopulos and Associates, Inc.,
7944 Wisconsin Avenue, Bethesda, Maryland 208 14-3620, U.S.A.


Division of Geological and Planetary Sciences,
California Institute of Technology. Pasadena, California 91125, U.S.A.


Department of Geology,
University of Massachusetts, Amherst, Massachusetts 01003, U.S.A.



Infrared absorption spectra of rutile crystals from a variety of geological environments (carbonatite, hydrothermal vein, kyanite + rutile + lazulite association, xenoliths that are kimberlite hosted and dominated by Nb- and Cr-rich rutile) exhibit strong absorption in the 3300 cm-1 region due to interstitial protons bonded to structure O. In general the proton is located at sites slightly displaced from 1/2 1/2 0 of the unit cell, although some samples show evidence of additional protons at tetrahedral interstitial sites. H contents of rutile range up to 0.8 wt% H2O, the highest concentrations occurring in mantle-derived Nb- and Cr-rich rutile of metasomatic origin. The role of H in rutile was examined, particularly with respect to its relations to other impurities. Protons are present in the rutile structure to compensate for trivalent substitutional cations (Cr,Fe,V,Al), which are only partly compensated by pentavalent ions (Nb,Ta). The possibility of using the H content of rutile as a geohygrometer is illustrated for the case of coexisting hematite and rutile.

American Mineralogist, Volume 78, pages 1181-1191, 1993