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

Abstract

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 1/21/20 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% H_2O, 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 AI) 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.