Abstract

Statistical analyses were carried out on the ion contents of 70 titanium pargasite and kaersutite samples from alkalic igneous rocks and mafic mantle xenoliths, in order to evaluate the extent to which Fe3+ and Ti are accommodated by substitution mechanisms involving oxy-component in these amphiboles. Sixty-five of the amphiboles occurred in extrusive rocks, the remaining five in dike rocks. Linear, bivariate regression and correlation analyses were carried out to relate the Fe3+ and Ti contents to the relevant ion contents that are required if Fe3+ and Ti, respectively, are accommodated by the following substitution mechanisms:
Fe2++OH−=Fe3++O2−
(1)
[6]A3++OH−=Ti4++O2−.
(2)

Statistically significant correlations (generally at the 99 + % level of significance) were observed only between variable pairs that are indicative of the oxy-component. Namely, there is an inverse correlation between the univalent anion content of the O3 site (i.e., OH + F + Cl) and both Fe3+ and Ti contents, an inverse correlation between [6]Al3+ and Ti, and a positive correlation between the amount of positive charge in excess of 46.0 per formula unit and both Fe3+ and Ti. The induced negative correlation that results from the fact that these are variables of a constant sum could lower the statistical significance of the inverse correlations but would increase the significance level of the positive correlations. A statistically significant correlation was not observed among any of the variables related to the accommodation of Fe3+ by the Al-substitution mechanism. It is concluded that the Fe3+-, Ti-, and OH contents in this population of amphiboles are controlled mainly by the coupled substitution mechanisms described in Equations 1 and 2.

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