Summary.The temperature at which dehydroxylation occurs in amphiboles is thought to be largely dependent on the type of cation that occupies the M a and M1 sites. Four amphibole types have been studied in which these sites are occupied, in various proportions, by Mg 2+ or Fe 2+. The temperature of dehydroxylation, in an inert atmosphere, is shown to rise with increasing Mg 2+ content in these sites.ECENTLY Hodgson (1965) has discussed the relationship between the chemical composition and the temperature of thermal decomposition of some crocidolites. He concluded from d.t.a, evidence that the position of the high temperature endotherm, that is, the temperature at which structural breakdown of the crocidolite occurs, was dependent on the Fe2+/Mg ~+ ratio, the temperature of the endotherm rising with rising Mg 2+ content.Croeidolite may be unique amongst the amphiboles in that the dehydroxylation process and the structural breakdown process are consecutive rather than concurrent (Freeman, 1962). Thus, in seeking correlations between the composition and thermal behaviour of amphiboles it is important to consider the temperature of dehydroxylation as well as the temperature of final structural breakdown.In the amphibole structure the hydroxyl group is bonded to the M 3 cation and to the M 1 cations (using Whittaker's (1949) notation). Whittaker (1949) found that in Bolivian crocidolite the MI-OI-I and Ms-OH bond distances were 2-10 A. The MI-OH bond distance was 2-10 3~ and the Ms-OH distance 2.07 3~ in eummingtonite (Ghose and I-Iellner, 1959). It is to be expected, then, that the strength of the M-OH bond, or the MO-I-I bond (where M represents the cation on the M a or M 1 sites), and consequently the temperature at which bond breakage occurs, will be dependent on the cations which occupy these sites. In order to show the etteet of site occupancy on stability the temperature of dehydroxylation of four amphibole types has been determined.Experimental results. The four amphibole types considered were:
The technique of 67Fe conversion-electron Mossbauer spectroscopy is shown to be a viable method for the study of the early stages of solid-state reactions which start in regions close to solid surfaces. In particular, increases in the Fe3+ : Fe2+ ratio can be detected in a region confined t o within 300 nm of the outer surfaces of a biotite flake which has been subjected to mild oxidation in air. These changes would have gone undetected if conventional transmission Mossbauer methods had been used alone. The reactionswhich occur in the surface regions of biotites during heat treatment in air are discussed.
THE use of 57FeMossbauer spectroscopy in the study of minerals and solid-state reactions is now well estab-1ished.l-3 Most Mossbauer spectra are accumulated by monitoring the y-radiation transmitted through a ' thin ' specimen and in this way information relating to bulk properties of the material under examination is obtained. In situations where the process of sample preparation and consequent alteration or destruction of the specimen is impracticable, ' thick ' iron-containing specimens can be studied in a reflection geometry by monitoring the back-scattered Xor y-ray photons which result from the decay of the I = $ nuclear spin state of 57Fe in the sample. This technique has been applied to mineral specimens including lunar samples 4-6 and in the study of corrosion products in sita.7 More attractive to the surface chemist is the possibility of increasing the surface sensitivity of 57Fe Mossbauer spectroscopy by counting the back-scattered conversion and Auger electrons.8-10 In this way the depth probed is liiiiited to ca. 300 nm.9911 The sensitivity of the method is such that (using existing equipment) ca. 80 A of natural iron (ca. 2% 57Fe) can be detected and, with
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