An Attempt to Determine the Fe-Mg Content of Hornblendes by X-ray Diffractometric Method

An attempt was made to devise a quick x-ray diffractometric method for determination of Fe-Mg contents of hornblendes based on reflection intensity as a function of composition. Experimental techniques and conditions and the procedure used in indexing reflections in the hornblende diffraction pattern, have been given in detail. The nature and range of various ionic substitutions in hornblendes have been fully discussed and it has been shown that variation in reflection intensity between samples, are caused almost entirely by differences in the extent of Fe-Mg substitution at the octahedral site. Effect on intensity caused by change in lattice parameters resultant from substitution, has also been considered lntensity of (h00) type reflections as a function of Fe-Mg content, was specifically investigated. Calculated and observed intensities of reflections from hornblendes of known compositions, are discussed. It is concluded that within the present limitations of even the best instrumental conditions obtainable in powder work, record of reflection intensity differences caused by compositional difference of up to two Fe ions per unit cell in hornblendes, cannot be measured to a degree of accuracy required to make this criterion useful in detecting compositional differences of the order commonly encountered in natural hornblendes.