Isothermal compressibility of kyanite, andalusite, and sillimanite from synthetic aggregates

Abstract

Many minerals are available only in small quantities and in a finely divided form that is not suitable for determination of compressibility in the usual ways. We describe a technique employing synthetic mixtures; the unknown mineral is mixed with a ductile solid of known characteristics. Then, from measurements on hot-pressed samples of the aggregate and the known properties of the matrix, properties of the unknown mineral can be derived by a Voigt-Reuss averaging procedure. The method has one weakness: an accurate result requires a close matching of the properties of matrix and unknown mineral. We used this method to obtain compressibility of kyanite, andalusite, and sillimanite to 10 kb in a hydrostatic system and to 40 kb in a solid medium system. Compressibility of all three at 1 bar and 25°C was within 0.77±0.04 Mb−1. Change of compressibility with pressure could not be satisfactorily determined. Because of this inability and because of the rather large probable errors in compressibility, our technique will perhaps serve principally as an exploratory tool. One advantage is that only about 2 grams of powdered material are required, even for fairly incompressible minerals. Some details are given of the hot-pressing procedure used to form aggregate samples of low porosity with lead or copper as the matrix and of the strain gage techniques used to measure compressibility to 10 kb.