A Note on "Stress-Minerals"

Abstract

It is the purpose of the present paper to criticize the hypothesis of stress-minerals proposed by Alfred Harker, to make it clear that it is unneccessary to use such a concept, and to attempt the explanation of metamorphic rock characteristics by regarding temperature, pressuer (hydrostatic), and concentration of components as the only essential factors controlling metamorphism., Petrological and physico-chemical considerations do not show the necessity of Harker's hypothesis of stress-minerals, in the present writers' view., All the minerals which Harker ranked in the category of stress-minerals, seem to be capable of formed under some conditions without stress., They may be divided into two groups, according to the condition of generation., The first group including kyanite, staurolite, almandine, and chloritoids, may be formed only under very high pressures., They all have very high density., They occur sometimes in ignenous rocks, pegmatitic veins, hornfelses, etc., which do not show any special evidence of stress., The second group, containing chlorite, muscovite, epidote, etc., may be formed only at low temperatures., This is shown by the fact that they are hydrothermal or deuteric minerals, the temperature of whose formation is much lower than that of ordinary pyrogenetic or thermal-metamorphic minerals., Harker's stress-minerals seem to be such as are formed only under conditions of lower temperatures and perhaps higher pressures than in ordinary hornfelses., Each of various kinds of metamorphism, such as thermal, regional, etc., takes place under a certain definite condition of temperature and pressure., The kinds of metamorphic minerals developed are determined by the prevailing temperature and pressure as well as by the chemical composition of the metamorphosed rocks., Therefore, each kind of metamorphism is distinguished from the others by a characteristic set of metamorphic minerals.,