A Simple Identification Reaction for Zeolites

or the length of time which the process requires. The allanite replacements in the Tan-y-Grisiau microgranite suggest that unmixing was completed before hydrothermal minerals began to crvstallize and suggests, therefore, that in some instances, at least, unmixing ma.v belong to an early, relatively high temperature post-magmatic phase' The reason for the selective replacement of K-feldspar b.v allanite in the Tan-y-Grisiau microgranite is uncertain, but it is signifi.cant that near the northwestern margin of the intrusion, where volatiles were concentrated, all of the K-feldspar is converted into muscovite, indicating its complete instabil ity in the presence of large quantit ies of volati les.