INTRODUCTION

System 3 - No SS, Incongruently Melting Compound

Incongruently Melting Compound is a solid phase which when heated does not melt to a liquid of its own composition, it melts to a liquid and another solid, e.g.


We will look at the Forsterite - Silica system, in which Enstatite is the Incongruently Melting Compound.

In examining the forsterite - silica system, at P = 1 atm., the following features are clearly evident:


  1. En melts incongruently to a Liquid + Fo.
  2. Liquid immiscibility at high temperature for silica-rich melts.
In examining this same system at progressively increasing pressures the following changes in the appearence of the system are evident:
  1. With increased pressure, from 1 atm to 7 kbar, En changes from incongruent to congruent melting behaviour.
  2. The field of liquid immiscibility, the 2L field on the 1 atm diagram, dissappears at slightly elevated pressures and is no longer evident in the 3 kbar diagram.

The Fo - Si system can be used to explain the incompatibility of olivine and quartz in igneous rocks and to simplify and model processes occurring in layered mafic intrusions, examples of which include the Skaergard Intrusion, Muskox Intrusion, Bushveld Intrusion.


Simplified Fo-Si System

The 1 atm system, represented in the above link, is not to scale in terms of temperature and is simplified to ignore the effects of liquid immiscibility at high silica contents. However on the diagram it can be seen that there exists:


  1. a single binary eutectic (E) at a temperature = 1543°C
  2. a binary peritectic (P) at a temperature = 1557°C.
The temperature of the peritectic corresponds to temperature at which the following reaction occurs.
En ===> liquid + Fo

Peritectic - is defined as a low temperature inflection point on the liquidus surface at which a unique liquid of specified composition is in equilibrium with two or more crystalline phases.


In this case the liquid has a composition of P and the two solids are En and Fo.