Minerals of the Composition MgSiO % . 411 



Table VII. 

 Change of Orthorhombic Amphibole to Monoclinic Pyroxene. 



Time. 



Temperature. 



Observations. 



1 day 



2 days 



20 hours 

 18 hours 



1020°-1040° 

 1055 -1077 



1100 -1100 

 1120 -1140 



No change. 

 Indications of slight 



change. 

 No change observed. 

 Fragments dotted with 



3 days 



18 hours 



2 days 



1127 -1153 

 1144 -1173 



1150 -1183 



pyroxene. 

 All changed. 

 Partly changed. 

 All changed. 



It is possible that at the lower temperatures the amphibole 

 passes into enstatite ; the product is there so fine-grained that 

 a microscopic distinction between the two pyroxenes cannot 

 be made. 



Part II. — Relations of the different forms to one another. 



Stability relations. — The four forms of magnesium silicate 

 having thus been prepared and studied in detail, the question 

 of their relative stability arises. It is well known that a sub- 

 stance chemically homogeneous may exist in several different 

 physical forms, which are called polymorphic when they yield 

 identical liquids, solutions and vapors, in which case their 

 chemical molecules, in distinction from those of isomers, are 

 identical. Polymorphic substances are enantio tropic when, by 

 heating, one form changes without melting into the other at a 

 definite temperature called the inversion point, and on cooling 

 again the reverse change takes place. In this case, the first 

 form is stable below the inversion point only, the second above 

 it. The form stable at lower temperatures inverts to the other 

 with absorption of heat, and the reverse change takes place 

 with evolution of heat.* An instance of the enantiotropic 

 relation is the mineral wollastonite, which inverts at about 

 1190°, with absorption of heat, into a second form, pseudo- 

 hexagonal in symmetry. 



In another class of 'polymorphic substances there is no such 

 inversion point, and one of the forms is more stable than the 

 other at all temperatures below the melting point. These 

 relations are best expressed by diagrams. 



* Given the relations of the vapor pressures, this follows from the second 

 law of thermodynamics. Van't Hoff, Vorlesungen liber Theoret. u. Phys. 

 Chemie, Braunschweig, 1901, ii, 128. 



