92 Allen and White — Polymorphic Forms of 



Properties. — The pure mineral is white in the mass and 

 shows when prepared by the process just described the fibrous 

 structure which is commonly characteristic of the natural 

 mineral. A perfectly transparent variety in beautifully formed 

 crystals of short prismatic habit was obtained later in an 

 entirely different manner, which will be described subse- 

 quently. A detailed microscopic study of both varieties by 

 Mr. Wright accompanies this paper. 



The density of the fibrous material varies considerably, 

 being dependent on the temperature of crystallization, and per- 

 haps upon other conditions. The variation is satisfactorily 

 accounted for by the presence of bubbles, or more probably 

 vacuous spaces, which the microscope shows are generally 

 present in the crystalline mass. They are probably the resid- 

 ual spaces which are left by the condensation of substance 

 about the crystal nuclei. In one of the preparations, Prep. IV, 

 which in crystallizing almost reached the inversion point, 

 these bubbles were not found, and the crystalline fibers were 

 much coarser. The following determinations were made by 

 the method of Day and Allen : * 



Specific Gravity of Fibrous Wollastonite, prepared by the devi- 

 trication of the glass. Determinations were made at 25° and 

 compared with water at 25°. 



Prep. I. Heated about 3 days at a maximum tempera- 2 -907 

 ture of 860°. 2-907 



Prep. II. Heated 17 hours at a maximum temperature of 2-876 

 950°. 2-876 



Prep. III. Heated 22 hours at a maximum temperature of 2*896 

 980°. 2-896 



Prep. IV. Formed from an under-cooled mass which began 



to crystallize at 1030° and rose in the pro- 2-9145 

 cess to a temperature of 1127°. The micro- 29156 



scope showed a coarsely fibrous mass without 



the bubbles which appeared in I, II and III. 2-915 



The density of the glass from which the wollastonite is pre- 

 pared is here given for purposes of comparison. The glass is 

 perfectly free from bubbles and hence shows a practically con- 

 stant density, which differs but little from that of wollastonite, 

 being a trifle lighter than the densest preparation. 



Specific Gravity of Glass of the Composition CaSi0 3 , deter- 

 mined at 25°, compared with water at 25°. 



Prep. a. 



Prep. b. 



Prep, c, 



2-901 



2-905 



2-905 



2-902 



2-905 



2-903 



* Isomorphism and Thermal Properties of the Feldspars, this Journal, xix, 

 125, 1905. Publication No. 31, Carnegie Institution of "Washington, p. 55. 



