13 
They are of a prehnite having the following composition: 
Prehnite from Ratschinges, Tyrol 
Per cent 
Si0 2 . 43-40 
AlsO*. ... * 24-53. 
FeaOs . .... 
CaO... 27*37 
HaO 4-48 
Total 99 -7& 
Dana’s Textbook 1 contains fourteen analyses of prehnite differing* 
considerably from one another, but only a slight variation of the axial angle 
and of the birefringence has been observed on prehnite from various local- 
ties, although the varying chemical composition strongly suggests that 
their refractive indices should appreciably differ. A few years ago, R. A. A. 
Johnston 2 described a prehnite from Adams sound containing 6*89 per 
cent of ferric oxide, to which variety the name ferroprehnite was given. 
This prehnite is somewhat similar in composition to a prehnite from Corn- 
wall, Pennsylvania, analysed by Genth. The present writer undertook 
the determination of the optical properties of the ferroprehnite from 
Adams sound in order to compare them with those of the Ratschinges 
prehnite which is free from iron. 
The ferroprehnite examined under the microscope came from the same 
specimen that furnished the material analysed by Johnston. The prin- 
cipal constituents of the specimen, which has a granitic structure, are 
quartz, calcite, and ferroprehnite, all intimately associated. The colour 
of the ferroprehnite is light apple green, but in thin section it is colourless 
Thus in physical appearance it does not differ from common prehnite. 
The axial plane is either parallel to b(010) or to a(100); the mineral 
is biaxial and positive; and the cleavage angle m A m = 80°. The indices 
of refraction carefully measured were found to be as follows: 
a = 1-632 ± 0-001; |8 = 1-642 ± 0-001; y = 1-665 ± 0-001. 
2V measured = 68° 10', and calculated from the above figures is 
=to 67° 38'. Thus the Ratschinges prehnite which represents one pole of 
a series may be compared as follows with the ferroprehnite of Adams sound 
representing the other pole. 
— 
Ratschinges, 
Tyrol 
Adam§ sound* 
Baffin island 
SiOs 
43-40 
41-85 
AlgOj 
24-53 
20-36 
6-89 
CaO... 
27-37 
26-70 
4-19 
H 2 0 
4-48 
Total 
99*78 
100-00 
Specific gravity 
2-90 
2-924 at I5*5°C. 
1-632 
CL . 
1-616 
8. 
1-626 
1-642 
y * 
1*649 
1-665 
2 V calculated 
67° 40' 
67° 38' 
5 Dana, B. S : "A System of Mineralogy,” p. 531. 
•Johnston, R. A, A.: Geol. Surv., Canada, Bull. No. 1, pp. 95-98 (1913). 
37233—24 
