HORNBLENDE AND BYTOWNITE FROM HYPERSTHENE GABBRO. 215 
The chemical composition obtained from the analysis 
corresponds closely with the formula Ca Wee Mgis Also 
(Na, K, H)14 Siss O121, with Ti replacing some Si. 
As in certain sodic amphiboles like arfvedsonite,’ the 
silica is insufficient to function as the only acid, and it 
seems probable that the acid-forming hydroxyl (OH) radical 
is present in the molecule, as suggested by Penfield and 
Stanley.” 
The ratios of SiO, to the bases R”,O, and R’O, the latter 
including K.O, Na,O and H,O, are as follows: 
Si@a 0) Re 5 On 25k: ©) (SiOsu. Ry O 
1665 o= 094." 345 862 gC ere ln (a | 
The latter ratio, along with the high iron percentage, allies 
the mineral] to the hornblende group of Penfield and Stanley, 
although the low alumina and high water connect it rather 
with the tremolite-actinolite group. 
(b) Optical Properties. 
Under the microscope the mineral is seen to occur as 
allotriomorphic grains which appear to be primary. It is 
translucent in thin section and has a brown colour. The 
cleavage is prismatic, the angle between the cleavage 
traces being 62°. There is also trace of a cross-fracture. 
The pleochroism is intense, X < Y< Z 
X = yellow 
Y = light brown 
Z = brown 
Y is parallel to crystallographic “‘b’”’ axis. ZAc=21°. 
II 
The mean refractive index was determined by the immer- 
sionof cleavage chipsina mixture of«-mono-clor-naphthalene 
and a-mono-brom-naphthalene. The refractive index of the 
liquid was measured on the Abbé refractometer and found 
to be about 1°66. 

* Iddings, Rock Minerals, p. 349. 7 Amer. Jour. Sci., Series IV, 23, 
1907, p. 30. 
