66 Transactions of the Royal Society of South Africa. 
in degrees of freshness in the pieces determined ; the freshest and least 
cracked specimen gave the value 3'18. 
There are three directions along which the mineral may readily be 
cleaved ; two of these are extremely good cleavages and have proved to be 
parallel to the faces of the unit monoclinic prism ; the third, which is not 
quite so perfect, is orthopinacoidal. The prism angles — as nearly as one 
can determine them from cleavage fragments with a simple contact 
goniometer — are 93° and 87°. Experiments carried out on a small cleavage 
fragment by means of a reflecting goniometer, gave a mean value of the 
principal prism angle, iTOAllO, of 93° 9' ; the orthopinacoidal cleavage 
face symmetrically cuts across the solid angle between these two prism 
faces. 
Thick sections of the mineral are only slightly pleochroic. Ortho- 
pinacoidal sections extinguish parallel to the traces of the prismatic 
cleavage, whilst other sections extinguish obliquely with regard to the 
same direction. 
From a clinopinacoidal section ^aC (given by the traces of the 
prismatic cleavage) was determined to be 26° 40' ; this very closely 
approximates to the value of Bx^aC given by Dana.* 
Comparison of Observations with Dana's Data. 
Namaqualand Material. i Dana's Data for Spodumene. 
G: 316-318 
H: 7 
Prismatic cleavage angle : 93° 9' 
Pleochroism : slight 
A 
Extinction angle : I A C = 26° 40' 
Refractive indices : 
Na« = 1-651 
Na/3 = 1-670 
Nay = 1-679 
Chemical tests : 
Proved presence of Li and of SiOj 
3-13-3-20 
6-5-7 
93° 0' 
Distinct in sections of dark green varieties 
B.r« A C = + 26° 
or A C = + 26° 
V 
Naa = 1 651 
Na/3 - 1-669 
Nay - 1-677 
Chemical formula : 
Li,0 . Al,03 . 4SiO, 
The mean refractive index as determined on a Herbert Smith refracto- 
meter was 1-66 ; the scale was not sufficiently large nor was the shadow 
sufficiently well defined to determine the refractive indices for the principal 
* " A System of Mineralogy," Dana. 6th Ed. 
