Gabbroid Rocks of Minnesota. — Wincliell. 177 
finally there is one other secondary mineral that is formed 
at the expense of the feldspar that I have found only in this 
rock. Its description follows: 
Pectolite was first correctly determined by von Kobell in 
1828 on material from the Tyrol. It is monoclinic and nearly 
isomorphous with the pyroxene, wollastonite. Nevertheless 
the constant notable content of water has caused it to be 
grouped frequently with the zeolites. At the present time it 
is generally recognized as a pyroxene, or a mineral closely al- 
lied to that group. It occurs in minute lamellae and fibers, 
often assuming nearly the spherulitic habit. The mineral pos- 
sesses two marked cleavages parallel to /(ooi) and/!'(ioo) re- 
spectively, making an angle of about 85°. 
Inasmuch as pectolite has not heretofore been described as 
a direct decomposition product of feldspar,* though its ordi- 
nary mode of occurrence is in cavites or seams in basic igneous 
rocks, a careful statement of the diagnostics relied upon for its 
determination will not be out of place. 
The mineral is found in the olivine gabbro from Birch lake 
(1136) occurring as an indubitable alteration product of the 
labradorite. It is frequently associated with the other alter- 
ation products such as calcite and penninite, and seems to be 
due like them to the action of percolating waters. It occurs 
by preference in those areas where the labradorite has been 
slightly fractured by some strain. It rarely occurs on the out- 
side of the feldspar grain, usually being found along some 
cleavage or fracture line. The mineral is transparent and 
colorless by transmitted light ; by reflected light it gives the 
whitish color so commonly seen on acid feldspars which are 
altering to kaolin. As mentioned above it is eminently fibr- 
ous, the fibers interlaced with their axes making very small 
angles with one another. Such a structure produces a hazy 
changeable extinction, familiar 'through its frequent occur- 
rence in the fibrous zeolites. This fact, together with the ex- 
ceedingly small size of the lamellse — for, fortunately, the fibers 
sometimes develop into lamellar habit — render the determin- 
ation of the optic properties a matter of great difficulty. In- 
*A transformation entirely analogous, however, has been described 
by Lacroix: (Mineralogie de la France: II, p. 46). It is the alteration 
of anorthite to wollastonite. 
