Monoclinic Habitus. 
cleavage 1| O and i-i^; angle about 90°. TABLE XIII. 
Colors of 
Folaj'iza- 
ti 07 l. 
Color. 
Sii'ucture. 
Association. 
Inclu- 
siofis. 
Altera- 
tions. 
Occurrence. 
Reinaj'ks . 
Rather 
brilliant, 
but less 
than as 
quartz. 
In thin 
sections 
often 
less bril- 
liant, 
bluish 
gray as in 
nepheline 
General- 
ly cloudy 
white or 
gray, 
rarely 
clear. 
Colored 
red by 
limonite. 
In large crys- 
tals or grains 
of the first 
order, more 
rarely in small 
grains or rods 
in eruptive 
rocks. Often 
coalesced 
with plagio- 
clase. Zonary 
structure is 
rare as is 
zonal arrange- 
ment of in- 
clusions. 
With 
quartz, 
biotite, 
muscovite, 
hornblende 
and rarely 
with augite. 
plagioclase 
and 
elseolite. 
In 
general 
poor. 
Specular 
iron, 
biotite 
scales, 
fluid 
inclu- 
sions, 
needles 
of 
apatite, 
zircon. 
Kaolini 
zation 
with 
forma- 
tion. 
of musco- 
vite or 
epidote. 
One of the 
most abun- 
dant compo- 
nents in gran- 
ular and por- 
phyritic older 
eruptives. 
Essential 
comp, in 
granite, syen- 
ite. quartz 
porphyry and 
accessoi-y in 
nearly all 
plagioclase 
rocks. Also 
in crystalline 
slates as 
gneiss, here 
often glas-y 
like sanidine 
Large crystals 
may be recog- 
nized by 
twinning in 
sections par- 
allel to 0 and 
i-i and by the 
oblique ex- 
tinction paral- 
lel i-E. Mi- 
nute rods of it 
and sanidine 
in the magma 
of rocks often 
greatly re- 
semble neph- 
eline and cer- 
tain melilites, 
Orthoclase 
lacks, how- 
ever, the iso- 
tropous hex- 
agonal sec- 
tions 
From plagio- 
clase it may 
be distin- 
guished by 
absence of the 
polysynthetic 
twins of the 
latter. 
Colorless 
clear. 
/3 p= 
1-5237 
In eruptive 
rocks as large 
crystals of 
first order and 
minute rods 
second order. 
The large 
crystals often 
fractured. 
Zonary 
structure. 
As 
orthoclase 
a'so with 
nepheline 
and leucite 
but never 
with 
muscovite. 
Very 
rich. 
Gla.ss in- 
clusions, 
usually 
zonal in 
arrange 
merit, 
augite 
micro- 
lites and 
needles 
of apatite 
Almost 
always 
unaltered 
Into opal 
in ande- 
sites and 
trachytes 
Essential 
comp, (pri- 
mary) in 
trachytes, 
rhyolites, 
phonolite, 
and the glass 
of orthoclase 
rocks. In 
nearly all 
later plagio- 
clase rocks. 
