TABLE XII. 
Colors of 
polarisa- 
tion 
Color ana 
index of re 
fraction. 
r 
Structure. 
Association 
: Inclusions. 
Alterations 
Occurrence 
’ Remarks. 
Not par 
ticularly 
brilliant. 
Browns. 
In thin 
plates ir- 
ridescent, 
carmine 
red. 
Brown 
black, 
dark green 
f3=i.6i. 
Pleochro- 
ism strong 
in long’t. 
sections a 
and b near 
ly alike 
In sections 
parallel to 
c — axis 
a=yellow- 
bright 
brown, at 
right an- 
gles c 
C=brown- 
black. 
c>b>«. 
Primary, as 
large crystals 
(in eruptive 
rocks) fre- 
quently frag- 
mentary or 
with opaque 
margin, 
(comp, of 
first order) ; 
also in minute 
scales particu- 
larly in crys- 
talline slates 
or scattered 
through the 
magma, as in 
basalts, etc. 
(comp of sec- 
ond order. 
Usually 
with 
quartz and 
ortho- 
clase, also 
with horn- 
blende, 
and more 
rar’Iy with 
augite and 
olivine. 
Generally 
free from 
inclusions, 
but not 
rarely 
with fas 
ciculate 
masses of 
epidote 
needles or 
regularly 
arranged 
slender 
needles of 
rutile. 
Into chlo- 
rite like 
mineral, 
with epi 
dote and 
calcite 
Loses its 
brown col- 
or, be- 
comes 
green, cal- 
cite pene 
trates be 
tween the 
scales. nee 
dies of ep- 
idote ap- 
pear. Also 
formation 
of limon- 
ite or mag- 
netite on 
periphery. 
In nearly 
all rocks. 
In many 
as neces- 
sary pri- 
mary con- 
stituent, 
one ot the 
first to ap- 
pear of the 
minerals. 
Product of 
alteration 
of augite, 
horn- 
blende, 
rarely ol- 
ivine. 
Contact 
mineral in 
metamor- 
phic slates 
Easily recog- 
nized by the 
cleavage and 
strong pleo- 
chroism. Cross 
sections not 
pleochroic as 
hornblende. 
Not green as 
chlorite, nor 
fasciculate. 
As merox- 
ene. 
Red-br'wn 
Pleochro- 
ism feeb 
ler than in 
meroxene. 
With oliv- 
ine and 
actinolite. 
Becoming 
green to 
colorless, 
as above. 
Rare in ol- 
ivine store. 
Difficult to 
distinguish 
microscopic- 
ally from talc. 
Sericite is but 
an unctuous, 
inelastic, light 
green musco- 
vite. which 
occurs in ir- 
regular scales 
in certain 
crystalline 
slates. 
Occurrence . 
Primary in granites 
(in particular in tour- 
maline granites) and 
crystalline slates, as 
gneiss, mica schists 
and slate. In no other 
rocks primary but as 
product of alteration 
in feldspars, chiasto- 
lite. liebenerite. etc. 
Very bril- 
iliant, irrid- 
escent, red 
to yellow. 
Colorless, 
light green 
oil green. 
As primary in 
large plates 
In fasciculate 
and radiate 
aggregate. As 
secondary in 
aggregates of 
minute irreg 
ular scales 
In crystalline 
slates. 
With 
quartz, or- 
thoclase 
tourma- 
line. 
Very poor 
Rarely 
needles of 
rutile, 
plates of 
specular 
iron, or 
columns ol 
tourma- 
lin, or Zir- 
con. 
Colorless, 
white, 
light 
green. 
Usually in ir 
regularly felt 
ed or rosette- 
shaped radi- 
ating aggre- 
gates of mi- 
nute scales. 
With 
quartz, or- 
thoclase, 
mica or 
with aug- 
ite and 
olivine. 
Very poor, 
biotite, ac- 
tinolite. 
As musco- 
vite. 
A primary constit 
ent in many crystal- 
line slates. Not abun- 
dant Secondary pro- 
duct of decomposition 
of augites poor in iron 
and hornblende, par- 
ticularly Enstatite al ; 
tered to olivine stone 
and serpentine. 
Microchemi- 
cal treatment 
of isolated 
scales with 
fluorosilicic 
acid is the 
surest test. 
Not very 
brilliant, 
blue — 
green. 
Light to 
dark green 
^^=i*5,75- 
Pleochro- 
ism feeble 
Not in large 
scales but like 
talc as aggre 
gates of mi- 
nute irregular 
scales radiate 
or disperse. 
With 
quartz, or- 
thoclase, 
biotite 
muscovite 
as primary 
const. 
Poor. hem- 
atite and 
limonite, 
needles of 
actinolite 
and rutile. 
Also hel- 
minth, pen- 
nine. kaem- 
mererite chlo- 
ritoid, sism on- 
line, and ott 
relite. Rare. 
Primary in chlorite 
slate. Decomposition . 
product of mica, au , 
gite. hornblende and , 
garnet. , 
