in the ordinary ray (that is, the one passing parallel to the optical axis and vibrat-
ing at right angles to the fundamental section, e= exponent of extraordinary ray
Digitized by
GoogI(
132 BULLETIN OP THE LABORATORIES
(i. e. that passing at right angle to the optical axis and vibrating in the principal
section. If the axis c coincides with the optical axis a and w> c the mineral is neg-
atively double refractive, while if r= c and 6)<c, the mineral is positively double
refractive. A thin section of a crystal of either the tetragonal or hexagonal sys-
tem, if basal, {i, e. parallel to O) acts as though isotropous and remains dark in
all positions between crossed Nicols. On the other hand, sections taken vertical
to O, or parallel to one of the prismatic faces, are dark !wice in a revolution and
these points occur when the sides of the section are parallel to the principal section
of the Nicol's prisms. In such a casp^the extinction is said to be perpendicular
or parallel to the crystallographic axis. Sections inclined to the vertical axis or
such as are parallel to a pyramidal face are extinguisned parallel to the vertical
axis, but not necessarily to all the sides. Whether the mineral proven to be uni-
axial is hexagonal or tetragonal can only be discovered by ascertaining the num-
ber of sides of a section transverse to the vertical axis.
In biaxial minerals there are two directions without double refraction. Three
axes are assumed, each of which is at right angles to the others and has a differ-
ent amount of elasticity of the ether. These elasticity axes are lettered a, b and
C, in order of elasticity. Two of the optical axes do not correspond with the
crystallographic axes, but the two sets of axes form with each other larger or
smaller angles. The line which bisects the acute angle = acute bisectrix^ that
which bisects the obtuse angle = obtuse bisectrix. The optical axes and both the
acute and obtuse bisectrix lie in the same plane, called the optical plane, perpen-
dicular to which is the optic normal. The axis of intermediate elasticity (b) co-
incides with the optic normal, while the axes of least and greatest elasticity may
coincide with either bisectrix. If a coincides with the acute bisectrix, then c co-
incides with the obtuse bisectrix and the mineral is negative. If, on the other
hand, c coincides with the acute bisectrix and a with the obtuse, it is positively
double refractive. a, ^, y are the indices of refraction corresponding to the three
axes of elasticity.
In the case of minerals of the Orthohombic system the three axes of elasticity
a>b>C correspond with the crystallographic axes a, b, and c, but not necessarily
in such a way that a always corresponds to 3,, etc. a and c are always bisectrices
and the optical plane is always parallel to one of the three pinacoids.
The following are possible :
If Opt. plane = O, a=a, b-=c \ / ^
a=c, b=ar -^
If Opt. plane =i-i,c^=a, a=c ) , ^j.
c^=c, &=a / ^
If Opt. plane = i-f, c^=a, b=c ) v j-
c'=c.b=a}''=*
Sections parallel to one of the three pinacoid surfaces (usually rectangular) extin-
guish perpendicularly, i, e. become dark between crossed Nicols when one side of the
rectangle or one of the pinacoid cleavage lines is parallel to the main section of the
Nicol. Distinguished from isometric minerals by the fact that basal sections (par-
allel O) are not isotropous Only such sections are isotropous as are exactly verti-
cal to the one or the other of the optical axes, which would be the case, accord-
Digitized by
GoogI(
OF DENI80N UNIVBRSITY. l$$
ing to the position of the optical plane, in the plane i-I, i-I or a prismatic surface.
Hence isotropous sections are more rare and less regular than in isometric minerals.
Sections parallel to thr vertical axis in the zone i-I, i-1 extinguish perpendicularly.
In monoctinic minerals only the ortho- diagonal {b) coincides with one of the
axes of elasticity, the other axes of elasticity form angles with a'' and of. The op-
tical plane is either parallel or at right angles to the plane of symmetry i-!.
If optic plane = i-1 the acute bisectrix = cl i »:
or •< •< " =.or="
then c and a are inclined toward c and a.
If optic plane is at right angles to i-t.
the acute bisectrix = b=a
«« «» =b=c .
or the obtuse *' = b=a
«♦ «' «« = b=c
then h and c or 6 and a are inclined toward of and zf ,
As a result of the inclination of the axes of elasticity to the crystallographic
axes certain of the longitudinal sections are not extinguished when the crystallo-
graphic axes coincide with the principal plane of the NicoPs prism. Sections in
the zone O. i-i, all are extinguished perpendicularly, for in this case the orthodi-
agonal coincides with an axis of elasticity.
In Triclinic minerals none of the three axes of elasticity coincides with the
crystallographic axes. All sections parallel to the three pinacoid surfaces, there-
fore, are obliquely extinguished. The angle of extinction in the planes O and
i-i is known for most triclinic minerals and forms a ready means of determining
these minerals. In thin sections the shape of the section will generally make it
clear whether it is or is not parallel to a pinacoid. Accurate measurements of the
angle of extinction must be made in cleavage plates parallel to i-i and O.
Twinning Phenomena.
Twins in the regular system cannot be recognized by the polarizer. Twins in
the tetragonal and hexagonal systems cannot be recognized unless the axes of the
twins are inclined toward each other. If one individual, in such a case, appears
dark between crossed Nicols the other may appear colored.
In some cases a crystal individual is made up of alternate lamellae of the same
mineral in different position relative to the axis. Such aggregates are called poly-
synthetic twins. In plagioclase feldspar, calcite and disthene this frequently oc-
curs. In such minerals the adjacent lamellae will not be extinguished simultane-
ously under polarized light.
In the rhombic system the twinning plane is usually one surface of a brachy-
dome, a pyramid or a prism. Pleochroism in colored minerals assists in deter-
mining the relation of twinned crystals.
In the monoclinic system the plane is often i-i''.
In the triclinic system the polysynthetic form appears very frequently. In
plagioclase the twinning plane is generally i-i (albite type). Sections at right
angles tp this plane in the zone O. i-i always exhibit the banded colors in polarized
light. Twins of this type are impossible in the monoclinic system, because the
Digitized by
GoogI(
J34 BULLETIN OF THE LABORATORIES
plane corresponding to i-I = i-K and coincides with a symmetiy plane, hence coal-
escence in this way would not produce twins. A second type is the pericline type
in which the twinning plane is at right angles to the zone O, i-f.
Pleochroism,
Double refracting minerals have the property of affording different colors when
looked through in different directions corresponding to the axes of elasticity, In
optically uni-axial crystals there are two such directions (/. e. the minerals are di-
chroic) and the color afforded by looking through in the direction parallel to the
vertical axis is called the basis color, that appearing when looked through at right
angles to this the axial color. A section of a uniaxial crystal will show no
change of color when rotated above the polarizer (analyzer being removed) if the
section is parallel to the vertical axis.
In tetragonal and hexagonal minerals, therefore, the directions where greatest
change of color ocQur coincide with the two axes of elasticity, in the orthorhombic
system they coincide with the three crystaHographic axes as wel!, but in the mono-
clinic and tnclinic this coincidence seems not to occur.
Use of convergent Polarized Light,
The occular is removed and a condensing lense is placed above the polarizer,
between it and the object. Interference figures of a nature varying with the
character of the mineral now appear. In regular and amorphous minerals no
such figures are produced. The same is true of sections parallel to the vertical
axis of the hexagonal and tetragonal minerals. In the transverse (isotropous)
sections of tetragonal and hexagoni.1 minerals an invariable dark interference cross
lies in the centre of the field. If the sections is oblicjue, the cross falls at one
side of the centre but is not otherwise altered. A rotation of the st^ge causes the
cross to apparently revolve in the same direction. If the section is so oblique
as to fall outside the field a rotation will bring first one limb and then the other
into view.
A section of. biaxial crystals taken perpendicularly to the bisectrices and
placed so that the optical plane coincides with the principal section of the Nicols
shows two closed curves enclosing the axial points. These curves are bordered
by other curves and crossed by a dark cross. The smaller limb of the cross
passes through, the axial points and indicates the position of the optical axis
plane. The broader limb of the cross is at right angles to it. When the
stage is revolved the cross does not remain invariable but is altered to form two
hyperbolas which move about the axial points and again form a cross after a
revolution of 90°.
The above account, condensed, in the main, from Hussak, will serve to in-
troduce the tables, while the student may be referred for more full explanations to
the works of Rosenbusch, Zirkle, Fouque'' and Levy, Cohen and especially
Hussak's ** Anleitung zum beslimmen der gesteinbildenden Mineralien."
D, Recapitulation of characters of the various crystal systems,
I. Amorphous and regular minerals are distinguished from all others by re-
maining dark in all positions between crossed Nicols, while the later may be
Digitized by
GoogI(
OF DENISON UNIVERSITY. 1 35
distinguished from the former, even in sections, by the regular contour and
cleavage.
2. The remainder of mineral species are anisotropous. Tetragonal and
hexagonal minerals have different elasticity in directions parallel and perpendicular
to vertical axis c. Rectangular or hexagonal longitudinal sections are extinguished
perpendicularly. In convergent polarized light transverse sections exhibit a
fixed axial cross, while in longitudinal sections no interference figures appear.
Sections obique to. the axis show lateral displacement of the optic axis. The
6xed axial cross appears to move in the same direction as the stage when revolved.
Hexagonal are distinguished from the tetragonal forms by the hexagonal and
twelve-sided transverse sections.
3. The remaining minerals are optically bi axial, and in those .sections which
are isotropous a black band appears in convergent polarized light, which appears
to move in a direction opposite to the stage. The axial point may lie within or
out of the field (depending on the inclination of the section.) If the band
(hyperbola) is bordered with red on the convex side and blue on the concave the
dispersion of the axes=/9>v, if the reverse p<Cv.
Orthorhombic minerals have both optic axes in a plane parallel a pinacoid.
In sections at right angles to the vertical axis extinction takes place parallel and
perpendicular to the sides. In sections parallel to the vertical axis extinction is
also perpendicular. No interference figure in the third pinacoid section in con-
vergent polarized light.
In Monoclinic minerals one axis of elasticity coincides with the orthodiagonal,
the others in the symetry plane i-f 'or perpendicular to it. Sections perpendicular
to the vertical axis are perpendicularly extinguished. Sections parallel to the
vertical axis if in the zone O . i-T are extinguished perpendicularly, sections
parallel to \-\ are obliquely extinguished. If the optical axial plane is parallel to
i-t none of the pinacoid sections exhibits perpendicular displacement of a bisectrix,
but the interference figure is displaced toward the vertical axis.
In Tridinic minerals neither of the axes of elasticity coincides with the crys-
tallographic axes. The optical axial plane is not peipendicular to pinacoid sur-
faces. No pinacoid section Is perpendicularly extinguished and none of these
sections exhibits perpendicular displacement of the bisectrix.
Explanation of Plate XL
Fig. I. Diagramatic section of Li thological Microscope as manufactured by
Fuess of Berhn. [See text.]
Fig. 2. Reflecting goniometer of WoUaston, for measuring angles of macro-
scopic crystals.
^^' 3- Polysynthetic twins of plagioclase ; section parallel i-i. (From
,Hussak.)
Ftg, 4. Plagioclase twinned according to both the albite and pericline type ;
section parallel i-i. (From Hussak.)
Fig. 5. Diagram illustrating relation of the optical axes etc., in twins.
(In A X = Normal.) It may be more clearly illustrated as follows : — A thin
plate of gyp&um is taken parallel the principal cUnodiagonal Out of this a
Digitized by
GoogI(
t$6 BULLCtlN Ot'^THE LABORATORIES
rhombic piece is obtained by using the conchoidal fracture parallel the ortho-
pinacoid and the satiny cleavage parallel to the -}- hemipyramid. The obtuse
angle =113° 46'' and the acute = 66° 14^ In as much as the optical axes in
gypsum lie in the clinodiagonal terminal plane, the surfaces of the plate may be
taken as the plane of the optical axes. The conchoidal fracture corresponds with
the main axis, and the optical axes above and anteriorly form angles of 23° and
83°. These are represented. By bisecting the acute and obtuse angles we may
secure the bisectrix (axis of least elasticity) and the optical normal (axis of
greatest elasticity) respectively. The bisectrix lorms an angle with the longer
diagonal of the plate of nearly exactly 20°. Every ray entering the plate per-
pendicularly is polarized, forming one ray vibrating in the plane of the bisectrix
and another in the plane of the optical normal. In a drawing of the exact shape
of the plate these lines are drawn and, in addition, the whole area checked into
squares by lines parallel to the bisectrix and the normal (on both sides.) The
plate is laid upon the drawing and both are bisected parallel the main axis and
one piece is revolved 180° and united by its other edge with the second. The
result is an artifical twin of gypsum with the optical determinants all indicated in
each (Rosenbusch.) The same method may be applied to illustrate other twins.
Jr^. 6 A. Orthoclase twins of Carlsbad type.
B ** Banover type.
Polysynthetic twins of calcite in marble.
Section of augite parallel to i-i, showing zonary structure. ( H ussak. )
Micro- crystals of gypsum (twinned.)
Inclusions showing Auction structure.
Hornblende crystal with opaque altered margin. (Hussak.)
Diagrams of interference figures in convergent polarized light.
(Adapted from Fouque''.)
The polarizer and condensor are used, but the occular is removed and the
Nicols are crossed. In regular and amorphic minerals no figure is seen. In
uniaxial crystals in isotropous sections there appears a fixed axial cross with more
or fewer concentric colored rings. These latter vary with the thickness of the
section. If the section is not exactly at right angles to the main axis the inter-
section lies out of the centre and revolves with the motion of the section. If
the inclination is still greater the intersection falls beyond the field and a revolu-
tion of 90° brings first one and then the other limb of the cross into view,
I A= a section slightly oblique, I B is the same section revolved 45°, and I C is
revolved °90. Biaxial crystals, if taken perpendicular to bisectrix or normal,
and if so placed that the opical axis plane coincides with the principal section of
the Nicols, show an interference figure consisting of two separate systems of curves
whose foci correspond with the two axes. These curves are surrounded each«
by a lemniscus and a black cross appears with a narrow arm passing through the
foci and a broader band between them (II A.) The slender band represents the
position of the optical axis plane. When the section is revolved the cross alters
its form and at 45° becomes an hyperbola passing about each focus while at 90° the
cross reappears but in a position trft06V«rse tp th»t formerly occupied.
fig.
7-
ftg.
8.
Fig.
9-
Fig.
10.
Fig.
II
Fig.
12.
Digitized by
GoogI(
TABLES FOR THE DETERMINATION OF THE PRINCIPAL
ROCKFORMIJ^G MIJ^TERALS,
Translated and Modified from Hussak's
Tabellen zur Bestimmung der Mineralien.
By C. L. Herrick.
Table of Abbreviations.
c ^= verlical crystallographic axis.
a = brachydiagonal in orthorhombic and triclinic systems.
2f =^ clinodiagonal in monoclinic system,
a, b. C, =^ axes of elasticity.
cj -^ exponent of refraction in ordinary ray. / In uniaxial
e -^ •* •• * in extraordinary ray. f crystals.
,i =- *' *' ** in intermediate axis, ^
/:> ^= •* *• '• in red light. V In biaxial crystals.
f = '* *• *• in violet light, J
Disp. = dispersion of axial points.
Symbols of planes essentially as in Dana's Text-book of Mineralogy.
Digitized by
GoogI(
A. Minerals opaque
Name.
I. Mag-
netite.
Chern Comp. and
reactions.
FeO-f-[Fej03.
soluble in H CI.
2. Titan-
ic Magne
tite.
3. Pyrtte.
Only distinguish-
able by chem.
analysis.
Fe S2
Soluble in nitric
acid separating
sulphur.
4. Titan-
ic Iron.
Fe Ti O, +
+[Fe2]'03.
Soluble in H CI
with dif. With
salt of phosph.
gives reaction for
Ti.
^ . Graph
'ite
(and Bitu-
men.)
6. Pyrr-
ho tite.
See also
Chro-^ b
mite, e
Pleo- I y
nast, j- o
Spec • n
ular d
Iron. '
C.
Bitumenous
black rocks, be-
coming gray on
heating.
Fe S ,
n n-fi
Specif.
Grav.
49-
5-2
4.8-
5.1
4.9-
5-2-
4.';6-
5-21
1.9-
2-3
4-54
4.64
Crys-
tal
In the
plane
I.
Hex.
Hex.
Hex.
Cleav-
age.
Usual conibina
tions and form of
Sections.
Granular and
I. Quadratic and
Triangular.
o [i-2j
Regular hexa-
gons and penta-
gons.
R and
OR.
con-
choi-
dal
frac-
ture.
I and granular.
Tabular R.oR
also-^ R, -2 R
and granular
grains elongate
rather than
rounded.
Sections chiefly
hexag. elongate
In zic-zacked or
reticulated forms
Very rare, ir
thin hexagonal
plates and irreg-
ular scales.
Irregularly gran
ular.
Twins.
In the
plane i.
as above
Interpene-
trating
twins
'A [i-2.]
Axes par-
allel, Poly-
synthetic
twins in
R.
Digitized by
Googk
even in thinest sections.
TABLE L
Color and
lustre.
Structure.
Association. Alterations,
Occurrence^ etc.
Iron black, Often in With nearly
blue-black beautiful cru- all rock -form
metallic lustre ciate a g g r e- ing minerals,
gates, or, as p a r t i cularly
result of alte augite, oli-
ration, about vine, plagio-
a mineral or close, nephe-
in its cleavage lin, and leu-
ilines. cite.
Very often
into limonite,
forming a red -
ish brown
band about
the crystal of
magnetite.
In reflected
light yellow,
metallic lustre
Bl ack ish
brown, metal-
lic lustre. If
altered, gray
in reflected
light. I
Iron black
with metallic
lustre.
Bronze yel
low. Metallic
lustre.
Into titanite
|a n d limonite
l(Forms the
transition to
ititanic iron.)
I
Into limon-
nite.
1. As primary ne-
cessary component of i
basic eruptive rocks
and accessory in near-
ly all crystallines.
2. As result of al-
teration of olivine,
augite, hornblende
and biotite.
Piimary, in basalts
and crystalline slates.
With plagi-
oclase, augite,
ho rn blende,
and olivine.
Rarely as accessory
secondary component
of altered basic erup-
tives and (also pri-
mary) in crystalline
slates.
Into titanite In basic eruptives.
and rutile with (particularly granu-
specular iron.jlar diabases, gabbros,
basalt, pikrit) ; also
in crystalline slates.
[Distinguished from
magnetite by form of
sections and altera-
tions.]
Rarely in cryst.
slates, clay and clay-
mica-slates, gneiss,
limestone; and as in-
clusions in staurolite,
andalusite, chiasto-
lite, dipyre, etc.
Very rare in cryst-
alline slates and con-
itact slates
Easily distinguished
from pyrite in re-
flected light by the
lustre.
Digitized by
GoogI(
B. Minerals transpa-
1. Minerals crystallizing in the Isometric System.
Name.
Chemical comp and
reactions.
specific
Gravity.
Cleav-
ctge.
i
Unsual combi-
nations dfform
of sections.
Gr.inular and
Twins.
Inter-
Color and
amount of
refraction.
Colorless,
I. Hauyn
3 (Na [Al,] Si,
08)+2 Na Ci. CI
2.13-2.29
Group.
i (rarely i. H)
pen'tra
red from
a. Sodalite
reaction. Easily
sections quad-
ting
presence or
soluble in H CI.
ratic and hex-
twins
Fe2 O3,
leaving a gelatin-
agonal.
in a tri-
greenish or
ous residue of Li
gonal-
bluish, blue.
O,. On evapora-
secon-
tion, cubes of salt.
dary
axis.
b. Hauyn
2. (Na, Ca) (Al,)
2.4-2.5
i
Crystals i and
Twins
Colorless,
Si, Og+(Na,, Ca^
I as the above
ini&as
blue, black.
and
SO4. Reacts for Ca
and H, SO^.
above
c. Nosean.
3 Na, (Al,) Si,
1.279-
Crystals usu-
Colorless,
Og-I-Na, SO^.
2.390.
ally corroded.
brown black
Reacts for H, SO^
Both are soluble
in HCl with sepa-
ration of gelatinous
LiO,
2 . Garnet
Fe,(Al2 ) Si^ O12.
3-78
Imper.
i, 2-2 and
Red,in very
Group.
iZ 1-4 2)
m
granular. Sec-
jthin secti'ns
a Alma
1
tions quadrat-
nearly color-
nadiiie.
ic, hexagonal
nt)= 1.772.
■
or octagonal.
.
b. Pyrope.
(Ca 0, Mg 0, Fe
Mn O) Al, O3
3Si O2 containing
chromium.
37-3.S
Imper
i
Granular.
Blood red.
Digitized by
GoogI(
rent in thin Sections. . table ii.
(Sec also amorphous-minerals, Opal and Hyalinamorph,)
Structure,
Intimately
united
with feld
spar and
horn blend
often with
a colorless
nucleus
and red
cortical
part closed
by oxides
of iron.
With micro
cline augite,
and mica in
syenites, with
sanidin and
augite in
Trachytes.
Association.
Very often
with a cor-
tical part
[colored dif
ferently
from the
centre !
which may
be darker!
opaque, t
Often col-'
ored by ;
iron in the!
fissures.
Chiefly with
leucite, nephe
line and au-
gite.
Inclusions.
Fluid inclu-
sions, gas
pores.
Glassy inclu-
sions, needles
of augite.
Innumerable
gas pores and
glass inclu-
sions ar-
ranged in
bands, minute
black grain.s
and needles,
often regular-
ly distributed,
tables of spec-
ular iron.
Alterations.
B'^comes tur-
bid from al-
teration into
zeolites.
As primary
constituent in
.syenites and
rarely in au
gite-trachytes
as secondary
product in
cavities of the
latter.
Alters into an
aggregate of
double re-
fracting need-
les and fibres
of zeolite and
calcite produ-
cing a destruc-
tion of the col-
or, and by sec
ondary colora-
tion by iron,
a change to
yellow.
Often uni-
ted with
quartz or
feld. spar
in micro-
pegmatite.
Usually with Cavities hav-
quartz, ortho iog the form
clase, biotite,iof the garnet
and horn- [crystal (nega
blende.
tive crystals)
I Fluid inclu-
sions, quartz
grains, rulile
grains ; often
' in zonary
bands.
with olivine
and augite.
Very poor.
Occurrence. Remarks.
Changes on
the surface
and in crevi-
ces to plates
of chlorite, or
(more rarely)
into fibrous
hornblende or
augite.
As primary
constituent ofj
later eruptive!
rocks (those
containing
sanidin as
well as those
bearing plagi-
oclase) such as
trachyte(rare-
ly) phonolite,
leucitophyre,
tephrite and
nephelin and
leucite-basalts
Occurs also
frequently in
volcanic tra-
chyte.
These three
minerals can
only be dis-
tinguished
with certainty
by microche-
mical qualita-
tive analysis.
Hauyn may
be distin-
guished from
sodalite by
the presence
of the charac-
teristic need
les of gypsum
on evapora-
tion of the ni-
tric acid solu-
tion. Sodal-
ite is charac-
ized by its
chlorine.
Hauyn and
Nosean are
chemically
difficult to dis-
tinguish and
mineralogical-
ly may be