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THE TEXTURE OF IGNEOUS ROCKS
WHITMAN CROSS, J. P. IDDINGS, L. V. PIRSSON, H. S. WASHINGTON
The vagueness that characterizes descriptions and conceptions
of textures of igneous rocks is due
in part to
a lack
of systematic
treatment
of the subject by
petrographers, and a consequent inexact-
ness in the terminology by which definite conceptions may be expres-
sed. It
is also due to
the intricacies of
the textures to be described
and to their
diversity.
The first may be remedied by
more systematic consideration
of
the fundamental factors that make up rock textures, and by the
introduction of more definite descriptive terms. The second diffi-
culty can be met
by more
accurate and
detailed descriptions of the
complex fabrics, and by the recognition of the more frequently
recurring ones
as types for reference.
In considering
the fundamental factors constituting rock texture
it is necessary to begin with the simpler, elemental factors, and also
the simpler fabrics; afterwards taking up the more complex and
intricate fabrics. This is not the order of the more common first,
and subsequently the
less frequent fabrics. On
the contrary, many
of
the commonest fabrics
are the most intricate and the most
difficult
to describe. This is probably a reason why the subject has never
been
satisfactorily developed. A systematic treatment must com-
mence with
more or less abstract, general,
principles, which eventually
may be applied to concrete
cases. It is hoped
that the following
discussion may advance
this part of petrography.
Fundamental factors constituting texture in igneous rocks.-
Defining texture
as the material features of rocks exhibited by
the
mineral components and by the groundmass of dense
or glassy
rocks,
whether they are viewed megascopically
or microscopically,
it has already been said' that these features fall into three categories:
i. Crystallinity-the degree of crystallization.
- Granularity-the magnitude of the crystals.
' This Journal, Vol. X (1902), p. 611.
692
TEXTURE OF
IGNEOUS ROCKS
- Fabric-the shape and arrangement of the crystalline and
non-crystalline parts.
It is
proposed to consider further
each of these
factors in the
general constitution of texture
separately, in order
to determine
what
characters in each need
special recognition
and specific des-
scriptive terms.
I. Crystallinity
The degree of
crystallization attained
by an igneous rock is
measured by the relative amounts of crystallized and glassy portions.
All
degrees of crystallinity are
known to
exist, from
perfectly glassy
rocks, through those consisting of glass with more or less crystals,
to
completely crystalline rocks. The size
of the
crystals is
not
involved in
the crystallinity, but is a factor in
the granularity.
The idea of completecrystallinity, as the condition most commonly
observed, has a definite term
in holocrystalline, while
the general
idea of partly crystalline is expressed by hypocrystalline. The
general idea
of glassiness is
expressed by
the terms vitreous and
hyaline; and partly
glassy is expressed
by hypohyaline. The term
subvitreous is commonly employed in describing the luster of some
minerals, but has
not been applied
to rock texture. Completely
glassy is expressed by holohyaline. There is a term signifying that
a rock is evidently crystalline to the unaided eye-phanerocrys-
talline-and one
that signifies that its
crystalline character can
not be recognized by the unaided eye-aphanitic.
It
would be
proper to
use the term phanerohyaline
for rocks
whose
glassiness is evident to the unaided eye; and the term phaneric should
be applied to those rock textures that are apparent megascopically,
whether crystalline or glassy; just as the term aphanitic is used for
textures that cannot be
discerned by the naked
eye, whether crystal-
line or glassy. But
there are no terms to indicate any intermediate
degrees of crystallinity. Such terms would permit more definite
descriptions which become more and
more necessary with the intro-
duction of quantitative standards.
Definitely determined proportions between glassy and crystalline
parts should be expressed in
exact mathematical terms, but it
may
frequently happen that approximate proportions only are known
or are considered sufficient for the
purpose of the
description. In
TEXTURE OF IGNEOUS
ROCKS
terms in use express certain general ideas of magnitude based on
the limits of vision, as follows:
Phanerocrystalline-All
sizes large enough
to be seen with the
unaided eye, that is,
megascopically.
Cryptocrystalline (aphanitic)-all sizes too small to be seen with
the unaided eye. Aphanitic is a
purely negative term
which
does not state
whether the rock is crystalline or glassy,
and is
very convenient when it is desirable to express simply the fact
that
the matter is
not determinable megascopically. Cryptocrys-
talline states that the rock is crystalline, and that the crystals
are
too small to
be seen megascopically.
Microcrystalline-all sizes recognizable only with a microscope,
and therefore embracing
most of those
that are megascopically
cryptocrystalline.
Microcryptocrystalline-sizes too small to be seen
with a microscope,
but recognized as crystal particles by the exhibition of
aggregate
polarization.
Microaphanitic-This term may
very well be employed to
cover those
doubtful cases of microscopic and submicroscopic aggregation
which have been
sometimes described as
microfelsitic.
Phanerocrystalline rocks have been described
as coarse, medium,
or fine grained, with little effort to fix absolute values for these
terms.
Zirkel's suggestion that
they be compared
with the size
of peas and
millet seed has led to the more definite suggestion, already made
by us, that the
average diameter of crystals in-
Fine-grained rocks be taken as
less than i
millimeter; in
Medium-grained rocks, between I and 5 millimeters;
and in
Coarse-grained rocks,
greater than 5 millimeters.
Coarse-grained
rocks are vaguely described as very coarse, or
extremely
coarse, without any indication of the
actual degree of
granularity. It would be more definite to express the approximate
size of the grain
of rocks in terms of the various units of the metric
system.
There are some extremely coarse rocks, such as pegmatites,
whose crystals may be
measured in meters;
more whose
crystals
are several decimeters in diameter;
many that may be measured
in centimeters. Using a decimal series of units, it would be reason-
able
to employ the
following terms for
approximate descriptions
696 CROSS,IDDINGS, PIRSSON, AND WASHINGTON
of
the grain of
rocks, remembering that in finer-grained
rocks the
millimeter is oftener the unit of measurement, especially in micro-
scopic work:
Meter-grained rocks, when the
average size of the
crystals is
over
I meter.
Decimeter-grained rocks, when the size of
the crystals is
from I to
o10decimeters.
Centimeter-grainedrocks, when the size is from i to 10 centimeters.
Millimeter-grained rocks,
when the size is
from i to
Io millimeters.
Millimeter-grained rocks would include medium-grained and
slightly coarse-grained rocks.
Decimillimeter-grained rocks; those in
which the average
size of
the crystals is from o. i to i. o millimeter.
Micron- (millimillimeter-) grained
rocks; those in
which it is
from
I to 10 microns, from 0.001 to o.o010iomillimeter.
Decimicron-
(decimillimillimeter-) grained rocks; those in
which it
is from
0.1 to .0o microns, o.0ooo to o.oo
millimeter.
The terms just described may be found useful in describing
rocks in a
general manner, but each includes a wide
range of varia-
tions in the size of grain, which in the extreme reaches a ratio of
10: for the diameters of the crystals, which
is a ratio of 100:I for
areas of crystal sections; differences which might be found in the
crystals of porphyritic
rocks. It follows that
the grain of the rocks
that may be described by any one of the terms mentioned-as, for
example, millimeter-grained-is not always of the same order of
magnitude, since
in one case it may be eight or nine times larger in
diameter than in
another.
III. Fabric
The arrangement of the crystalline parts of a rock, or of the crys-
talline and glassy
parts when glass is
present, which we have called
the fabric, depends on the relative sizes of the parts, on the shape of
the crystals, and on
the positions with respect to one
another and to
the glass base when present. The significance of these factors will
appear upon further consideration.
Relative size of crystals.-While it never happens that all crystals
in a rock are of one
size, it often happens that they are approximately
698 CROSS,
IDDINGS,
PIRSSON,
AND WASHINGTON
Basing the comparison of the sizes of crystals on those of the
great number of
crystals that give character to the
fabric of the
rock,
the following
general distinctions
may be recognized:
I. Equigranular rocks-composed chiefly of crystals of like orders
of magnitude.
II. Inequigranular rocks-composed of crystals
of different orders
of magnitude.
I. EQUIGRANULARROCKS.-The fabric of equigranular rocks
depends further on the-
(A) Shapes
of the crystals.
(B) Arrangement or
distribution of them.
(A) Shapes of crystals
may be described in
general terms with
reference to the presence or absence of crystallographic faces as:
Euhedral-completely bounded by crystal faces; automorphic,
idiomorphic.
Subhedral-partly bounded
by crystal faces, hypautomorphic,
hypidiomorphic.
Anhedral-without crystal faces, xenomorphic, allotrimorphic.
The
crystals of
equigranular rocks may be:
Equiform-all the same shape, or nearly
so; or
Multiform-having various shapes.
With reference to their dimensions the shapes may be described as:
Equant-equidimensional or nearly so.
Tabular-in plates or
tables.
Prismoid-in
prismatic forms.
Irregular-not one of the three preceding divisions.
More specifically
crystals may
be described further
under each
of
the general
shapes just named,
as follows:
Equant.-Cuboidal, polyhedral, spheroidal, equant
anhedral,
equant
subhedral.-The simplest examples of
equant, equiform,
equigranular, fabric are found
in an
evenly granular quartz vein,
in some quartzites and
crystalline limestones,
and in
certain anor-
thosites, dunites, and pyroxenites.
Tabular.-Tabular, platy, foliated.
The crystals may be in
plates, tables,
disks, folia, scales.-The best examples of tabular,
equiform, equigranular fabric
are found in certain feldspar
rocks:
syenite with so-called
trachytic fabric, hedrumite, and certain anor-
TEXTURE OF IGNEOUS ROCKS
thosites.
It will be found in some micaceous
metamorphic rocks
with highly developed foliated texture.
Prismoid.-Parallelopipedons, lath-shaped blades, prisms, spin-
dles, fibers.-Examples of
prismoid, equiform,equigranular, fabric
are
rare among igneous
rocks. The most familiar
examples are certain
metamorphic, actinolite rocks: strahlstein.
The great
majority of equigranular
rocks are multiform, since
they consist of several kinds of minerals whose characteristic shapes
are
not alike. Quartz is
almost always
equant, micas
are nearly always
tabular, amphiboles frequently prismoid; whereas feldspars may be
equant, tabular, or
prismoid, and
pyroxenes may be equant or pris-
moid. The various combinations of these shapes produce fabrics
which have
no simple character
and require specific description.
They frequently recur and should be distinguished by
simple terms
for
convenience.
(B) Arrangement or distribution of
crystals in equigranular
rocks
produces differences of fabric.
I. Equant.-Where crystals are equant there can be no varia-
bility in the arrangement which would affect the fabric, so far as
form alone is concerned. But in
case there are more than
one kind
of mineral
present, especially if they are
of different colors, the dis-
tribution of
equant crystals of different kinds produces variations
in fabric as regards color-varieties
of color pattern.
- Tabular.-The tabular crystals may be arranged in the follow-
ing ways:
Parallel.
Subparallel.
Diverse,
in all
directions (omniversal).
Radial, divergent, fan-like, spherulitic, concentric, imbricated.
- Prismoid.-The
prismoid crystals
may lie in the
following
positions:
Parallel.
Subparallel.
Diverse,
in all directions (omniversal).
Radial, divergent, spherulitic, axiolitic.
Tangential, around a nuclear crystal.
Irregular.-Crystals
of irregular shapes
forming equiform,
TEXTURE OF IGNEOUS
ROCKS
In porphyries the phenocrysts may be of any size, microscopic or
megascopic. Groundmass and phenocrysts are the essential parts
of porphyritic fabric, variations in which produce different porphy-
ritic fabrics. The
kinds of variation are as follows:
(a) Relative amounts of groundmass and phenocrysts.
(b) Character of the
phenocrysts.
Sizes.
Shapes.
Arrangement.
(c) Character of the groundmass.
Texture: Crystallinity.
Granularity.
Fabric.
(A) The relative amounts of phenocrysts and groundmass.-Differ-
ences in
the amounts of phenocrysts
give a noticeable character to
porphyritic rocks, which up to this time has found very
little expres-
sion in
petrographic literature. At one extreme are porphyries with
very few phenocrysts;
at the
other, those crowded with pheno-
crysts and having very little groundmass. Employing the French
word for
groundmass, pate=" paste," and sem= "sown"
or
"sprinkled," comparisons are easily described by the terms:
Perpatic, extremely rich in
groundmass.
Dopatic, groundmass dominant.
Sempatic, groundmass and
phenocrysts equal or nearly
equal.
Dosemic, phenocrysts dominant.
Persemic, extremely rich in phenocrysts.
Having
noted the relative
amounts of groundmass and pheno-
crysts, the
relative sizes, shapes, and arrangement of the phenocrysts
may be taken into account.
(b)
Character of phenocrysts.-Size.
Phenocrysts may
be large
enough
to be seen
megascopically, or they may
be so small as to
be
only visible
microscopically; for these distinctions
we have
the
terms:
Megascopic phenocrysts, or megaphenocrysts.
g.m.
ph.
g.m.
ph.
g.m.
ph.
g.m.
ph.
g.m.
ph.
I
i 3
3 5
<3i,
57
I
702 CROSS,IDDINGS, PIRSSON, AND WASHINGTON
Microscopic phenocrysts, or microphenocrysts.
In the
first case the rock may
be said to
be-
Megaphyric, having megascopic phenocrysts;
and in the second case it may be called--
Microphyric, having
microscopic phenocrysts.
Considering
the megaphyric rocks, the phenocrysts may
be large
or small, of any size. Following the custom in describing the grain
of equigranular
rocks, we
might establish three common distinctions:
Magnophyric-coarsely porphyritic, the phenocrysts
greater than
5mm
in longest
diameter.
Mediophyric-moderately porphyritic, the phenocrysts between
5mm and Imm in
longest diameter.
Minophyric-minutely
porphyritic, the phenocrysts from
Imm to
o. 2mm in longest diameter.
In case corresponding distinctions in the size of microscopic
phenocrysts are desirable-that
is, in microphyric rocks-the same
terms may be used with the change of the letter o to i, and a corres-
ponding decrease in the value for the diameters.
Magniphyric-coarsely microphyric, the
phenocrysts having longest
diameters from 0.2 to o.o4mm
Mediiphyric-moderately microphyric, phenocrysts having longest
diameters from 0.04 to o.oo8mm.
Miniphyric-minutely microphyric,
phenocrysts having longest diam-
eters less
than o.
oo8"m.
Such minute phenocrysts are microlites in a glass base.
There is further the distinction as to whether
all the phenocrysts are:
Of like magnitude, or
Of different magnitudes.
Shape.-The
shape or form of phenocrysts
is an
important
factor in determining fabric. While the specific shape of a pheno-
cryst
may be
a crystallographically complex form requiring
some-
what elaborate description, there are certain terms which serve to
describe the
crystals without reference to crystallographic detail.
Some
of these
are as follows:
Equant-with surfaces about equidistant from the center, sometimes
cuboidal or spheroidal.
Tabular-flattened
in one plane.
704 CROSS, IDDINGS, PIRSSON, AND
WASHINGTON
It has
been suggested
that this term be used
when the rock is
granopatic phanerocrystalline,
and-
Graniphyric, when the
groundmass is microcrystalline, or granipatic.
The texture. called granophyre by Rosenbusch is called by us
graphophyre or graphiphyre,
according as the
groundmass is mega-
graphic or micrographic.
Granularity.-The terms that may be applied to the grain of
equigranular rocks
are applicable to
the groundmass
of porphyritic
rocks, and do not
need to be repeated in this place. A distinction
between megascopic
and microscopic sizes may be effected by using
the letters o and i,
respectively, when the
textural term is used as a
prefix, as in grano- and grani-.
Fabric.-The fabric of
the groundmass of a
porphyry may be
the same as that of a rock taken as a whole, so that any term applied
to one may be used in describing the other.
- Poikilitic fabric is
that kind of hiatal fabric in which the matrix
consists of relatively large crystals, through which are scattered
relatively small
crystals. The rock may be called a poikilite, in
contrast to a porphyry.
The two factors in this fabric, inclosing
crystals or oikocrysts and inclosed crystals, xenocrysts,may vary (a) in
relative proportions, and (b) in relative size; (c) the xenocrysts may
vary in shape, and
(d) in arrangement or distribution.
(a) Relative
proportions between oikocrysts and xenocrysts. All
possible proportions exist, from that in which the host constitutes
the slightest interstitial cement between the
xenocrysts, which in
such a
case may form the greater part of the rock, to that in
which
the xenocrysts occupy a very small space in the oikocrysts.
To
express the relative amount of outer
and inner crystals in poikilitic
fabric, the
following terms may be used:
Peroikic,
oikocryst extremely abundant.
Domoikic, oikocryst
dominant.
Xenoikic, oikocryst
and xenocrysts equal
or nearly equal.
Doxenic, xenocrysts dominant.
Perxenic, xenocrysts extremely abundant.
oikocryst
xenocryst
oikocryst
xenocryst
oikocrvst
xenocryst
oikocryst
xenocryst
oikocryst
xenocryst
'
i 3
3 5
5 7
I
7
TEXTURE OF IGNEOUS ROCKS
(b) The sizes
of the xenocrysts as compared with the matrix
crystal in which they occur may be described as
relatively large,
medium, and small or fine. The relative sizes are
best expressed in
terms of the average diameters of the inner
crystals and host.
FIG. 4 FIG. 5 FIG. 6 FIG. 7
Relatively
large xenocrysts-those whose average
diameter is greater
than
one-eighth the average diameter of the host (Fig. 4.)
Relatively medium-sized xenocrysts-having average diameters be-
tween one-eighth and one-twelfth that of the host. (Figs.
5, 6.)
Relatively small xenocrysts-having
average diameters
less than
one-twelfth
that of the
host.
. (Fig. 7.)
Poikilitic fabric may be megascopic or microscopic, when its
granularity may
be described with reference to
the size of
the oiko-
crysts in the terms applied to equigranular rocks as coarse, medium,
or fine grained.
(c) The shapes of the xenocrysts lead to modifications of poikilitic
fabric, as in porphyritic fabric. The general shapes may be-
Equant.
Tabular.
Prismatic.
All the
xenocrysts may have similar shapes; they may be equiform.
Or they may have different
shapes; they may be
multiform.
(d) The
arrangement of the
xenocrysts further modifies
the fabric.
They
may be-
(1) Scattered more
or less uniformly through
the oikocryst; or
(2) Grouped
together in various
ways.
Those xenocrysts
that are tabular or
prismatic may stand in
all
possible positions, or they may be parallel or nearly parallel to one
another, sub-parallel, or their positions
may be diverse.
Ophitic fabric is a particular case of poikilitic in which the xeno-
TEXTURE OF IGNEOUS
ROCKS 707
fabric is
found
to recur frequently,it may
be given
a specificname,
which
may
possess
more
or
less elasticity,
accordingto
the
definiteness
of the descriptionto whichit has been applied.
It will be observed that the foregoing discussion is confined
almost
wholly to general features of textures, and
that
the
terms
suggestedhave a generalapplication. They contributeto the general
descriptiveequipment
of
petrography.
The
specific
description
of
a
definite rock
texture
must
of necessity be
a
complex
expression,
which, if needed
frequently
for other rocks, may be representedby a
denotiveterm, derivedfrom a geographicalname, with
the
termina-
tion
al,
as
suggested
in our essay
on a QuantitativeClassificationof
Igneous Rocks.'
' This
Journal, Vol. X (1902),
p.
