Myron G. Best. Igneous and metamorphic 2003 Blackwell Science

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6.4 Describe the role of diffusion in petrologic

processes and how a knowledge of diffusion

rates can provide insight into rock-forming

phenomena.

6.5 Account for the large contrast in rates of chemi-

cal and thermal diffusion.

6.6 Discuss the nature of a phase interface and the

importance of surface free energy in the kinetic

paths and textural equilibria that inﬂuence rock

fabric.

6.7 Contrast homogeneous and heterogeneous nu-

cleation.

6.8 Describe a homogeneous nucleation model for a

one-component melt and indicate limitations in

applying this model to natural multicomponent

melts.

6.9 Comment on the apparent relative nucleation

rates of olivine, pyroxene, and plagioclase in the

Makaopuhi basalt (Plate III), assuming growth

rates are uniform.

6.10 Describe factors that control the growth and

dissolution of crystals in a melt.

6.11 Discuss factors that govern grain size in mag-

matic rocks; grain shape.

6.12 What similarities and contrasts exist between

nucleation and growth of bubbles and crystals?

6.13 Describe possible kinetic controls on fragmenta-

tion of melts and explosive volcanism.

6.14 What kinetic factors might be responsible for

the highly explosive eruptions of some silicic

magmas?

PROBLEMS

6.1 From Figure 6.2: (a) Plot viscosities, extrapo-

lated if necessary, at 1100°C versus weight per-

centage silica for dry silicate melts. Explain your

plot relative to the chemical composition of the

melts and their atomic structure. (b) Plot vis-

cosities of rhyolite melts with 77 wt.% silica at

800°C versus dissolved water concentration. Ex-

plain the drastic decrease in viscosity in the ﬁrst

2 wt.% or so of dissolved water, followed by a

lesser effect at higher water concentrations.

How does the speciation of dissolved water, as

(OH)



or as molecular H

O, enter into your ex-

planation?

6.2 Calculate the viscosity of Makaopuhi basalt melt

at 1200°C using the chemical composition listed

in Table 5.1 and the Shaw equation in Advanced

Topic Box 6.1. How does your answer compare

with the viscosity of 10

1.51

Pa s measured exper-

imentally by Shaw (1969)?

6.3 Calculate the activation energy of diffusion of

through anorthite crystals (Figure 6.4).

6.4 Compare the diffusivities of water, H

, and O

in a basalt melt at 1200°C. Calculate the average

diffusion distance for these three species over 1

day. Could this have any bearing on the com-

mon red color of basaltic lapilli around explo-

sive vents? Explain.

6.5 Freer (1981) cites D  9.66  10

16

/s at

1373K (1100°C) for interdiffusion of Fe and

Mg in Mg-rich olivine. Calculate the order of

time to homogenize a compositionally zoned

olivine crystal whose radius is 2 mm.

6.6 Demonstrate graphically that the ratio of surface

area to volume, 4r

/(4/3)r

, for spheres in-

creases rapidly as the radius, r, diminishes to

submillimeter dimensions. (Suggestion: Plot se-

lect values of r from 0.0002 to 0.015 mm against

corresponding values of the ratio.)

6.7 Draw hypothetical homogeneous nucleation

and crystal growth rate curves for a single min-

eral as a function of undercooling, and use

these to illustrate the contrasts in the kinetic

paths of glassy, aphanitic, and phaneritic

textures.

Chemical Dynamics of Melts and Crystals

147

UNDAMENTAL

UESTIONS

ONSIDERED IN

HIS

HAPTER

1. How do the time-dependent kinetic phenomena

discussed in the previous chapter control the

fabric of magmatic rocks?

2. How can the fabric of a magmatic rock be used to

obtain insights and sometimes speciﬁc information

regarding the kinetic path that the magma

followed during creation of the rock?

INTRODUCTION

Like magmatic rock compositions, magmatic rock fab-

rics comprise a wide and continuous spectrum of at-

tributes. Fabrics provide rich petrologic information

regarding the time-dependent kinetic path of the tran-

sition between the magmatic state and the ﬁnal solidi-

ﬁed rock, as well as its subsolidus history after solid-

iﬁcation. The kinetic history recorded in rock fabrics

takes different paths (Figure 7.1). In human affairs,

there are many possible “paths” for a journey (i.e., a

change in a person’s “state”) between San Francisco

and Paris, but an observer in Paris (at the ﬁnal “state”)

can readily differentiate the relaxed, well-tanned trav-

eler who made a week-long stopover on the beach of a

Caribbean island from the haggard red-eyed overnight

traveler. In similar manner, the appearance of a rock—

its fabric—is predominantly a record of the kinetic

path taken during the solidiﬁcation of the magmatic

system. Other factors can inﬂuence fabric besides ki-

netics, as in the contrast in water pressure in develop-

ment of hypersolvus versus subsolvus texture in granite

(Figure 5.18). Of course, contrasting kinetic paths can

result from contrasting magma compositions, as in the

contrast between paths in highly viscous silicic versus

much less viscous basaltic magma.

The importance of diverse kinetic paths in devel-

opment of fabric is most obvious when comparing fab-

rics in one type of solidifying magma. For example,

the same identical granitic (rhyolitic) magma can solid-

ify into the following fabric heteromorphs: phaneritic,

porphyritic phaneritic, aphanitic, porphyritic aphanitic,

glassy, vitrophyric, pumiceous, or pyroclastic. As this

chapter will show, additional distinct fabrics can be

found in the granitic-rhyolitic composition type. Fabrics

record kinetic paths in much the same way that mineral

compositions record intensive variables in the ﬁnal

thermodynamic state of the system.

Some kinetic paths recorded in rocks are relatively

simple, such as the creation of glassy fabric by drastic

undercooling of a melt. Other paths involve more than

one kinetic process operating simultaneously or in

close sequence, such as rather rapid crystallization and

volatile exsolution that result in an aphanitic vesicular

fabric. A particular fabric attribute, such as grain size,

can evolve along multiple paths, including crystalliza-

tion, crystal dissolution, textural equilibration, and frag-

mentation processes.

Fabric encompasses noncompositional properties of

a rock that comprise textures and generally larger-scale

structures. There is no sharp distinction between these

two. Textures, also called microstructures, are based

on the proportions of glass relative to mineral grains

and their sizes, shapes, and mutual arrangements that

are observable on the scale of a hand sample or thin

section under a microscope. Structures are larger-size

features generally seen in an outcrop, such as bedding

in a pyroclastic deposit or pillows in a submarine lava

ﬂow. Features related to exsolution of volatiles and

fragmentation of magma can occur on a wide range of

scales. One rock can have more than one texture and

one or more structures. Altogether, textures and struc-

CHAPTER

Kinetic Paths

and Fabric of

Magmatic Rocks

tures constitute the rock fabric, for which responsible

multiple kinetic paths of formation can be interpreted.

There is also no sharp distinction between fabric

and ﬁeld relations. But the latter pertain to aspects of

the whole rock body as a genetic entity, usually on a

scale of observation no smaller than an outcrop, such as

a lava ﬂow from its source to terminus. Field relations

of intrusive magmatic rocks are speciﬁcally addressed

in Chapter 9 and of extrusive rocks in Chapter 10.

The links between major kinetic paths and resulting

principal fabric attributes are shown schematically in

Figure 7.1. Following is an outline of how this chapter

is organized according to these kinetic paths. The out-

line lists speciﬁc fabrics to be described. Numbers in

parentheses refer to ﬁgures and plates illustrating the

speciﬁc fabric.

Section 7.1 Crystallization path → crystallinity and

grain size

7.1.1 Glassy (7.2–7.6)

Vitrophyric (7.4, Plate IIIa)

Perlitic (7.4, 7.5)

Spherulitic (devitriﬁcation of glass) (7.6,

6.15d)

7.1.2 Aphanitic (7.7–7.9, 7.26)

Aphyric

Cryptocrystalline

Microcrystalline (7.7–7.9, 7.35)

Felsitic (7.7, 7.26, 7.35)

Felty (7.8b)

Intergranular (7.9)

Intersertal (Plate IIId)

7.1.3 Phaneritic (7.10, 7.11, 7.13, 7.14, 7.16,

7.17, 7.20)

Equigranular (7.17)

Inequigranular (7.10, 7.11, 7.13, 7.14)

Seriate (7.10)

Pegmatitic (7.11)

7.1.4 Porphyritic (7.4, 7.7–7.9, 7.13)

Vitrophyric (7.4, Plate IIIa)

Cumulophyric (6.17, 7.34)

7.1.5 Poikilitic and ophitic (7.14, 7.15, 7.22)

Section 7.2 Crystallization path → crystal shape

Euhedral (7.3b, 7.15, 7.16, Plate IIIa)

Subhedral (7.10, 7.17)

Anhedral (7.10, 7.16)

Skeletal (7.3c, 6.15b)

Dendritic (6.15c)

Feathery (7.3b)

Spherulitic (7.6, 6.15d)

Glass (melt) inclusions (6.18)

Hypidiomorphic-granular (7.16, 5.18b,

Plate IV)

Aplitic (7.17)

Partial resorption (5.32, 6.19, Plate II)

Section 7.3 Crystallization path → inhomogeneous

grains

7.3.1 Normal zoning (5.14b)

Oscillatory zoning (6.17)

7.3.2 Reaction rim (5.32, 6.20, Plate II)

Rapakivi (7.18)

7.3.3 Decomposition (rim) (5.34)

Exsolution (5.14b, 5.18a)

Section 7.4 Textural equilibration → secondary

grain boundary modiﬁcation

Graphic (7.20)

Granophyric (7.21)

Myrmekitic (5.18b)

Section 7.6 Nonexplosive exsolution of volatiles →

volatile-ﬂuid cavities

Vesicular (7.23, 7.24, 7.28b, c)

Pumiceous (7.23)

Scoriaceous

Amygdaloidal (7.24)

Vuggy (7.25)

Kinetic Paths and Fabric of Magmatic Rocks

149

VOLATILE FLUID

NUCLEATION AND

CAVITY GROWTH

FRAGMENTATION

TEXTURAL

EQUILIBRATION

CRYSTALLIZATION:

CRYSTAL NUCLEATION

AND GROWTH

VISCOUS MAGMA

FLOW

Principal

fabric

attribute

Grain

orientation

Layering

Crystallinity

Grain

(particle)

size

Grain

(particle)

shape

Fragmental

Fluid

cavities

KINETIC PATH

metastableInitial melt

7.1 Generalized kinetic paths involved in the creation of magmatic

fabric as a metastable melt transforms to a solid magmatic

rock. Most fabric attributes evolve along multiple paths. For

example, layering can develop by magma ﬂow or by crystal-

lization processes.

Miarolitic

Diktytaxitic

Lithophysal (7.26)

Section 7.7 Fragmentation → volcaniclastic fabric

(7.27–7.35)

7.7.1 Pyroclastic (vitroclastic) (7.28–7.30,

7.32, 7.34, 7.35)

7.7.2 Autoclastic (7.33)

Section 7.8 Consolidation of volcaniclasts

Welding and fusion

Eutaxitic (7.34)

Rheomorphic

Devitriﬁcation and vapor-phase

crystallization

Felsitic (7.35)

Spherulitic (6.15d, 7.6, 7.35)

Section 7.9 Anisotropic fabrics (7.36–7.45, 7.47)

Foliated (7.36–7.45, 7.47)

Lineated (7.36)

Layering (7.36, 7.37, 7.40–7.45)

Igneous lamination (7.16b, 7.38)

Trachytic

Schlieren (7.39)

Flow layering (7.2, 7.4, 7.41, 7.42)

Section 7.10 Maﬁc inclusions (enclaves) (7.48)

Additional textural and structural features speciﬁc to

restricted rock types and their formative processes

are discussed in later chapters. For additional descrip-

tion and illustrations of textures the classic work of

Williams, Turner, and Gilbert (1982) or the shorter

treatment by Nockolds, Knox, and Chinner (1978) can

be consulted.

This chapter is designed to be a companion and se-

quel to Chapter 6, which introduced some concepts of

the kinetics or chemical dynamics of solidifying and

vesiculating melts. The intent is to apply these concepts,

150 Igneous and Metamorphic Petrology

(b)

(a)

02cm 0 0.04mm

7.2 Glassy texture in high-silica rhyolite obsidian. (a) Hand sample shows characteristic conchoidal fracture of the broken amorphous glass.

Note faint, alternating lighter and darker ﬂow layers whose deﬁnition is apparent in the photomicrograph in plane polarized light (b), which

shows minute, spiderlike crystallites that are too small to display any interference colors under cross-polarized light. Differing concentra-

tions of these abundantly nucleated crystallites, perhaps related to differing amounts of shear during viscous ﬂow, create the alternating

lighter and darker layers seen in the hand sample.

however tentative they may be, to facilitate a better un-

derstanding of the origin of magmatic rock fabrics.

7.1 FABRICS RELATED TO

CRYSTALLIZATION PATH:

CRYSTALLINITY AND GRAIN SIZE

The modal percentage of mineral grains relative to

glass—their crystallinity—ranges from 0% to 100% in

magmatic rocks. Grain size ranges widely, from submi-

croscopic grains much less than about 0.001 mm, which

is about the smallest that can be discerned with an op-

tical microscope, to the giant crystals of pegmatites,

which can be as much as several meters in dimension.

Crystallinity and grain size depend upon rates of crystal

nucleation and growth, as discussed in Section 6.5.4.

7.1.1 Glassy Texture

Glass is basically a highly viscous liquid, disordered on

an atomic scale, formed from a polymerized silicate

melt that was cooled too rapidly for crystallization to

occur. Although natural, massive high-silica glass, or

obsidian (Figure 7.2a), appears in hand samples to

have zero crystallinity, few natural glasses are entirely

lacking in crystals. High magniﬁcation (Figure 7.2b) re-

veals that obsidian contains abundantly nucleated sub-

micrometer-size crystallites that experienced limited

growth in the highly viscous glass. Though much less

viscous, basaltic melt solidiﬁes as a glass in drastically

undercooled margins of submarine lava pillows ex-

truded on the seaﬂoor (Figure 7.3) and in thin paho-

ehoe lava ﬂows on land. Thin streamers of basalt melt

Kinetic Paths and Fabric of Magmatic Rocks

151

Skeletal and

feathery crystals (c)

Glassy rind (b)

Pipe vesicle

(a)

7.3 Fabric of a basalt pillow. (a) Idealized cross section showing concentric zonal variation from outermost glassy rind, underlying zone of

skeletal crystals, and inner, more crystallized part with radial pipe vesicles and shrinkage cracks. Pipe vesicles may form normal to an in-

ward advancing front of crystallization that promotes volatile exsolution (Philpotts and Lewis, 1987). (b) The glassy outer rind a few mil-

limeters thick was produced by quenching of the hot magma against cold seawater and consists of basaltic glass in which are embedded

sparse phenocrysts of euhedral olivine that crystallized before extrusion on the seaﬂoor. Feathery crystals of clinopyroxene apparently

nucleated heterogeneously on the olivine during the rapid quenching and substantial undercooling of the melt. (Photograph of a sample

collected from the submersible Alvin along the Mid-Atlantic Ridge courtesy of A. E. Bence.) (c) Beneath the glassy rind is a mosaic of

skeletal crystals of plagioclase, pyroxene, and olivine produced by slightly less, but still substantial, undercooling. The innermost, and

most voluminous, zone of fabric development (not shown), reﬂecting the relatively slowest rate of heat loss in the core of a typical pil-

low, is a partly glassy to holocrystalline, intersertal to intergranular aggregate of plagioclase, olivine, pyroxene, and spinel, usually ex-

hibiting some noncompact crystal shapes. (Photograph from a Deep Sea Drilling Project core sample courtesy of A. E. Bence.)

0.1

(b) (c)

ejected from lava fountains in Hawaiian-type eruptions

also quench to glass (see Figure 7.29b). Thin dikes and

margins of thicker dikes of a wide range of composition

that are emplaced in the cool shallow crust can also be

of glass.

All glass is metastable and therefore susceptible to

secondary hydration, devitriﬁcation, and other types of

alteration (Bous

ka, 1993) that progress over time to

achieve a more stable state. Hydration and devitriﬁca-

tion are kinetic processes that depend on diffusion of

water molecules or (OH)



and H



ions and on nucle-

ation and growth of crystals, respectively, in the glass.

Hydration is faster than devitriﬁcation in silicic glasses.

Some massive glass having a waxy luster and dark

color in hand sample into which 6–16 wt.% water has

been absorbed is called pitchstone. More common hy-

drated silicic glass, occurring in subaerial environ-

ments, is called perlite because it typically has a pearl-

gray color, but some is green, red, or brown. As

much as 6 wt.% of perlite is H



absorbed at near-

atmospheric T and can be liberated by heating the glass

to only 110°C; the isotopic composition of this water

indicates that it is meteoric, not magmatic, in origin.

152 Igneous and Metamorphic Petrology

0 0.5mm

Feldspar

phenocryst

Perlitic

cracks

7.4 A combination of vitric (glassy) and phyric (porphyritic) tex-

ture is vitrophyric. In this photomicrograph of a silicic vitro-

phyre, phenocrysts of feldspar and opaque (black) Fe-Ti ox-

ides lie in a ﬂow-layered glass that has hydrated to produce

perlitic cracks (see also Figure 7.5).

7.5 Nests of concentric curved fractures in glass constitute perlitic

texture. Internal reﬂectance of light imparts a lustrous, pearly-

gray color to hand samples. Small nodules of black, unhy-

drated, and uncracked obsidian (called Apache tears) may be

located in the core of the nested cracks.

02cm

7.6 Spherulitic texture in high-silica rhyolite obsidian. Spherulites

are spherical to ellipsoidal clusters of radiating ﬁbrous alkali

feldspar and a polymorph of SiO

, here in a black glassy ma-

trix. Faint, relict ﬂow layering that extends from lower left to

upper right is accentuated in the spherulites in this hand sam-

ple, indicating their growth after active ﬂow of the magma. In-

dividual spherulites in volcanic rocks can range in diameter

from less than 1 mm to 1 m or so. A phenocryst may be located

at the center of the spherulite, where, in the original glass or

drastically undercooled melt, it allowed heterogeneous nucle-

ation of crystals to occur. Spherulites are secondary devitriﬁ-

cation features, not phenocrysts.

Grains in cryptocrystalline texture are too small to

be resolved optically but are visible with an electron

microscope and can be identiﬁed by X-ray diffraction

analysis. Larger grains in microcrystalline texture (Fig-

ures 7.7–7.9) can be discerned with a petrographic mi-

croscope. Microcrystalline rocks in which elongate rec-

tangular grains of feldspar are dominant have felty

Kinetic Paths and Fabric of Magmatic Rocks

153

(a)

(b)

7.7 Felsitic, microcrystalline, porphyritic aphanitic texture in an

intrusive granodiorite porphyry. Photomicrograph under

cross-polarized light. Weak oscillatory zoning in plagioclase

phenocryst just below center of view.

On the other hand, obsidian generally has 1 wt.%



of a magmatic isotopic composition that was

dissolved in the high-silica melt prior to solidiﬁcation.

It is part of the atomic structure of the glass, is held

tightly in the glass, and can only be liberated from the

glass by heating to hundreds of degrees. Perlitic tex-

ture (Figures 7.4 and 7.5) develops by hydration of ob-

sidian on fracture surfaces that are exposed to moisture

in the atmosphere or to meteoric water (groundwater).

As the outer rind hydrates, it expands and separates

along a crack from the nonhydrated substrate. Inward

repetition of this process creates a sequence of concen-

tric perlitic cracks that reﬂect light, creating the charac-

teristic pearl-gray color. Perlitic hydration occurs by

atomic diffusion so the rate of inward advance is de-

scribed by t  x

/C, where t is time, x is distance,

n is about 2, and C is a constant for a given area that

is chieﬂy a function of mean ambient T (Friedman et

al., 1966). Fast rates occur in hot climates and are

about 1–2 micrometers/1000 years, according to mea-

surements of the thickness of the hydrated rind on ob-

sidian artifacts of known age. Once a calibration has

been established in a particular climate, obsidian arti-

facts of unknown age can be dated by measurement of

the thickness of the hydrated rind.

Devitriﬁcation, or delayed crystallization, of silicic

glasses produces two crystalline textures, felsitic (dis-

cussed later) and spherulitic (Figures 7.6 and 6.15d).

Hydration and devitriﬁcation tend to be simul-

taneous in basaltic glass, producing the alteration

product called palagonite. In thin section, this is

isotropic to weakly birefringent, variably orange to

brown, and commonly concentrically layered in col-

loform fashion. Relative to pristine basalt glass (Plate

IIIa), palagonite is vastly but variably enriched in

water, to as much as 30 wt.%; most of the Fe is oxi-

dized to the ferric state; and most other elements

have perturbed concentrations. Palagonite is a com-

plex mixture of clay and zeolite minerals and hydrated

ferric oxides.

7.1.2 Aphanitic Texture

Aphanitic texture (Figures 7.7–7.9), typical of surface

extrusions and near-surface small intrusions of magma,

consists of a mosaic of crystals too small to be identiﬁ-

able by the naked eye without magniﬁcation by a hand

lens or a microscope. Aphanitic texture implies high

crystal nucleation rates relative to growth rates, such as

occur during rapid reduction in T or water content of

the magma system.

Relatively few aphanitic rocks are aphyric, or non-

porphyritic. The presence of phenocrysts in most

aphanitic rocks testiﬁes to the fact that few magmas

reaching near the surface of the Earth are superheated

above liquidus temperatures.

154 Igneous and Metamorphic Petrology

Hornblende

phenocryst

Plagioclase phenocryst

with corroded core

Pyroxene

phenocryst

Microlites

of feldspar

in felty matrix

Hornblende phenocrysts

with decomposed rims

rich in opaque Fe–Ti oxides

(a)

(b)

01cm 0 0.5mm

7.8 The most common texture in volcanic rocks is porphyritic aphanitic, as in this andesite from a lava ﬂow. (a) Hand sample in which phe-

nocrysts of white plagioclase and black hornblende lie in a gray aphanitic groundmass. (b) Photomicrograph of same rock in plane-

polarized light showing phenocrysts surrounded by a microcrystalline matrix made largely of small birefringent microlites of euhedral

plagioclase that are interwoven in a felty texture. For a magniﬁed view of the partially decomposed hornblende phenocrysts see

Figure 5.34a.

Olivine phenocrysts

with minute semiopaque

inclusions of Cr-spinel

Part of

olivine phenocryst

Plagioclase

Pyroxene

Magnetite

mm 0

0.2

(a)

(b)

7.9 Porphyritic aphanitic basalt from a lava ﬂow. Intergranular texture of the aphanitic groundmass, shown in the same thin section at two

different magniﬁcations in plane-polarized light, is an interlocking network of randomly oriented plagioclase microlites and abundant

more equant pyroxene and Fe-Ti oxide grains. Euhedral to subhedral olivine phenocrysts may be present in some basalts. Intergranular

texture resembles the felty texture of the andesite in Figure 7.8, except that maﬁc minerals are more abundant.

texture (also called pilotaxitic texture; Figure 7.8). The

feldspar microlites in such rocks are large enough to dis-

play optical birefringence in polarized light. Felty tex-

ture is common in the aphanitic matrix of andesitic

rocks and some basaltic rocks; in both, the feldspar is

plagioclase. If feldspar microlites are oriented in a com-

mon direction the texture is trachytic (see Section 7.9.1).

Felsitic texture is that of any aphanitic mosaic of

mostly felsic minerals—feldspars and quartz—found

commonly in rhyolite, dacite, and trachyte. Some de-

vitriﬁed glass has a felsitic texture. Basaltic rocks com-

monly have intergranular texture, wherein randomly

oriented microlitic plagioclases and abundant, more

equant pyroxenes, Fe-Ti oxides, and, in some rocks,

olivines, form a tight interlocking mosaic (Figure 7.9).

Many aphanitic rocks appear to be holocrystalline in

hand sample though examination of a thin section re-

veals the presence of minor glass. In many basaltic

rocks, intersertal texture resembles intergranular tex-

ture except that brown glass is interspersed among the

microcrystalline grains as a result of incomplete crys-

tallization (Plate IIId).

7.1.3 Phaneritic Texture

Phaneritic texture occurs in rocks in which grains of

major rock-forming minerals are all large enough to be

identiﬁable with the unaided eye (Figures 7.10–7.14).

Smaller Fe-Ti oxides and accessory minerals, such as

zircon and apatite, are typically not visible without a

microscope. Phaneritic rocks are typically found in

magmatic intrusions and reﬂect crystallization at small

degrees of undercooling, perhaps only a few degrees;

nucleation rates are relatively low regardless of crystal

growth rates. Magmatic intrusions worldwide of differ-

Kinetic Paths and Fabric of Magmatic Rocks

155

2.5

0cm

7.10 Seriate texture in a polished slab of inequigranular phaneritic granite. White, mostly rectangular subhedral grains are plagioclase. Light

gray, more equant grains are potassium-rich alkali feldspar; note the oscillatory zoning in the larger ones made evident by delicate alter-

nating layers of whiter and grayer color concentric to their margins. Dark gray anhedral grains without apparent crystal faces are quartz.

Black grains are biotite. Slight differences in nucleation and growth rates during crystallization of constituent minerals at small under-

cooling are responsible for this seriate fabric.

ent compositions, sizes, magma viscosities, and depths

of emplacement (dictating cooling rates) all have a re-

stricted range of grain sizes, generally 1–20 mm. This

order-of-magnitude range suggests that nucleation and

growth rates are not signiﬁcantly different in different

magmas at small degrees of undercooling. Otherwise,

more variable grain sizes might be anticipated.

Phaneritic rocks have an equigranular (Figure 7.12),

or simply a granular, texture if the grains are of similar

size, within an order of magnitude, or so. Each crys-

talline phase must have experienced similar nuclea-

tion and growth rates. Other phaneritic rocks have

an inequigranular texture in that they contain grains

of conspicuously variable size. These include rocks of

porphyritic texture in which there is a bimodal size

population (large distinct from small; see below) and

rocks having seriate texture in which grains have a

more or less continuously ranging size (Figure 7.12).

Many granites, such as shown in Figure 7.10, possess

seriate texture made up of apatites and zircons visible

only under the microscope, somewhat larger but gen-

erally 1-mm Fe-Ti oxides, larger maﬁc silicates, com-

monly still larger plagioclase and quartz grains, and al-

kali feldspars that are as much as 2–3 cm, or more. The

156 Igneous and Metamorphic Petrology

7.11 Pegmatitic fabric is exceptionally coarse, but variable seriate

grain size. Most grains are 1 cm. Pegmatitic fabric is best dis-

cerned in outcrops, rather than hand samples, because grains

are commonly as large as, or larger than, a typical hand sam-

ple. Giant crystals in some pegmatites attain extraordinary di-

mensions of 10 m or more. The photograph shows a quarry

face in the Harding pegmatite, Taos County, New Mexico; the

gallon jug beside the 0.6-m-long box in the lower left corner of

photo attests to the giant size of the white crystals of spo-

dumene (LiAlSi

). (Photograph courtesy of Richard H.

Jahns. Reproduced with permission from Jahns RH and

Ewing RC, 1977. The Harding Mine, Taos County, New

Mexico, Min. Record 1977; March, April: 123.)

Aphanitic Phaneritic

Equigranular

Inequigranular

seriate

Inequigranular

porphyritic

phaneritic

Porphyritic aphanitic

Vitrophyric (glassy matrix)

Phenocrysts

PhenocrystsMicrolites

Grain size

Frequency of a particular grain size

7.12 Schematic grain size populations in common magmatic rocks.