Myron G. Best. Igneous and metamorphic 2003 Blackwell Science

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more or less continuous variation in grain size probably

reﬂects differing ease of nucleation among the copre-

cipitating minerals in the slightly undercooled magma;

Fe-Ti oxides nucleate readily, alkali feldspar nucleates

at the slowest rate, and the other minerals in interme-

diate manner.

Small intrusive bodies and parts of larger ones made

of exceptionally large, but heterogeneously sized, crys-

tals whose dimensions are at least several centimeters

and locally meters deﬁne pegmatitic fabric (Figure

7.11). Outcrops are generally required to identify this

fabric, which must reﬂect limited nucleation and fast

crystal growth rates.

7.1.4 Porphyritic Texture

Most aphanitic rocks, many glassy rocks, and some

phaneritic rocks contain large, more or less euhedral

phenocrysts embedded in a distinctly ﬁner-grained or

glassy matrix, or groundmass. This is porphyritic tex-

ture. Porphyritic aphanitic textures are shown in Figures

7.7–7.9; porphyritic glassy, or vitrophyric, texture in Fig-

ure 7.4 and Plate IIIa; and porphyritic phaneritic texture

in Figure 7.13. Phenocrysts rarely constitute more than

50% of aphanitic and glassy rocks formed in extruded

magma because abundant crystals immobilize magma

and retard extrusive ﬂow. In hand sample, phenocryst-

rich (35–50%) aphanitic and glassy rocks may superﬁ-

cially appear to be phaneritic, careful examination with

a hand lens or microscope will reveal otherwise.

Porphyritic textures originate in different ways: That

is, they are polygenetic. Probably the most common

origin for porphyritic aphanitic and vitrophyric tex-

tures involves a two-stage cooling history for the melt.

An initial episode of slow cooling rate (small under-

cooling) yields few nuclei just below liquidus tempera-

tures in a thermally insulated plutonic environment be-

low the surface of the Earth. These grow to produce

relatively large phenocrysts. After this partial crystal-

lization, the magma experiences an episode of relatively

rapid heat loss in a small intrusion in the shallow cool

crust or in an extrusion onto the surface; both create

the aphanitic or glassy matrix around the phenocrysts.

However, other kinetic paths can create porphyritic

texture under more or less uniform cooling of plutonic

magma bodies:

1. A two-stage cooling path cannot account for deep

plutonic phaneritic rocks that must have cooled

slowly at a rather uniform rate but contain large

phenocrysts of one mineral in a ﬁner matrix of

others. In this case, different nucleation rates for

different minerals may be involved. An example is

euhedral alkali feldspar phenocrysts in porphyritic

phaneritic granodiorite (Figure 7.13). These large

alkali feldspars have provoked decades of debate as

to their origin, but it is now believed that they nu-

cleate more slowly just above solidus temperatures

than other constituent minerals (Vernon, 1986).

Many granitic magmas, such as granodiorite in

Figure 5.29, reach alkali feldspar and quartz satura-

tion within only a few degrees to tens of degrees

above solidus temperatures. As much as half of the

magma by volume may still be melt at this stage

because of the considerable solubility of alkali

feldspar and quartz. Therefore, growing alkali feld-

spars have ample space to produce a large crystal as

they grow from sparse nuclei. Experiments suggest

that alkali feldspars nucleate more slowly than

quartz and plagioclase in granitic magmas (Swan-

son, 1977).

2. Porphyritic aphanitic texture can also develop in a

shallow granitic intrusion that experiences an es-

sentially isothermal reduction in water pressure.

The texture of the resulting porphyry rock evolves

through two levels of volatile concentration, not by

two-stage cooling. Initial nucleation and growth of

phenocrysts of plagioclase and other high-T miner-

als promote volatile oversaturation in the residual

melt. If the exsolved ﬂuid rapidly escapes from the

system, such as by rupturing roof rocks and venting

ﬂuid through the cracks, the remaining melt be-

comes supersaturated with less soluble crystalline

phases. In effect, the solidus of the system rotates

clockwise in P-T space (Figure 5.2; see also Figures

5.11, 5.29, and 5.33), stranding the melt in the crys-

talline stability ﬁeld, so a shower of nuclei develops,

yielding an aphanitic matrix around the pheno-

crysts.

Kinetic Paths and Fabric of Magmatic Rocks

157

0cm

7.13 Porphyritic phaneritic texture in granodiorite. The commonly

pink, subhedral Carlsbad-twinned phenocrysts of K-rich alkali

feldspar are as much as 6 cm in longest dimension and are sur-

rounded by a ﬁner but phaneritic matrix of gray quartz and

feldspar and black biotite. One of the Carlsbad-twin individu-

als is oriented so as to reﬂect light off its cleavage.

3. Bowen (1914) pointed out that continuous uniform

cooling of some melts can produce porphyritic tex-

ture. Consider crystallization of a melt containing

90% of the CaMgSi

component in the simple bi-

nary system CaAl

-CaMgSi

(Figure 5.4).

For some amount of undercooling below the liq-

uidus, diopside nucleates. Growth of these pyrox-

ene crystals at decreasing T occurs until, at the

eutectic T, anorthite plagioclase begins to coprecip-

itate with diopside. Meanwhile, the near-liquidus

diopside crystals have had a long growth period

and could exist as large phenocrysts in a ﬁner ma-

trix of anorthite and possibly diopside crystals if

more of these had nucleated near the eutectic.

Many aphanitic and glassy rocks contain clots, or

polygranular aggregates, commonly of the same miner-

als of the same size that occur as isolated phenocrysts

in the same rock. The clots in this cumulophyric tex-

ture (Figures 6.17 and 7.34) can originate in all of the

ways that phenocrysts can. Clots originate as sus-

pended crystals attach to each other, or they may be de-

rived from breakup of the more crystallized wall of the

magma chamber where precipitated crystals accumu-

lated. Alternatively, some clots may be restite material

dislodged from the site of magma generation.

7.1.5 Poikilitic and Ophitic Textures

In both ophitic and poikilitic textures, larger crystals

enclose smaller, randomly oriented crystals. The larger

crystals form from fewer nuclei than the smaller

enclosed mineral grains. In poikilitic texture, large

oikocrysts (literally, house crystals) completely sur-

round many smaller grains. Poikilitic texture occurs

in a wide range of rock compositions. For example,

in phaneritic ultramaﬁc rocks, oikocrysts of amphi-

bole or pyroxene many centimeters in diameter enclose

millimeter-size olivines, chromites, and other minerals

(Figure 7.14). In some granitic rocks, near-solidus al-

kali feldspar oikocrysts surround minerals precipitated

at higher T. Ophitic texture in rocks of basaltic com-

position is found in some aphanitic lava ﬂows and more

typically in slightly coarser, marginally phaneritic dikes

of diabase. In this texture, large clinopyroxenes par-

tially to completely enclose smaller euhedral plagio-

clases (Figure 7.15; see also Figure 7.22).

7.2 FABRICS RELATED TO CRYSTAL-

LIZATION PATH: GRAIN SHAPE

Most, but not all, aspects of the shape of magmatic

crystals result from the way they crystallize from melts.

Under near-equilibrium conditions at small amounts

of undercooling, slow uninhibited crystallization of a

melt yields euhedral crystals bounded by characteristic

crystal-face forms. Euhedral is synonymous with idio-

morphic (Greek, “one’s own form”). Euhedral crys-

tals are commonly found as isolated phenocrysts in

aphanitic and glassy rocks. Thus, euhedral olivines in

Plate IIIa and Figures 7.3b and 7.9a are bounded by

prism and pinacoid forms; prismatic pyroxenes, tabu-

lar feldspars, and platy micas can be seen in other ﬁg-

ures in this chapter. Faster cooling rates (greater

undercooling) yield progressively less compact grains

of greater surface area relative to their volume. Figures

6.14 and 6.15 show a hierarchy of plagioclase grain

shapes with increasing amounts of undercooling—

from compact tabular crystals (see also Figures 7.15

and 7.16) to skeletal, to dendritic and feathery, to

spherulitic grains.

As some crystals grow in a skeletal fashion they can

completely encompass pockets of melt, forming melt

inclusions. If the host crystal is rapidly quenched as the

magma is extruded, inclusions solidify as glass rather

158 Igneous and Metamorphic Petrology

7.14 Poikilitic texture in peridotite, Stillwater Complex, Montana.

A single large pyroxene oikocryst, with light reﬂecting off its

cleavage, encloses numerous smaller, randomly oriented dark

grains of olivine. A single pyroxene nucleus formed and grew

in the residual melt, surrounding numerous earlier-formed

olivine crystals.

than crystallizing. Some inclusions have an inverted

crystallographic habit of their host crystal with which

they are in equilibrium—a sort of “negative crystal”

(Figure 6.18).

Less perfectly formed compact grains are subhedral

if only partly bounded by crystal faces (plagioclases in

Figure 7.17) and anhedral, or xenomorphic (foreign

form), if not bounded by any characteristic crystal face

(pyroxenes in Figure 7.16). These noneuhedral grains

can form by restricted growth from melts in highly

crystallized magmas where neighboring grains interfere

with development of characteristic growth forms. Note

that grains within cumulophyric clots (Figure 6.17) are

commonly subhedral to anhedral, and if, for example,

a clot breaks up during extrusion of the host magma,

the resulting isolated phenocrysts are anhedral to sub-

hedral. Other noneuhedral grains result from fragmen-

tation processes discussed in Section 7.7 and from par-

tial dissolution (resorption) of grains into the enclosing

melt with which they are unstable (Plate II and Figures

6.19 and 6.20).

Phaneritic aggregates of grains showing varying eu-

hedralism deﬁne two rock textures. In hypidiomor-

phic-granular texture there is a mix of euhedral, sub-

hedral, and anhedral grains. It is seen in a wide range

of rock types, from gabbro (Figure 7.16) to granite

(Figure 5.18b and Plate IV). In granodiorites, Fe-Ti ox-

ides, hornblendes, biotites, and plagioclases tend to be

euhedral to subhedral, whereas quartz and alkali

feldspar grains are typically anhedral. As discussed in

relation to porphyritic phaneritic rocks, melts in grano-

dioritic magma systems become saturated with alkali

feldspar and quartz within a few tens of degrees above

solidus temperatures (Figure 5.29). Just above the

solidus, only about half of the magma by volume

has crystallized as free-formed, euhedral maﬁc miner-

als and plagioclase. Consequently, during near-solidus

growth of quartz and alkali feldspar there is less free-

dom for growth among the existing higher-T euhedral

grains and they become anhedral. If only sparse nu-

clei of alkali feldspar form, these grains become phe-

nocrysts if earlier formed grains are moved aside or

oikocrysts if they are enveloped.

In equigranular phaneritic rocks, exempliﬁed by

granite aplites consisting of generally ﬁne leucocratic ag-

gregates of alkali feldspar and quartz, virtually all grains

are equant and anhedral to subhedral (Figure 7.17).

This texture is appropriately known as aplitic. It ap-

pears likely that all of the grains crystallized essentially

simultaneously from the melt and competed equally for

space.

Kinetic Paths and Fabric of Magmatic Rocks

159

0 0.5mm

Plagioclase

Olivine

Augite

Plagioclase

7.15 Ophitic texture in a diabase (coarse basalt) lava ﬂow. Several

randomly oriented, euhedral plagioclases are partially to com-

pletely surrounded by a single large crystal of clinopyroxene.

Compare Figure 7.22.

(a)

(b)

7.16 Hypidiomorphic-granular texture in gabbro. Euhedral mag-

netites (black), plagioclases (prominent cleavage), and olivines

interspersed among anhedral pyroxenes. (a) Isotropic fabric.

(b) Preferred orientation of tabular plagioclases deﬁnes ig-

neous lamination, a type of anisotropic fabric.

7.3 FABRICS RELATED TO

CRYSTALLIZATION PATH:

INHOMOGENEOUS GRAINS

In a system at equilibrium, every phase must be homo-

geneous, including each mineral grain. But in most

crystallizing magmas, sluggish reaction rates between

melt and crystals lag behind rates of changing intensive

parameters. Accordingly, many grains in magmatic

rocks are inhomogeneous. Several types of zoned and

composite grains manifest states of disequilibrium.

7.3.1 Zoned Crystals

A systematic pattern of chemical variation within a solid

solution mineral is called zoning. It is a record of in-

complete continuous reaction relations between a melt

and the crystallizing solid solution as intensive parame-

ters were changing in the magma system faster than ki-

netic rates could maintain equilibrium (Section 5.5.2). In

some minerals, zoning forms a sector pattern that is

hourglass shape in prismatic crystals, such as clinopy-

roxene. In feldspars, zoning is concentric to the exterior

grain margin. Even in the most slowly cooled, hottest

magmatic intrusions, plagioclases are normally zoned

from calcic cores to more sodic rims, testifying to the

very sluggish rates of diffusion of NaSi and CaAl ions

during crystallization (Figure 5.14b). Reverse zoning is

rarely evident in plagioclases. Oscillatory zoning, espe-

cially widespread in plagioclases in intermediate compo-

sition magmatic rocks (Figure 6.17), most likely origi-

nates in the sluggish kinetics of crystal growth (e.g.,

Bottinga et al., 1966), but the exact process is not clearly

understood. However, some oscillatory zoned feldspars

might reﬂect cyclic changes in intensive parameters

throughout the magma chamber, such as episodic emis-

sion of steam from an overlying volcanic vent, causing

ﬂuctuations in water concentration and consequent

shifts in crystallization conditions (Figure 5.16).

7.3.2 Reaction Rims

Incomplete discontinuous reaction relations in fraction-

ating magmas are recorded in a reaction rim that sur-

rounds an anhedral, partially resorbed grain of another

160 Igneous and Metamorphic Petrology

K-rich

alkali feldspar

Quartz

Plagioclase

7.17 Aplitic texture in a granite aplite dike that intruded grano-

diorite. Photomicrograph under cross-polarized light. The

quartz and feldspar grains are of subhedral to anhedral shape

and of similar size, so the rock is equigranular. This texture

likely results from similar rates of nucleation and growth of the

felsic minerals, all of which were growing more or less simul-

taneously. In hand sample, the texture appears sugary, like

sandstone, but, unlike in sedimentary rock, the grains are

somewhat interlocking and pore spaces or secondary cement is

nonexistent.

7.18 Rapakivi feldspar in dacite, Clear Lake, California. Photomi-

crograph in cross-polarized light showing sanidine (high-T,

K-rich alkali feldspar with a sector twin on left) surrounded by

polysynthetically twinned oligoclase plagioclase. Stimac and

Wark (1992) believe these mantled feldspars were produced

by mixing of a sanidine-bearing rhyolite magma with basaltic

andesite magma. (Photograph courtesy of James A. Stimac.

Reproduced from Stimac JA and Wark DA. Plagioclase man-

tles on sanidine in silicic lavas, Clear Lake, California: Impli-

cations for the origin of rapkivi texture. Geol. Soc. Am. Bull.

1992; 104:728–744, with permission of the publisher, the Geo-

logical Society of America, Boulder, Colorado, USA. Copy-

mineral. Such reaction rims develop during fractiona-

tion in the model Mg

SiO

-SiO

system (Section 5.3.2)

and are found in many magmatic rocks where rim and

core phases are solid solutions. In Plate II, decreasing

T and increasing water concentration in the crystalliz-

ing magma destabilized the clinopyroxene so that it re-

acted with the melt to form hornblende reaction rims.

If magmatic conditions had changed more slowly so

that the diffusion-controlled reaction relation could

have been completed, achieving a state of equilibrium,

all of the pyroxene would have been consumed in the

reaction. In Figure 5.32, reduction in water pressure in

the volcanic system promoted a reaction between the

unstable resorbing hornblende and the enclosing melt,

creating a reaction rim of stable pyroxene, plagioclase,

and Fe-Ti oxides; the developing rim curtailed fur-

ther resorption in the time available. In Figure 6.20, a

xenocryst of quartz incorporated into basalt magma

reacted with the silica-undersaturated melt, forming

a reaction rim of stable clinopyroxene crystals. Their

needlelike habit probably reﬂects rapid crystallization

as the hot melt was quenched around the cool xeno-

cryst. Rim crystals nucleated heterogeneously on the

unstable core crystal.

Observations of volcanic rocks and complementary

experiments suggest that rapakivi texture, in which an

alkali feldspar grain is rimmed by plagioclase (Figure

7.18), can originate during mixing of magmas (Stimac

and Wark, 1992). Alternatively, Nekvasil (1991) pro-

poses that magma decompression during intrusion can

create plagioclase-mantled alkali feldspars.

Another history of reactions with enclosing melt is

revealed in some plagioclases (Figure 7.19), which con-

tain an internal concentric zone crowded with irregular

inclusions of minute mineral grains like those consti-

tuting the microcrystalline matrix.

7.3.3 Subsolidus Decomposition and Exsolution

in Unstable Minerals

Partial to complete replacement of hydrous maﬁc phe-

nocrysts is common in volcanic deposits (Figure 5.34).

Because such replacements occur in the interior and es-

pecially upper part of the deposits but are lacking in

Kinetic Paths and Fabric of Magmatic Rocks

161

0.2

(a) (b)

Fe-Ti oxide

Pyroxene

Plagioclase

7.19 Complexly zoned plagioclase phenocryst in basaltic andesite lava ﬂow. Photomicrographs in plane-polarized light at two different mag-

niﬁcations. Clear anhedral core is andesine and thin clear rim is labradorite. “Spongy” zone is crowded with irregular glass inclusions

and minute mineral grains like those in groundmass of rock. (b) Enlarged view of bottom of (a) rotated 90° counterclockwise.

the rapidly quenched, commonly glassy bases, it is in-

ferred that these replacements developed after extru-

sion at subsolidus temperatures. As the interior and

upper parts of the deposit cool and adjust toward the

higher oxygen and lower water fugacity of the atmo-

sphere, biotites and amphiboles are destabilized, and

very ﬁne-grained, anhydrous replacements that consist

of pyroxenes, Fe-Ti oxides, and feldspars develop in

their place. Partial replacement creates what may be

called decomposition rims, but over time the entire

grain can be replaced, or pseudomorphed. In the same

volcanic rocks, pyroxenes are generally not decom-

posed. Olivines in maﬁc lava ﬂows are commonly

rimmed or completely pseudomorphed by red-brown

“iddingsite,” which is a cryptocrystalline mixture of

ferric oxides and clay minerals.

Pyroxenes, magnetite-ulvöspinel solid solutions,

and alkali feldspars that crystallized in slowly cooled

plutons as more or less homogeneous minerals from

the melt can exsolve at subsolidus temperatures into

more stable coexisting crystalline phases. The most

widespread exsolution texture is perthite (Figure

5.18a), an intergrowth of Na- and K-rich alkali feld-

spars formed by unmixing below the alkali feldspar

solvus (Figure 5.17). Exsolution texture is also seen in

some pyroxenes in gabbro (Figure 5.14b), where they

unmixed below a solvus.

7.4 FABRIC RELATED TO TEXTURAL

EQUILIBRATION: SECONDARY

GRAIN-BOUNDARY MODIFICATION

In slowly cooled magma systems, textural equilibration

may modify grain size and shape after the initial

episode of crystallization. Textural equilibration is

more likely if temperatures remain high so that move-

ment of atoms can minimize the surface free energy of

the crystalline aggregate (Section 6.4).

Puzzling intergrowths of quartz grains in an alkali

feldspar host are called graphic texture (Figure 7.20).

This appellation is suggested by the wedge- and

hook-shaped quartz that calls to mind ancient (circa

3000 BC) cuneiform writing. It occurs in some leuco-

cratic granite pegmatites, called graphic granites. A

similar micrographic, or granophyric, texture (Figure

7.21) occurs in felsic rocks—usually intrusive—called

granophyres. A long-favored origin, among many that

have been proposed, involves simultaneous crystalliza-

tion of quartz and feldspar from a melt; this process has

been conﬁrmed experimentally by Fenn (1986) and

MacLellan and Trembath (1991). As a euhedral plagio-

clase begins to grow in a viscous granitic melt, a silica-

162 Igneous and Metamorphic Petrology

02cm

7.20 Graphic texture in hand sample of graphic granite. The scat-

tered quartz grains in a single-crystal host of perthitic alkali

feldspar resemble poikilitic texture. However, the quartz

grains are more or less uniformly spaced and are not randomly

oriented. They are all crystallographically continuous, as may

be veriﬁed by their optical continuity viewed in thin section

under cross-polarized light, and grew simultaneously with the

alkali feldspar as an intergrowth.

Plagioclase

0.50mm

7.21 Granophyric texture in granophyre under cross-polarized

light. Note somewhat altered euhedral plagioclase at center of

sprays of micrographic quartz and alkali feldspar. The sufﬁx

-phyric implies a porphyritic texture in which the ﬁne-grained

intergrowth surrounds a phenocryst.

sure, and fugacities of volatiles, especially water. It

would be useful for the petrologist if inexpensive pet-

rographic examination of rock fabric could determine

the order of crystallization and thereby provide insight

into the nature of changing intensive parameters with-

out recourse to costly, tedious, less available laboratory

equipment and techniques. Four textural criteria have

been employed during the past decades to ascertain or-

der of crystallization:

1. Comparison of phenocrystic and groundmass min-

erals in quenched magmas, usually volcanic rocks

2. Reaction rims

3. Mineral inclusions in larger grains

4. Relative euhedralism at mutual grain contacts

The latter two criteria are examined here (see also

Flood and Vernon [1988, pp. 105–116] for a more

thorough critique).

Inclusions of, for example, pyroxene poikilitically

enclosed in a larger oikocryst of plagioclase are com-

monly interpreted to mean that pyroxene crystallized

before plagioclase. But this interpretation is not neces-

sarily correct in every case. At least three alternate pos-

sibilities are:

1. Late pyroxenes might have crystallized from melt

inclusions entrapped within skeletal plagioclase af-

ter it ceased crystallizing.

2. Late pyroxenes might have crystallized from melt-

ﬁlled cracks that cut thorough the plagioclase after

it grew.

3. The inclusion-ﬁlled plagioclase might be a xeno-

cryst or a restite grain derived from metamorphic

rocks in the deep continental crust where the

magma was generated.

The order of crystallization indicated by the relative

euhedralism of grains in mutual contact can also be

ambiguous (Figure 6.26). Ophitic texture (Figure 7.15)

and hypidiomorphic-granular texture (Figure 7.16)

have mutual grain contacts that tempt one to interpret

order of growth. For ophitic texture, it is commonly as-

sumed that the euhedral plagioclases grew freely in the

melt, followed by pyroxene, which is anhedral because

of the restricted available space within the network of

earlier plagioclases. However, this is only one of at least

three different possible orders of crystallization of pla-

gioclase and pyroxene. In Figure 7.22a, a few nuclei

formed in the melt allow early growth of pyroxene, fol-

lowed by abundant nucleation of plagioclase and

growth of both phases; the ﬁnal resulting ophitic tex-

ture is shown on the right. In Figure 7.22b, pyroxene

and plagioclase nucleate in the melt and grow simulta-

neously to create the ﬁnal ophitic texture. In Figure

7.22c is the conventionally assumed order of crystal-

lization in which plagioclase nucleates and grows be-

fore pyroxene does. In all three instances, pyroxene nu-

cleates less abundantly than plagioclase; that can be

taken as the only valid conclusion for ophitic texture. A

and alkali-enriched boundary layer (Figure 6.16) forms

around it. Continued crystallization may create a den-

dritic to spherulitic intergrowth of alkali feldspar and

quartz, the former crystallographically continuous with

and projecting radially outward from the euhedral pla-

gioclase core. Some Ostwald ripening might coarsen the

large-surface-area intergrowth (Figure 6.21), producing

granophyric texture. Whether phaneritic graphic gran-

ite might be created by further Ostwald ripening is a

speculative possibility.

A superﬁcially similar intergrowth in granitic rocks

consists of an intergrowth of vermicular (“wormy”)

quartz in a sodic feldspar host typically in contact with

K-rich alkali feldspar (Figure 5.18b). The origin of this

myrmekite has been ascribed to direct crystallization

or to replacement but Castle and Lindsley (1993)

believe it may originate in subsolidus unmixing of

K-bearing plagioclase in a ternary feldspar system open

to excess Si.

7.5 A WORD OF CAUTION ON

THE INTERPRETATION OF

CRYSTALLINE TEXTURES

Kinetic investigations of the dynamic behavior of melts

in the laboratory, coupled with theoretical models and,

as always, evaluated by observations of real rocks, pro-

vide insights into the origin of magmatic fabrics. How-

ever, the behavior of natural magmas is impossible to

duplicate wholly by experiments and models. A deﬁ-

ciency in experiments is an appropriate handling of the

time scale of rock-forming processes; the slow crystal-

lization of plutonic magma bodies can never be dupli-

cated in one researcher’s professional career! In plu-

tonic rocks, and to some extent in volcanic as well, the

observed fabric is the cumulative product of a long pe-

riod of changing geologic conditions and declining ki-

netic rates that took place above and below the solidus;

the effects of overprinting processes are not easily un-

raveled. The spontaneous drive toward a state of tex-

tural equilibrium that potentially can modify grain

boundaries is largely unexplored in slowly cooled mag-

matic rocks. The kinetic history of magmatic systems as

indicated by the rock fabric is frequently ambiguous

except in the simplest of cases.

To emphasize and illustrate this sense of caution we

next brieﬂy discuss a line of interpretive petrology that

has been a part of the discipline virtually since its in-

ception, namely, the determination of the order of crys-

tallization of minerals from the magmatic rock texture.

7.5.1 Magmatic Rock Texture and

Order of Crystallization

The sequence in which minerals crystallize with chang-

ing intensive parameters, most commonly declining T,

between the liquidus and solidus of the magma, de-

pends upon the magma composition, conﬁning pres-

Kinetic Paths and Fabric of Magmatic Rocks

163

less important factor is the relative growth rates for the

two crystalline phases that vary somewhat among the

three scenarios.

One of the pitfalls in using relative euhedralism of

two or more crystals to decide their order of crystal-

lization is to assume that the later anhedral grain is

molded onto the earlier euhedral one. However, the

two grains could have grown simultaneously, or even in

reverse order; the contrast in shape may simply reﬂect

the lower surface free energy of rational faces of the eu-

164 Igneous and Metamorphic Petrology

hedral crystal that prevailed in minimizing the overall

energy of the grain contact (Figure 6.26).

Interpretation of the order of crystallization for the

hypidiomorphic-granular granodiorite previously dis-

cussed proﬁted from a determination of the crystal

melt equilibria in a similar model system. This is an ex-

ample of how observations of real rocks and experi-

mental data can be integrated to arrive at a more accu-

rate conclusion than can be achieved by using only one

source of information.

Pyroxene and plagioclase nucleate and

grow simultaneously

FINAL

Ophitic texture

Few nuclei of

pyroxene, then

growth

Continued growth of

pyroxene; abundant

nucleation of plagioclase,

then growth

EARLY

SUBSEQUENT

(a)

(b)

Abundant nuclei of

plagioclase, then

growth

Continued plagioclase

growth; few pyroxene

nuclei, then rapid growth

(c)

7.22 Three alternate kinetic paths for creating ophitic texture.

Kinetic Paths and Fabric of Magmatic Rocks

165

7.6 FABRICS RELATED TO

NONEXPLOSIVE EXSOLUTION

OF VOLATILE FLUIDS

This section deals with fabrics produced by volatile ex-

solution from a melt (Sections 4.2.3 and 6.7) that did

not result in fragmentation of the magma. Fragmenta-

tion is considered in Section 7.7.

Volatile-ﬂuid bubbles become vesicles as magma

solidiﬁes. The corresponding texture is vesicular.

The surface tension of a static melt against a ﬂuid

phase makes bubbles spherical—the shape of least

surface area relative to volume. Movement of either

the bubble in the melt or the melt containing the

bubble can distort this equilibrium shape. Elongate

pipe vesicles (Figure 7.3a) are one result. Highly

vesicular silicic glass, or pumice (Figure 7.23), has

a pumiceous texture. Scoria and scoriaceous are

parallel terms for andesite and basalt. Vesicles can

be larger in basaltic lavas (to as much as 10 m or so)

than in silicic, not necessarily because they contain

more dissolved volatiles, but rather because basaltic

melts have a less viscous nature, which allows the

bubbles to expand and coalesce before the magma

solidiﬁes.

Some smooth-walled vesicles may be ﬁlled with sec-

ondary minerals precipitated from ﬂuid solutions per-

colating through the rock, producing amygdules and

amygdaloidal texture (Figure 7.24).

0cm

0.2

0mm

Glass

Vesicle

7.23 Highly vesicular glass deﬁning pumiceous texture in silicic

pumice. Stretched vesicles are apparent in hand sample (a)

and thin section photomicrograph in plane-polarized light (b).

02cm

Amygdules

Unfilled

vesicles

7.24 Amygdaloidal texture in basalt. Vesicles ﬁlled with secondary

minerals that precipitated from percolating aqueous solutions

are amygdules. Filling minerals may be carbonate or zeolite

minerals, or some form of silica, such as quartz, chalcedony, or

opal. Amygdules are not phenocrysts.

166 Igneous and Metamorphic Petrology

Vapor-phase precipitates are common in lithophysae

and include quartz, topaz (in F-rich metaluminous

rocks), and specular hematite. Lithophysae are locally

ﬁlled with secondary silica (quartz, chalcedony), form-

ing geodes.

7.7 VOLCANICLASTIC FABRICS

RELATED TO FRAGMENTATION

OF MAGMA

The term volcaniclastic refers to a broad category of

fragmental material, called volcaniclasts, composed at

least in part of volcanic rock and formed by any parti-

cle-generating process and subsequently transported

and deposited by any mechanism (Fisher and Smith,

1991). Volcaniclastic processes can operate over a wide

range of scales. Therefore, their products are repre-

sented not only in rock fabrics but also in larger-scale

stratigraphic ﬁeld relations, described in Chapter 10.

In this continuum of fabric and stratigraphic charac-

teristics, distinquishing one from the other is somewhat

arbitrary.

Volcaniclasts can be considered from four per-

spectives:

1. Size of fragments is the most fundamental attribute

of volcaniclastic deposits, as it is in sedimentary de-

posits (Section 2.4.1): (a) Ash particles are 2 mm

(corresponds to sand, silt, and clay sizes of sedi-

ment); (b) lapilli (singular, lapillus) are 2–64 mm

(pebble and granule); (c) blocks and bombs are

64 mm (boulders and cobbles). Because work

In highly crystalline magmas, exsolved ﬂuid can col-

lect in angular vugs between crystals, creating vuggy

fabric (Figure 7.25). Euhedral vapor-phase crystals,

most commonly quartz in silicic magmas, nucleate on

the crystalline walls of the vug and grow freely into the

cavity. Miarolitic cavities are vugs in phaneritic rocks,

especially in granitic rocks, that are widely spaced,

many grain diameters apart. Diktytaxitic texture is

found in some coarse-grained basalt (diabase) lava

ﬂows and consists of small angular vugs interspersed

pervasively among the slightly larger plagioclase and

pyroxene grains.

Hollow, bubblelike gas cavities in rhyolite lava

ﬂows and compacted tuffs are lithophysae, creating

lithophysal fabric (Figure 7.26). In lava ﬂows, litho-

physae are subspherical masses generally a few cen-

timeters in diameter composed of concentric onion-

like shells of aphanitic material. Their origin is

uncertain, but rhythmic exsolution and expansion of

volatiles during crystallization may be involved. In

compacted welded tuffs, lithophysae tend to be more

irregular discoidal cavities commonly lacking the con-

centric structure seen in lava ﬂows. They may form

by collection of exsolving gas into pumice fragments.

0 mm 5

7.25 Vuggy fabric in felsitic aphanitic rhyolite. Collection of ex-

solved volatiles into irregularly shaped pockets in a highly

crystalline matrix produces vugs. They are commonly lined

with euhedral vapor-phase crystals that grew freely in the en-

trapped volatile ﬂuid. Vapor-phase precipitates include quartz

(seen here), specular hematite, and, in some peraluminous

rhyolites, topaz and Fe-Mn garnet.

0 cm 1

7.26 Lithophysal fabric in felsitic aphanitic rhyolite. Lithophysae

(singular, lithophysa, meaning “stone bubble”) occur in partly

glassy to aphanitic rhyolitic rocks and consist of onionlike

masses a few centimeters in diameter with concentric shells of

aphanitic quartz and alkali feldspar. (U.S. Geological Survey

photograph courtesy of Robert L. Smith and Susan L. Russell-

Robinson.)