grapic graanit

7/24/2019 grapic graanit

1/6

American Mineralogist, Volume

71,

pages

325-330,

1986

On the origin of

graphic

granite

Pnrr,rp M. FBNU

Sullivan

Park

FR-01-8,Corning

Glass

Works,Corning,

New York 14831

Ansrnecr

For

over a century, he

origin ofthe

quartz-feldspar

ntergrowth known as

graphic

granite

has

beendebated

n

the

iterature.

These

extures

have

been

produced

experimentally

during

a study ofthe

phase

equilibria and nucleation and

growth

characteristics

ofgranitic

peg-

matites. Analysis

of temperature,

pressure,

and compositional

variables suggestshat the

texture

is

produced

by the

simultaneous

growth

of

quartz

and feldspar

in

a

kinetically

driven,

nonequilibrium

situation. The

growing

interface of the

host

phase,

a sodic

alkali

feldspar,

s

degraded rom

planar

to cellular by the development

of a SiOr-

(and

probably

HrO-)

enrichedboundary ayer. Between

he cell boundaries, he

SiO, content of the

residual

liquid

achievesa

level

of supersaturation

hat allows

quartz

to

nucleate

and

grow

along

with the feldspar. The

bulk composition may well be on a cotectic

surface,but under this

mechanism t is not necessary. he development of this texture is dependentupon local

kinetics

at the interface rather

than being solely tied to the thermochemical

equilibrium

of the

bulk composition.

IurnooucrroN

The

coarse ntergrowth

of

quartz

in

a

potassic

alkali

or

sodic

plagioclase

eldspar host

that displays

runic or

cu-

neiform

texture s

commonly referred

o as

graphic

gran-

ite. Found

predominantly

in

granitic pegmatites,

graphic

granite

s volumetrically

insignificant

in

the

family

of ig-

neous ocks,

but the

process

nvolved in

the development

of

this texture may

give

considerable nsight into

the crys-

tallization ofgranitic rocks. n spite ofthis low abundance,

graphic

granites

have

been the

topic of considerable

de-

bate n

the

literature.

Not

only

is

their origin in

question

but also

the existence

of a crystallographic relationship

between he

two

phases.

t is

beyond the

scope of this

paper

o engagen

any detailed description

ofthe

texture

or

a

review

ofthe literature:

the reader s referred

to the

excellentdiscussion n

Smith's

(1974)

reatiseon the feld-

spar

minerals.

The

only

previous

experimental work

pertinent

to the

origin

ofthis

intergrowth

s

that of Schloemer I

962, rans-

Iated in

1964). n

a study of the hydrothermal

devitrifi-

cation of

glasses

n the K2O-AlrO3-SiO,system, he pro-

duced

extures hat are

suggestive fgraphic

granite,

e.g.,

his Figure

49a.

Schloemer

proposed

that these

textures

were

a eutectic

structure resulting from

the

simultaneous

$owth

of orthoclaseand

quartz.

His

P-Zconditions

and

bulk compositions

do not correspond

o

liquidus

cotectics

in

published

phase

equilibria

of this system

(Tuttle

and

Bowen,

I 958);

thus,

n

the strict

sense

is

textures

cannot

be called eutectic. In

another series

of experiments, he

demonstrated

the feasibility

of the

infiltration

and

re-

placement

of

orthoclase by

quartz

under hydrothermal

conditions. Thus

both

proposed

models for

the origin

of

graphic granite

were demonstrated

but neither in

such a

manneras o exclude he other from future consideration

and

argument.

0003{04)V86/0304{325$02.00

ExprnrvrnNrs

The

pegmatite

samples

used as starting

materials for

the experiments

described

n this

paper

were

provided

by

the

late R.

H.

Jahns.

They are splits of

the samplesused

by BurnhamandJahns(1962),ahns

nd

Burnham

1969),

and

Vaughan

(1963).

The

samples

are composites

made

up ofabulk

sampleofa

pegmatitic

pod

from Chalk Moun-

tain in the Spruce

Pine district,

North Carolina, and

from

diamond drill cores of the Harding pegmatite in Taos

County,

New Mexico.

These composites

were

prepared

to

represent

estimates

of the bulk

compositions of the

parent pegmatitic

magmas. The bulk

compositions and

normative mineral contents

expressed

n

the

haplogran-

odiorite tetrahedron

are

listed in Table l. Details of the

phase

equilibria

ofthese samples

at

5000

bars are

repro-

duced

n Figures 1 and

2.

The

Spruce

Pine and

Harding

pegmatite

samples

were

ground

to an average

grain

size of 5

pm,

loaded nto

gold

or

platinum

capsules

with

a

measuredamount of distilled,

deionized

water, and

finally sealedand

placed

n

an

in-

ternally heated

pressure

vessel. The sampleswere ho-

mogenized

n the iquid or

liquid

plus

aqueous

apor

phase

field at 900qC or a

period

of 72 h, then the temperature

and

pressure

were rapidly

readjusted to the nucleation

and

growth

conditions.

All of the experiments

reported

in this

paper

were

performed

at a confining

pressure

of 5

kbar. After

a

predetermined

period,

the runs

were rapidly

quenched

o ambient conditions,

he capsules

pened,and

thin sections

prepared

or

petrographic

analysis.

Rnsur,rs

During

the

examination of

the run

products,

it was

noted

hat

under

restricted

conditions

of temperatureand/

or water content, intergrowths ofquartz and feldspar re-

sembling

gaphic granite

were

produced.

Figure

3

illus-

325


2/6

326

FENN: ON THE ORIGIN OF GRAPHIC GRANITE

Table l.

Compositions of

starting

materials

and normative

mineralogy

s i 0 2

T i O 2

A l

2O3

Fe2O3

F eO

llno

['lgo

CaO

Na2O

K 2o

P2o5

F

coz

T o t a l

1 5 2 4

0 . 0 5

L 4 . 4 2

0 . 1 4

0 . 3 5

0 . 1 8

0 . 0 1

0 . 2 0

4 . 2 3

0 . 1 3

0 . 6 4

0 . 0 3

9 8 . 3 6

3 0 . 3 8

18 1.9

0 . 0 0

5 1 . 4 3

0 . 0 5

1 5 . L 1

i ) A

0 .

L 6

0 . 0 5

0 . 0 7

0 9 7

4

1 L

4 . 0 2

0 . 0 1

0 . 0 1

0 . 0 2

9 9 . 2 3

4 1 . 0 7

2 4

4 8

4 6 9

a J . t o

Ha rd -

i ng

SDruce

P in e

to00

950

900

850

800

T

(t )

750

700

650

600

550

trates a typical

assemblage bserved

n

the Spruce Pine

samples.

The

capsulewall is in

the upper

right

portion

of

the

photograph,

and two distinct

morphologies

of sodic

plagioclase

are observed: single laths

and the complex

intergrowths

seen

near

the center.

Figure

4 illustrates hat

these are

formed

by the intergrowth

of two single crystals;

the external morphology

and the differences n index of

refraction ndicate

hat the

host

phase

s

a sodic

plagioclase

and the

internal

phase

s

quartz.

Note

that the

intergrowth

is

best developed n feldspar

crystals hat

grow

away

rom

the capsule

walls,

surrounded

by bulk

melt. The relative

ease

with which

the

feldsparsnucleate

on the walls of the

capsule

s not

pertinent

to

the

topic

ofthis

paper,

but

it

may have

application to the

gxowth

of large apered eld-

spars

rom

the hanging wall

of

many

granitic pegmatite

dikes

(Jahns,

1953).

The

growth

conditions of the experiment llustrated in

Figures

3 and 4 indicate

that the

initial

undercooling of

the

plagioclase

was 165'C

and that of

quartz

I 5'C. This

places he bulk composition on the feldspar-quartzco-

tectic, but

it

should

be

noted

that there

is no free

quartz

6 8 r O

wT.

7.

HzO

Fic. . Phase quilibria

f Harding

egmatite

omposition

t

5

kbar asa

functionoftemperature

nd HrO added o

capsule.

L : liquid, V : aqueous apor,Q : e\artz (a- or p-poly-

morphs),

Ab

:

sodicalkali

feldspar,Kf

:

potassic

lkali feldspar,

Mu: muscovite, p

Spodumene.

ecause eryl

was

present

in all charges p

to liquidusconditions,

t

was

mpossible o

determine

hether

uartz

r beryl

s

he

iquidus

hase.

ocation

of solidus

s approximate

ut

is

believed

o be

within lOqC

f

location hown.

in the sample.

n

the

experiment shown

in Figure 5,

free

quartz

and

feldspar

are

visible,

but

graphic

intergrowths

are

not found under these

conditions.

Here an extra 50oC

ofundercooling

hasallowed he

nucleationofboth

phases,

but

the feldspar

grows

initially as

finely-bladed, dense

spherulites

along he

capsule

wall

and the

quartz

ascoarse

dendrites.

Figure 6

illustrates

the

results ofa

nucleation

and

growth

experimenton the

Harding composition.

This

bulk composition

lies

in

the

liquidus field of beta

quartz

under

the P-Zconditions

of the experiment,and

the

pres-

ence of

individual dendritic

quartz

crystals scattered

throughout

the

glass

confirms this.

The

graphic

inter-

$owth

displays

the external

morphology

of a

feldspar,

and there

s no indication

ofthe

growth

of feldspararound

any of the

pre-existing quartz

dendrites.

This demon-

strates hat

the feldspar

(nearly

pure

albite

in this system)

is the host

phase

for the

intergrowth and

must begin to

grow beforeany graphic texture can develop. Under these

circumstances,

he

bulk composition

is

on

the feldspar-

Ab

O r

AN

HARDING P E G M A T I T E

5

ki lobors

aQ+Ab+Mu+L+V

aQ

+ Ab + Mu . K f + L + V

aQ

+

Ab

+ Mu + K f + SD + L + V

FQ,Ab)

M u +Kf .

aQ+Ab

Kf+Mur

sP

tL


3/6

SPRUCE

INE

PEGMATITE

PttAf

+pQ+Mu+L

P l

+aQ+

Af t Mu lL + V

FENN:

ON THE ORIGIN OF GRAPHIC

GRANITT,

32 7

850

800

r

( )

750

700

650

600

550

o 2 4 6 8 r O t 2 t 4 t 6

wT. 7. HzO

Fig.2.

Phase

quilibria

or

the Spruce ine

pegmatite

om-

position.

Phase

dentification

s

n Fig. I

exceptPl: sodic

pla-

gioclaseeldsparprobably n oligoclase).

quartz

cotectic,

but even though

this

is

true,

the texture

is not

developed everywhere n

the charge

as evidenced

by the separate

quartz

dendrites

and euhedral

albite crys-

tals in near-juxtaposition.

DrscussroN

An

evaluation of the literature

concerning

origin of

graphic

granites

ndicates

hat there are two

basic

models

for

the

formation

of the texture: the replacement

of

por-

tions of a

pre-existing

feldspar

crystal by

quartz

or the

simultaneous

crystallization of

quartz

and

feldspar.

Most

ofthe

recent iterature

accepts he concept

ofsimultaneous

crystallization,

and the subject

of current debatehas

shift-

ed to the

role

of eutectic

or cotectic crystallization

in the

development

of the texture.

The

similarity

of the texture

to the

eutectic structuresdeveloped n metallic

systems

s

compelling, but the lack

of a strong correlation

between

the

available

analysesofgraphic

granites

and

the exper-

imentally

determined liquidus

and solidus

surfacessug-

gests

hat the

origin

is not

so simple

(Barker,

1970).

The

data

presented

n

this study

suggestan origin involving

simultaneous rystallization

driven by the kinetics

of crys-

tal gowth and diffusion. The developmentof the graphic

texture

commences

with

growth

of a

feldspar

crystal

under

Fig. 3. SprucePine

pegmatiteplus

4.5 wto/o rO held

at

750'C

for

96

h. The

capsule

wall was in the upper left corner. Scale

bar:

1.0 mm;

cross-polarized

ight.

Fig.

4.

Sameexperiment as

Fig. I showing enlarged

view of

quartz-feldsparntergrowth.Scale ar : 0.1 mm; cross-polarized

light.

Note

the

rods and ends of

rods in other crystals.


4/6

328

FENN:

ON

THE

ORIGIN OF GRAPHIC GRANITE

Fig. 5. SprucePine

pegmatite plus

4.5

wtVo

HrO held

at

700'C for

240 h. Capsule

wall was at

right edgeof

photomicrograph.

The large dendrite

at the center

is

beta

quartz,

and feldspar appears as dense

spherulites

at the capsule

walls and as thin

lath-shaped

crystals

around the

base of the dendrite.

Scale

bar

:

0.1 mm; cross-polarized

ight.

conditions that stabilizea

planar-growth

nterface;

hat

is,

a temperature and composition that

yield

a low under-

cooling

relative

to the upper stability

limit

of the

feldspar.

Ifthe conditions

are appropriate, the

rate

ofadvance of

the crystalJiquid interface will exceed he rate at which

nonfeldspar

components, specifically

HrO

and excessSiOr,

are able to diftrse

away

into

the bulk

melt.

Under

these

conditions, the relative

undercooling

at the

growth

inter-

face

will

decrease s he composition ofthe interface iquid

moves toward one with a lower liquidus temperature,

eventually approaching equilibrium

with

the

feldspar

(zero

undercooling). The rate of

advance

of the interface

will

be slowed by this

local

decrease

n

undercooling,but

ifa

perturbation

develops hat

projects

aheadofthe

interface

into melt compositions more closely resembling he bulk

composition, the tip of this

perturbation

will

experience

a relatively

greater

undercooling and thus may

grow

for-

ward

at a

higher rate. This leads

o the breakdown of the

planar

interface and may initiate the

formation

of a cel-

lular

interface

Tiller

and

Rutter,

I

956).

During the

growth

of a cellular

interface, the excess

components,

SiO, and

HrO, are

rejected

not

only

from the

tip of the

growing

perturbation

but also

from the sides.

Within the

grooves

betweengrowingperturbations, he concentrationof SiO,

and

HrO will build up

faster han at

the tip.

In metal

alloy

systems,

his

lateral segregation t

the cell boundaries

has

been

shown to

yield

a

fifteenfold increase

n solute con-

centration

(Biloni

and

Bolling, 1963).

n the

growth

of a

feldspar

from

a

granitic

melt, the concentrations

of SiO,

and

HrO in the cell boundary

regions may approach

or

even exceed

he saturation

evels of either or both

com-

ponents.

If enough supersaturation

s developed

in the

grooves,

he

nucleation

ofvapor bubbles

or

quartz

is

pos-

sible even

though

the bulk composition

of

the system

might suggest hat

neither

phase

s stable.

The nonequi-

librium supersaturation

f HrO at the

interfaceof a

grow-

ing crystal

has

been

demonstrated

experimentallyand re-

ported

by Fenn and

Luth

(1973).


5/6

Fig.

6. Harding

pegmatite

lus

6.0

w9o HrO held

at

750qC

for 168h.

Capsule all was

acrossop ofphotomicrograph.

ine

dendriticcrystals

re beta

quartz,

and the large

ntergrowh s

madeup

of

quartz

n an

albite

ath.

Undercoolings

f the two

phases

rebeta

quartz-

l379C

nd

albite*60"C.Scale

ar

:

0.1

mm; cross-polarized

ight.

In

the experiment

shown in Figures

3 and

4,

the low

degreeofundercooling (15"C)has apparently prohibited

the

nucleation

ofquartz except n

the

graphic

ntergrowth

where

a higher

degreeofsupersaturation

has

allowed its

nucleation

and subsequent

growth.

Once the

quartz

nu-

cleatesand

begins o

grow,

the coupled

growth

of the two

phases

may

ncrease

he

growth

rate

ofthe

aggregate bove

that of

either of

the components

e.g.,

Carstens,1983).

This would

explain why

the

intergrowths

shown in

the

photomicrographs

are considerably

arger

than either

of

the single

phases

n

the same charge.

The local

environment

at the interface

of the

growing

feldspar

crystal must

create

he conditions necessary

or

the

transition from

a

planar-

to a cellular-interfacemor-

phology

and thus

permit

the

formation

of the ntergrowth.

The

formation

of a

planar

interface

on

feldspars

grown

from

granitic

melts

has

been shown to

occur at low

to

moderate

undercoolings

Fenn,

1977;

Swanson, 1977).

Coupled

with

this

restriction

on

proximity

to the feldspar

liquidus,

there s also he requirement

hat the

growth

rate

must

be high relative

to the

diffusivity of

silica in the

residual

melt. In

the

previously

mentioned

studies

on the

nucleation

and

growth

offeldspars,

the

data suggest

hat

as he

HrO

content

of the system ncreases,

he maximum

growth

rate

shifts o lower undercoolings.

This

trend raises

the

possibility

that the development

of

graphic

textures

is enhanced y high HrO contentsandmay help to explain

the association

fgraphic

granites

and

granitic

pegmatites.

329

Fig.7.

Same ompositon s

Figure

6 but

held

at

750'C

or

48

h. The albite ath s intergrown

with

quartz

and surrounded

at

its

base y dendrites fbeta

quartz.

The

capsule

all

was

o

the ight.Scale ar:0.1

mm;

cross-polarized

ight.

Another feature of the

proposed

mechanism concerns

the time

required

to

form

the

necessarynterface

enrich-

ment and its relationship to the anisotropic nature

of the

feldspar

attice and the

variation

of

growth

rate

along the

crystallographicdirections.

Because inite

time

is

neces-

sary to build up the boundary layer, the

center of the

feldsparcrystal should be free of intergrowths,as hasbeen

described

n

the

literature

see

Smith,1974,

p.

601-602).

Also

because he

growth

rates

of the alkali

feldspars

de-

pend

upon

the

crystallographic

direction

(Fenn,

1977),

the extent ofboundary

layer

segregation

will

also depend

upon the direction ofgrowth. Thus, the appearance

fthe

intergrowth will

vary

with

the orientation of the observed

section. This feature has also been documented in

the

literature

and

in

the experiments

reported in

this study.

Figure 7 illustratesa feldspar ath showing a

sharpbound-

ary between he

nonintegrown

core and the

graphic

zone.

CoNcr,usrous

The origin of the intergrowth of quartz and alkali feld-

spar known as

graphic granite

has been shown to

be the

FENN:

ON

THE

ORIGIN OF GRAPHIC GRANITE


6/6

330

FENN:

ON

THE

ORIGIN OF GRAPHIC

GRANITE

result

of simultaneous

growth

of the two

phases

under

conditions that

favor

the

planar growth

of the

feldspar

host.An imbalance

between he

growth

rate

of the

feldspar

and he diffusivity of silica

n

the bulk

melt

creates silica-

enriched boundary layer

that

in

turn

causes he interface

to degrade

rom

planar

to cellular. Upon

further

erowth

of this cellular interface, the

grooves

between adjoining

cells

are

greatly

enriched

in SiOr, and this

enrichment

causes he nucleation and

growth

of

quartz

along

with

the

feldspar. This

simultaneous

growth

was not

caused by

classicaleutectic crystallization but by the

kinetic

phe-

nomena in

the boundary

layer

adjacent to the

interface

of

the

growing

crystal.

Acxxowr-nocMENTs

The author deeply egrets he

passing

f two

good

riendsand

colleagues,

ick

Jahns

nd

Peter

Gordon,both of

whom have

made

major

contributionso thisstudy.

t

is unfortunatehat he

publication

f these

esultsmust

come nder uch ircumstances.

His unparalleledeaching bility,boundlessnthusiasm,nd he

ability

o

pass

n this enthusiasm

adeDick

Jahns

ne of the

most

espected

eologists

f his ime.PeterGordon's kill n the

design,

abrication,

nd

maintenance

f experimentalquipment

made his and

dozens f otherstudies

ossible.

The

experiments

ere

performed

n

the

Tuttle-Jahns abo-

ratory for ExperimentalPetrologyat

StanfordUniversity.

The

support fthe

National

Science

oundation

hrough

ant

EAR

7

6-22688

s

gratefully

acknowledged.

RnrBnrNces

Barker,D.S.

1970)

Composition f

granophyre,

yrmekite,

nd

graphicgranite.

Geological ociety f

AmericaBulletin,

81,

3339-3350.

Biloni,H., and Bolling,G.F.(1963)A metallographictudyof

solute egregationuringcontrolled olidificationn lead-tin

alloys.Metallurgical

ociety f

AIME Transactions,27,l35l-

l

360.

Burnham, C.W.,

and

Jahns,

R.H.

(1962)

A method for deter-

mining the solubility

ofwater

in

silicate

melts.

American

Jour-

nal of Science,

60,

721-'1 5.

Carstens,

H.

(1983)

Simultaneous

crystallization

of

quartz-feld-

spar intergrowths

from

granitic

magmas. Geology,

11,

339-

341.

Fenn, P.M.

(1977)

The nucleationand

growth

of aklaki

feldspars

from hydrous

melts.Canadian

Mineralogist,

15, 135-161.

- (1979)On the origin ofgraphic intergrowths. abs.)Geo-

logical Society

of

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