GRAPHIC QUARTZ-FELDSPAR INTERGROWTHS IN PEGMATITES: DIFFUSION AND GROWTH KINETICS MIGIF-HAFAFIT AREA, SOUTH EASTERN DESERT EGYPT

Al-Azhar Bull. Sci. Vol.15, No.1(June): pp 83-98,2004
GRAPHIC QUARTZ-FELDSPAR INTERGROWTHS IN PEGMATITES:
DIFFUSION AND GROWTH KINETICS MIGIF-HAFAFIT AREA, SOUTH
EASTERN DESERT EGYPT
HASSAN Z. HARRAZ and Adel M. HASSAN
Department of Geology, Faculty of Science, Tanta University, Tanta-Egypt
ABSTRACT
During the formation of pegmatites in the Migif-Hafafit area, conditions of crystallization
were such that widespread graphic quartz-feldspar intergrowths were formed. The quartz is
interpreted to have nucleated epinastically on rough edges and corners of alkali feldspar
crystals. The existence of rugose inner feldspar-quartz boundaries and euhedral outer
boundaries evidence that the graphic texture is a primary magmatic feature. Rapid growth, at
or near volatile-saturated conditions, resulted in quartz saturation along the irregular melt-
feldspar inner interface. Slow diffusion of Si and Al species (network formers) in the boundary-
layer melt was likely the rate-controlling step for quartz saturation, which occurred along
corners and edges, where the feldspar grew most rapidly. Diffusion-limited growth resulted in
SiO2 buildup at the interface, producing oscillations from quartz-oversaturated to quartz-
undersaturated conditions and thus the rhythmic quartz-feldspar intergrowths. The transition
from planar, to edge, to cellular growth, and changes in the lobate inner feldspar-quartz
boundary occurred in response to changes caused by crystallization that affect rates of Si-Al
diffusion. Evidence of saturation in a volatile phase in these pegmatites indicates that water
was a catalyst for feldspar growth and that lower activities of H2O in the melt decrease Si
diffusivity at the crystal interface.
Introduction
The graphic texture refers to the regular cuneiform intergrowth of quartz and alkali
feldspar. The granophyric texture is the irregular finer-grained counterpart of the graphic
texture. Detailed textural, petrographic, and chemical examination of forty-six (46) samples
of graphic granitic pegmatites from seven (7) localities in the Hafafit area, southern Eastern
Desert of Egypt has provided insight into their formation.
Graphic and granophyric textures have been interpreted to result from rapid
crystallization at volatile-saturated conditions (Barker, 1970; Černy and Meintzer, 1988;
Kirkham and Sinclair, 1988). Both these textures occur within marginal facies of pegmatite,
pegmatite pods, aplites, and also associated with hydrothermal mineralization.
Refreed by: Ebrahim Abu Ali & Ahmed Moustafa El-Bousely

HARRAZ and HASSAN 84
Numerous hypotheses have been proposed for the formation of the graphic texture,
but simultaneous growth of quartz and feldspar near the thermal minimum, originally
proposed by Vogt (1931), is considered to be the controlling mechanism (e.g., Barker,
1970; Černy, 1971; Hughes, 1971). Replacement of feldspar by quartz-saturated solutions
(Augustithis, 1962; Seclaman and Constantinescu, 1972) and vapour-phase crystallization
(Simpson 1962) also has been proposed. Lofgren (1974) also observed the texture by
devitrification of rhyolitic glass in experimental charges. Carstens (1983), Fenn (1986) and
London et al. (1989) have proposed that textures are formed near the cotectic or eutectic
for the system but are not necessarily an equilibrium phenomenon. Fenn (1986) and
London et al. (1989) experimentally produced the texture from granitic bulk compositions
within the primary field of K-feldspar and concluded that the texture is related to the interplay
of diffusion and growth kinetics. Their mechanism involves short-range continuous diffusion
of quartz- and feldspar-forming components to a crystallizing surface containing both
minerals.
The common association of graphic and granophyric textures in granites and
pegmatites indicates that understanding the nature of the magmatic-hydrothermal transition
is key to the formation of these textures.
We attempt in this paper to handle the morphological, petrographical and
compositional features of the graphic quartz-feldspar intergrowths in granitic pegmatites at
Hafafit area, Southeastern Egypt to evaluate the petrogenetic process(es). The effects of a
dissolved and free volatile phase are examined because there is an imperial relationship
between the development of the graphic texture and evidence of volatile saturation in this
pegmatite.
This paper identifies morphological and compositional features of the intergrowths
using a diffusion-controlled model of growth (London et al., 1989). However, observation of
the lobate growth-boundaries between feldspar and quartz suggested that quartz nucleated
on feldspar growth-surface (Lentz and Fowler, 1992).
Geological Setting
The pegmatites in the Hafafit area, south Eastern Desert of Egypt contain abundant
examples of a well-developed graphic texture. In general, granitic pegmatites form both
concordant and discordant dykes, veins and lenses in the metasediments and gneisses.
They can attain widths of tens of meters and have lengths up to several hundreds of meters.
Some of the larger pegmatites are associated with mineral deposits, for examples the Sikait
Abu Rasheid, and Nugrus uraniferous granitic pegmatites in Wadi Hafafit (Wadi Sikait-
Wadi El-Gemal) area (Fig.1). Figure 1 illustrates the distribution of sample localities within
the Hafafit Gneiss Belt. Several of the samples of graphic granitic pegmatites are from
discordant and concordant pegmatites that hosted mainly by gneisses (Biotite-bearing
quartzofeldathic gneisses), and metasediments (metapelitic). The local geology and setting
of these occurrences may be found in Rashwan (1991); Mohamed (1993); Mohamed and
Hassanen (1997) and El-Sharkawy and Harraz (2001).

GRAPHIC QUARTZ-FELDSPAR INTERGROWTHS IN PEGMATITES 85
The pegmatites under consideration are late tectonic (565 to 600 Ma, Sr-Rb Zircon;
Stern and Hedge, 1985) with respect to the Hafafit orogeny (~ 682 Ma) and were emplaced
into the Hafafit gneiss rocks at pressures and temperatures less than peak metamorphic
conditions (Rashwan, 1991), which is compatible with their generally undeformed nature.
Rb/Sr of the Hafafit gneisses belt (Rashwan, 1991) suggest that the pegmatites were
emplaced while temperatures remained between D5 and D4, which is consistent with the
development of mineral assemblages in the exogranitic metasediments
Pegmatites
The investigated pegmatites are composed essentially of quartz, microcline and
sodic plagioclase, with lesser amounts of garnet, muscovite, kyanite, sillmenite, allanite,
apatite, zircon, fluorite, calcite and monazite (Rashwan, 1995). Field evidence suggests
that garnet, locally concentrated along the contact zone, originated by a complex process
of hybridization (Rashwan, 1991). Partial melting of hosted metapelitic rocks during post-
tectonic uplift produced H2O-undersaturated, anatectic partial melt deep within the crust
(Mohamed and Hassanen, 1997). Fractional crystallization of anhydrous silicates (quartz
and feldspars) during the ascent of these melts within the crust caused their enrichment in
rare metal and volatile components, to point of saturation (Mohamed, 1993).
Pegmatite Textures
Numerous textural variations occur within Hafafit pegmatites, indicative of the
variable conditions of crystallization. The pegmatites occur as both zoned and unzoned
types (Kamshsy, 2004). Most bodies are unzoned and contain fine-grained aplitic (<1 mm)
to coarse-grained pegmatite rocks (8 cm); however, the average grain-size is 1 to 3 cm.
The crystals are dominantly subhedral and equigranular. In the zoned bodies of granitic
pegmatites, grain size varies from medium to coarse-grained (2 to > 10 cm). Quartz and
feldspar are subhedral to anhedral, except in the core zone. Microperthitic and perthitic
microcline are unbiquituous. Feldspar exsolution postdated the formation of the graphic
texture, as the traces of exsolution lamellae intersect the intergrown quartz.
Description of graphic granite pegmatites
In the Migif vermiculite mining sites of feldspar and quartz from the zoned
pegmatites ceased after coarse- to fine graphic granitic pegmatite and can observed for
several hundreds of meters along strikes. The graphic texture is extremely variable even
on the outcrop scale; grain sizes range from 2 mm to greater than 8 cm. The average
distance between adjacent quartz domains varies from fine (0.25 to 1 mm) to coarse (> 10
mm). The quartz forms discontinuous plates, triangular rods, and angular corners (arrows),
which impart the characteristic cuneiform texture in two dimensions (Figs. 2 and 3). The

graphic quartz has an anhedral (irregular) inner boundary, whereas the outer boundary is
euhedral (planar). Lentz and Fowler (1992) interpreted this observation as indicating to
quartz nucleated on the anhedral boundaries of the host feldspar and grew to a more
euhedral form (Fig.3). The cuspate protuberances are particularly obvious in the coarser-
grained varieties of the graphic texture (Fig.3B). In the finer-grained samples, quartz has a
saw-tooth-shaped inner surface (Fig.3E).
The irregular nature of the inner surface of the quartz indicates that quartz nucleated
onto an irregular feldspar interface and grew to a more regular (prismatic) outer surface.
The formation of feldspar protuberances into the melt is a result of change from planar
(equilibrium) to cellular growth (nonequilibrium; e.g., Fenn, 1986). The hook-shaped
protuberances at the feldspar-melt interface (irregular inner Qtz-Kfs surface) have the
appearance of screw-dislocation growth in side view. Quartz saturation did not occur until
the outermost portion of the feldspar protuberances crystallized. However, the presents
bulbous tips of the protuberances was considered by Lentz and Fowler (1992) as indicated
to 1) quartz nucleated in the embayment and overgrew the protuberances, or 2) the feldspar
protuberances extended beyond the silica-enriched zone but were overgrown upon quartz
saturation. Moreover, the existence of rugose inner feldspar-quartz boundaries and
euhedral outer boundaries evidence that the graphic texture is a primary magmatic feature
(Stel, 1992).
Composition of graphic granitic pegmatites
A previously investigated the chemical compositions of Migif-Hafafit pegmatites
indicates an average composition near the volatile saturated minimum or eutectic in the
haplogranite system at approximately 400 Mpa (Mohamed and Hassanen, 1993).
Representative samples consisting of single feldspar crystals and their contained
quartz intergrowths were analyzed by X-ray fluorescence ( Rigaku R1X 2100) . The samples
varied from 2 to 5 kg depending on the grain size. Analyses for major- and trace elements
were performed on fused pellets at the Department of Geosciences, Osaka City
University, Osaka-Japan . Ga and G-2 were used as internal standards to determine
accuracy, and several duplicates were used to determine precision. Estimated precision for
major elements; less than 2%, except for Na2O (3%); for trace elements, precision is less
than 5%. Results of fifty chemical analyses of graphic granitic pegmatite from 10 localities
(Table 1) are presented in Figure 4a. The samples contain less than 1 vol.% of additional
phases. This was confirmed by the normative mineralogy (>99 wt.% quartz, orthoclase,
albite and anorthite in the norm; Table 1). The low abundance of anorthite (0 to 2 wt.% in
the norm) is a result of the analysis of perthitic microcline. Therefore, a separate plagioclase
phase was not incorporated into the graphic texture.

The chemical compositions of Hafafit pegmatites indicate an average composition
near the volatile-saturated minimum or eutectic in the haplogranite system at approximately
400 MPa (Fig.4A). The system quartz-albite-orthoclase-H2O (Fig.4B) is used to illustrate
the phase relations because of the considerable experimental work published on equilibria
in the haplogranitic system. The quartz-K-feldspar intergrowths consistently fall on the
feldspar side of coetectic. There is obvious relationship between the grain size of the
graphic texture and composition with respect to the eutectic (Fig.4A). The quartz-microcline
intergrowths have average quartz content of 28.0  3.9 wt.% (n=50).
Phase-equilibria considerations
The transition from a granitic melt saturated in quartz and feldspar to one saturated
in feldspar alone is problematic. In order for the graphic texture to grow, it is necessary that
equilibrium saturation in quartz not occur throughout the melt; therefore, the appropriate
phase-equilibrium relationships must be considered.
At the approximate pressure and temperatures corresponding to pegmatite
crystallization (H2O- saturated), the quartz and alkali feldspar coetectic at 500 MPa (Luth et
al. 1964) is located at 28% Qz (eutectic, 650o
C) to 36% Qz (Qz-Or pseudobinary eutectic,
735o
C). At 300 MPa, the coetectic lies between 33% Qz (eutectic, 665o
C) to 42% Qz (Qz-
Or pseudobinary eutectic, 745o
C) (Tuttle and Browen, 1958). By interpolation, an alkali
feldspar composition of Or70 at 400 MPa corresponds to 35% Qz along cotectic. The minor
amount of fluorine inferred to be present, based mainly on fluorite in the associated fluorine-
rich beryl deposit and beryl-biotite-schist in the pegmatites (Zaki, 1956), would reduce the
liquidus and solidus temperatures in the Qz-Or-Ab system and expand the primary field of
quartz. At a pressure of 275 MPa, Wyllie and Tuttle (1961) noted a decrease in the volatile-
saturated thermal minimum from 675o
to 595o
C with the addition of 4 wt.% fluorine. Manning
(1981) reported a substantial shift to higher proportions of feldspar with addition of fluorine
at 100 MPa (volatile-saturated system). These observations suggest that similar shifts in
the quartz-feldspar equilibria would occur at 400 MPa pressure with the addition of fluorine.
The inferred concentration of fluorine in the granitic melts that crystallized to form the Hafafit
pegmatites is variable but is estimated to be less than 1 wt.%. The maximum effect on the
solidus is, by analogy, on the order of 30o
C, yielding a 600o
C eutectic temperature, thus
enhancing the stability field of quartz, mainly at the expense of albite.
In contrast, a decrease in the confining pressure can displace the quartz-orthoclase
coetectic, enhancing the primary field of feldspar with respect to that of quartz. The
occurrence of large alkali feldspar phenocrysts only does not preclude quartz saturation
elsewhere in the granitic melt; however, the phase equilibria indicate that crystallization in
the feldspar field is possible under specific conditions.

A Model of Dynamic Crystallization For Graphic Intergrowth
The observation of lobate inner boundaries on feldspar-hosted quartz is critical
(Figs. 2 and 3). Rough boundaries arise during conditions of high super-cooling (Woodruff,
1973), when the supply of growth atoms is restricted to low diffusivities or depletion of
crystallizing constituents in the boundary layer. These results in nonequilibrium growth,
protuberances on the crystal face become stabilized and grow (e.g., Nittmann and Stanley,
1986). The appearance of the lobate inner boundary can be interpreted as the onset of
diffusion-limited conditions during feldspar growth (e.g., Lentz and Fowler, 1992). Feldspar-
hosted quartz in graphic intergrowths has characteristic prism, arrow, and rod
morphologies. However, the growth morphology of feldspar changes from equilibrium
planar to skeletal growth as a function of the degree of undercooling (Fenn, 1977). Crystal
corners and edges subtend a greater angle in the melt than planar crystal faces and
preferentially grow because they have access to a greater volume of melt. The striking
patterns of the graphic texture are the result of consistent angular relationships between
the intergrown quartz and the host feldspar crystal imposed during growth. These may due
to preferential nucleation and growth of quartz along corners, edges and to a lesser extent,
faces of the alkali feldspar (e.g., Lentz and Fowler, 1992). The observed narrow arrows and
prisms of quartz are two-dimensional sections of quartz that grew along edges of the
feldspar, whereas rods represent two-dimensional slices of edge or surface growth of
quartz on the host feldspar.
As shown previously, growth of the graphic texture in pegmatites of the Hafafit area
occurred at an approximate pressure of 400 MPa and a minimum melt temperature near
600o
C, close to H2O-saturated conditions. Figures 4B and 5 illustrate the dynamics of
growth. Alkali feldspar was the mineral on the liquidus. Growth of the feldspar in this melt
required the rejection of H2O and excess silica. An increase in dissolved volatiles alone
should increase slightly the diffusivity of all feldspar components by decreasing the degree
of polymerization of the melt (Scarfe, 1986). This has the effect of increasing rates of crystal
growth as result of increasing activity of water at the melt interface (Fenn, 1986). Thus, the
net effect of rejected H2O is to catalyze feldspar growth in the immediate vicinity of the
crystal. Initially, this process is self-propagating, because growth with cause rejection of
H2O, which increases the rate of growth, and so on. Depletion of Al-bearing species in the
boundary layer (i.e., Si enrichment) results from more rapid growth of feldspar and slow
diffusion of Al-bearing species from the bulk melt into the boundary layer. Therefore,
diffusion-limited growth of feldspar arises (Fig.5). Thus, edges and corners become sites of
preferred growth, and the feldspar faces become rough. Eventually, SiO2 builds up to
saturation at these sites, and quartz is epinastically nucleated and grown on the feldspar
structure. The growth of quartz depletes the boundary layer in Si (Fig.5C) relative to Al,

resulting a renewed crystallization of alkali feldspar.
Table 1: Composition of graphic granitic pegmatite in the south Eastern Desert, Egypt
Texture
Sample 35 27 28 16 15 22 29 30 32 36 37
wt.%
SiO2 74.95 75.57 74.65 74.60 74.75 74.78 75.60 74.95 75.70 75.06 74.79
TiO2 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.02 0.00 0.01 0.00
Al2O3 14.30 14.32 14.30 14.26 14.42 13.95 14.00 14.50 13.76 14.15 14.32
Fe2O3 0.18 0.20 0.14 0.24 0.17 0.19 0.15 0.31 0.10 0.19 0.15
MnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
MgO 0.10 0.10 0.12 0.12 0.21 0.13 0.00 0.12 0.00 0.01 0.05
CaO 0.32 0.12 0.21 0.19 0.24 0.33 0.17 0.20 0.08 0.20 0.22
Na2O 3.20 3.45 3.25 3.31 3.05 2.97 3.08 2.80 2.95 3.03 3.20
K2O 6.15 6.00 6.40 6.05 6.30 6.14 6.10 6.03 6.35 6.16 6.12
P2O5 0.00 0.00 0.01 0.00 0.02 0.01 0.00 0.00 0.00 0.01 0.01
Total 99.20 99.76 99.08 98.77 99.17 98.50 99.10 98.93 98.94 98.82 98.86
ppm
Ba 465 350 370 417 532 351 432 515 350 320 323
Sr 79 90 86 47 32 113 38 44 95 68 100
Rb 683 548 475 490 519 630 529 720 540 495 680
Rb/Sr 8.65 6.09 5.52 10.43 16.22 5.58 13.92 16.36 5.68 7.28 6.80
CIPW
Qz 32.21 32.20 30.93 31.98 32.39 33.62 34.28 35.33 34.43 33.84 32.60
Or 36.64 35.54 38.18 36.19 37.55 36.84 36.38 36.02 37.94 36.85 36.59
Ab 27.24 29.20 27.71 28.29 25.98 25.46 26.25 23.90 25.19 25.90 27.34
An 1.60 0.60 0.99 0.95 1.07 1.60 0.85 1.00 0.40 0.94 1.04
C 1.79 1.92 1.66 1.93 2.19 1.85 2.02 3.02 1.89 2.16 2.06
Mt 0.26 0.29 0.20 0.35 0.22 0.28 0.22 0.40 0.15 0.25 0.22
Il 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.04 0.00 0.02 0.00
Hy 0.25 0.25 0.30 0.30 0.53 0.33 0.00 0.30 0.00 0.03 0.13
ap 0.00 0.00 0.02 0.00 0.05 0.02 0.00 0.00 0.00 0.02 0.02
Texture
Sample 1 2 3 4 5 6 10 11 12 13 19 20 21 23 24 34 38 39 40
wt.%
SiO2 74.07 74.28 74.05 74.44 74.51 74.35 75.27 75.24 74.28 74.18 73.55 73.49 73.96 73.59 73.98 74.27 73.86 74.07 74.27
TiO2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.00
Al2O3 13.95 13.66 13.64 13.90 13.72 13.76 13.55 13.54 13.69 13.86 13.98 13.95 13.58 13.84 13.96 13.62 13.90 13.30 13.77
Fe2O3 0.11 0.19 0.20 0.17 0.18 0.18 0.14 0.14 0.19 0.21 0.16 0.24 0.22 0.18 0.17 0.15 0.19 0.12 0.10
MnO 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.01 0.00 0.01 0.00
MgO 0.13 0.00 0.01 0.05 0.03 0.02 0.12 0.06 0.08 0.20 0.08 0.18 0.15 0.10 0.00 0.06 0.10 0.09 0.04
CaO 0.13 0.21 0.15 0.17 0.13 0.15 0.07 0.09 0.09 0.07 0.12 0.19 0.16 0.21 0.19 0.32 0.21 0.10 0.16
Na2O 2.50 2.24 2.52 2.31 2.60 2.50 2.73 2.48 2.50 2.53 2.60 2.70 2.61 2.50 2.51 2.48 2.56 2.56 2.43
K2O 8.08 8.33 7.59 8.00 7.79 7.97 7.69 7.59 7.55 7.90 8.01 7.80 7.54 7.93 7.85 7.70 7.76 7.70 7.96
P2O5 0.01 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.02 0.00 0.01 0.01 0.01 0.01 0.02 0.00 0.01 0.01 0.02
Total 98.98 98.94 98.18 99.06 98.97 98.94 99.59 99.17 98.41 98.95 98.52 98.57 98.24 98.36 98.69 98.61 98.59 97.96 98.75
ppm
Ba 360 367 322 349 336 363 330 345 330 247 339 251 259 242 317 208 235 305 315
Sr 102 137 144 132 125 202 240 247 181 235 210 230 255 200 192 235 200 225 178
Rb 729 1150 1120 439 650 575 434 410 408 446 460 408 326 301 440 403 381 370 387
Rb/Sr 7.15 8.39 7.78 3.33 5.20 2.85 1.81 1.66 2.25 1.90 2.19 1.77 1.28 1.51 2.29 1.71 1.91 1.64 2.17
CIPW
Qz 28.44 29.26 30.60 30.26 29.56 29.27 29.76 31.76 31.07 29.06 27.81 27.65 29.89 28.58 29.36 29.99 29.09 29.98 29.69
Or 48.26 49.74 45.68 47.73 46.52 47.61 45.63 45.24 45.35 47.18 48.04 46.74 45.36 47.65 47.02 46.14 46.52 46.46 47.66
Ab 21.34 19.11 21.67 19.69 22.18 21.34 23.15 21.12 21.46 21.59 22.28 23.12 22.44 21.46 21.48 21.24 21.93 22.07 20.79
An 0.59 0.92 0.62 0.72 0.59 0.69 0.28 0.38 0.32 0.35 0.54 0.89 0.74 0.99 0.82 1.61 0.99 0.44 0.67
C 0.86 0.61 1.05 1.17 0.78 0.76 0.61 1.09 1.29 1.01 0.83 0.73 0.85 0.78 1.03 0.61 0.92 0.59 0.90
Mt 0.16 0.31 0.30 0.26 0.26 0.26 0.24 0.21 0.25 0.31 0.27 0.39 0.29 0.27 0.22 0.25 0.28 0.21 0.15
Il 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.02 0.00 0.00 0.00 0.02 0.00 0.02 0.00 0.00 0.00 0.00
Hy 0.33 0.00 0.03 0.13 0.08 0.05 0.30 0.15 0.20 0.50 0.20 0.46 0.38 0.25 0.00 0.15 0.25 0.23 0.10
ap 0.02 0.05 0.05 0.05 0.02 0.02 0.02 0.02 0.05 0.00 0.02 0.02 0.02 0.02 0.05 0.00 0.02 0.02 0.05
Texture
Sample 8 7 18 33 17 31 14 9 26 25 45 46 47 48 49 50
wt.%
SiO2 72.30 72.10 72.00 72.08 71.50 73.08 71.20 72.83 72.27 72.38 72.90 72.46 72.80 73.50 72.61 72.54
TiO2 0.01 0.01 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00
Al2O3 14.34 14.24 14.20 14.80 14.47 14.16 14.36 14.13 14.01 14.04 14.07 13.80 14.00 13.45 14.13 14.14
Fe2O3 0.10 0.07 0.07 0.11 0.18 0.11 0.39 0.17 0.13 0.14 0.11 0.13 0.10 0.15 0.05 0.10
MnO 0.00 0.02 0.00 0.00 0.01 0.01 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.00
MgO 0.21 0.24 0.00 0.00 0.06 0.10 0.27 0.10 0.07 0.06 0.07 0.10 0.05 0.10 0.10 0.10
CaO 0.09 0.17 0.21 0.05 0.10 0.26 0.11 0.20 0.15 0.08 0.14 0.10 0.17 0.16 0.15 0.14
Na2O 1.78 2.02 1.75 1.96 1.80 1.99 2.00 1.70 1.35 1.32 1.82 1.80 1.75 1.35 1.94 1.90
K2O 10.25 9.75 9.95 10.52 10.40 9.60 9.85 9.97 10.39 10.55 9.95 9.75 9.90 10.06 9.58 9.65
P2O5 0.01 0.00 0.02 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.02 0.00 0.01
Total 99.09 98.62 98.20 99.52 98.54 99.32 98.18 99.10 98.38 98.58 99.07 98.14 98.78 98.80 98.56 98.58
ppm
Ba 60 52 82 90 142 135 87 67 135 135 194 129 182 133 115 130
Sr 210 198 239 225 212 345 240 322 290 312 267 380 297 371 360 312
Rb 460 410 430 380 370 260 188 235 230 108 203 220 165 160 160 120
Rb/Sr 2.19 2.07 1.80 1.69 1.75 0.75 0.78 0.73 0.79 0.35 0.76 0.58 0.56 0.43 0.44 0.38
CIPW
Qz 22.43 22.61 23.76 20.39 21.23 24.23 21.58 24.40 24.59 24.39 24.06 24.75 24.60 27.00 24.53 24.46
Or 61.17 58.44 59.91 62.49 62.37 57.13 59.24 59.46 62.42 63.27 59.38 58.73 59.25 60.17 57.48 57.87
Ab 15.18 17.30 15.06 16.64 15.43 16.92 17.19 14.49 11.59 11.31 15.52 15.49 14.97 11.54 16.63 16.28
An 0.38 0.86 0.93 0.25 0.44 1.23 0.56 1.00 0.76 0.40 0.64 0.51 0.79 0.67 0.76 0.64
C 0.15 0.03 0.19 0.07 0.07 0.02 0.18 0.15 0.25 0.28 0.05 0.08 0.09 0.07 0.27 0.32
Mt 0.12 0.14 0.10 0.16 0.27 0.19 0.57 0.25 0.22 0.18 0.16 0.19 0.15 0.25 0.07 0.15
Il 0.02 0.02 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00
Hy 0.53 0.61 0.00 0.00 0.15 0.25 0.69 0.25 0.18 0.15 0.18 0.25 0.13 0.25 0.25 0.25
ap 0.02 0.00 0.05 0.00 0.02 0.02 0.00 0.00 0.00 0.00 0.02 0.00 0.02 0.05 0.00 0.02
Fine graphic (fg)
Medium graphic (mg)
Coarse graphic (cg)

Table 2: Composition of graphic granitic pegmatite in the south Eastern Desert, Egypt.
Fig.(1): Sample localities of pegmatite displaying the graphic texture within the Migif-
Hafafit area, Southern Eastern Desert, Egypt (after Harraz and El-Sharkawy,
2001).

Fig.(2): Photos of polished slabs showing different forms and sizes of coarse-grained graphic
textures in pegmatites from Abu Rasheid area ( A and B); Um Kheran area (C and D);
and Sikait area (D, E, and F).
Fig.(3): (A) Tracings of coarse-grained graphic texture with irregular lobate quartz-feldspar
interface (parallel edge) (sample 105-1). (B) Sketch of coarse-grained graphic texture
with faceted quartz outer interface and irregular interface (perpendicular to edge or at
corner) (sample 57F). (C) Sketch of coarse-grained graphic granite pegmatite from
(Sikait) pegmatite field (perpendicular to edge). (D) Sketch of medium-grained graphic
granite pegmatite with sawtooth and barded shaped inner quartz-feldspar margin (Sikait
area). (E) Sketch of coarse-grained lobate-texture graphic granite pegmatite from Abu
Rasheid.

Fig.(4): (A) The pseudoternary system quartz (Qz)-albite (Ab)-K-feldspar (Or), showing the
normative mineralogy of 50 samples of graphic granite pegmatite. (B) The
estimated pseudoternary quartz (Qz)-albite (Ab)-K-feldspar (Or) diagram with the
phase boundaries for the haplogranite system at 0.4 kb PH2O {400 MPa (H2O)}.
The dot represents the approximate composition of melt. The dashed-dottedline
indicates the integrated evolution of the liquid over the coarse of crystallization
of the graphic texture. The jagged line represents the composition of the
boundary-layer melt at the crystal-melt interface.
Fig.(5): Growth of alkali feldspar crystal. (A) Shown schematically are the diffusion of
rejected Si-species and H2O (Solid vectors) and the diffusionof Al-species added
to the crystal (hachured vectors). (B) Magnification of crystal edge at corner of
(of (A), showing growth of protuberances (displaced diagonal hachure) to form
rough boundary. The relative fugacity of water [fH2O] and activity of silica [aSiO2]
are shown in the boundary-layer melt (BLM) and bulk melt. (C) Crystallization of
quartz has reduced aSiO2 , so that alkali feldspar crystallizes next.
Discussion
The shape of quartz in graphic intergrowths of quartz and feldspar is a function of
orientation of the feldspar (Simpson, 1962; Smith, 1974; Lentz and Fowler, 1992), as is
observed here. Detailed work by Simpsom (1962) and Lentz and Fowler (1992) shows that
quartz rods change shape along their length and the irregular inner face of the rod quartz.
Mehnert (1968) argued that a quartz- to –feldspar ratio of 27: 73 was not compatible with a
melt on the quartz-feldspar cotectic. Within each data set, the volume proportion of quartz

is approximately constant, indicative of a recurrent process.
There are consistent proportions of quartz to feldspar (28 wt.% normative quartz)
in graphic pegmatite from the Hafafit area (Table 1). Similarly, Erdmannsdorfer (1942)
found an average of 27 vol.% quartz and a range of 10-to 65 vol.% quartz in quartz-
microcline intergrowths. de Schmid (1916) determined an average 28.9 wt.% quartz for
Lower St. Lawrence pegmatites. Černy (1971) found similar proportions of quartz to
feldspar in a study of graphic pegmatites from Czechoslovakia, whereas Lentz and Fowler
(1992) determined an average 271.5 wt.% (n=23
The proposed model is driven by interplay between crystal growth and diffusion
rates of rejected solutes. The diffusivity of Si- and Al-bearing species is very low in low-
temperature granitic systems (Hofmann 1980; Dunn 1986); however, the depolymerization
of the melt by network modifiers (H2O and HF) reduces viscosity and increases diffusion in
volatile-rich granitic melts (Scarfe, 1986). The effect of depolymerization of the network of
Si(Al)-O tetrahedra and coordination with OH-
could considerably reduce the diffusivity of
OH-
in the melt and increase the diffusivity of Si and Al because of their coherent behaviour.
The nucleation and growth of either quartz or feldspar at the interface may be too rapid for
the rejected H2O to diffuse into the volatile-undersaturated melt, and may create locally
volatile-saturated conditions at the interface. The limiting factor in diffusion-controlled
crystal growth is most likely the diffusion of Si away from the feldspar and diffusion of Al
toward the feldspar interface, since the diffusivity of potassium and sodium is considerably
greater (Hofmann, 1980; Dunn, 1986).
At high volatile content in the melt, the nucleation and growth curves for feldspar
shift toward the liquidus (Fenn, 1977). The formation of epitactic quartz on feldspar may be
indicative of a relatively low density of nuclei for quartz, which is expected at very low
undercoolings. Heterogeneous nucleation in pegmatites, particularly along the wall zones,
will initiate crystal growth at low degree of supercooling. This inference was substantiated
by Fenn (1986) and London et al. (1989), who found the graphic texture along the walls of
their reaction vessels.
Perhaps the most unusual feature of the proposed model is its oscillatory nature.
Therefore, the liquid line of descent (Fig.4B) is distinctly different from the familiar
equilibrium case. The graphic intergrowths are a nonequilibrium phenomenon. If the
crystallization were equilibrium (i.e., if the system was well mixed), one would observe a
granitic texture produced by the crystallization of discrete crystals of alkali feldspar and
quartz. The proposed model for graphic intergrowths identifies H2O as the catalyst that
changes the diffusion properties of the melt with crystallization. The corner and edge habit,
and inner lobate boundaries of quartz, indicate that the growth of feldspar was diffusion-
limited. Conversely, the euhedral outer boundaries of the quartz with the feldspar are
indicative of a return to an equilibrium style of growth (Fenn, 1977). Oscillations in far-form-
equilibrium growth commonly are characterized by such growth, involving a transition from
stable to unstable growth of one phase followed by an abrupt and stable growth of a
different phase (e.g., Gray and Scott, 1990).
Conclusions
- The physical conditions within the pegmatites of the Hafafit area were
responsible for the formation of graphic quartz-feldspar intergrowths.
- The spatial distribution of the graphic texture in these pegmatite bodies, near
the contact with the host rocks, suggests that the saturation of a volatile

phase during crystallization is important to understanding their formation.
- The whole rock geochemical, the volume proportions of quartz to K-feldspar,
and phase-equilibrium data in the fluorine-bearing haplogranite system
favour nearly simultaneous crystallization of quartz and K-feldspar from a
melt approaching the coetectic toward quartz saturation.
- Close observation of the textures reveals an irregular quartz-K feldspar
boundary and indicates that quartz preferentially grew on the coroners and
edges of the feldspar.
- During feldspar growth, the area immediately surrounding alkali feldspar
crystals became depleted in Al and enriched in Si. This resulted in growth
being concentrated at crystal edges and corners and the production of rough
crystal faces (i.e., feldspar protuberances). The buildup of rejected silica
resulted in quartz saturation and epitactic nucleation. Growth of quartz
depleted the silica concentrated in the boundary layer melt and restored
feldspar-saturated growth. This process repeated and resulted in the
oscillation of the melt composition in the boundary layer from quartz-
oversaturated to quartz-undersaturated conditions, producing rhythmic
quartz-feldspar intergrowths.
References
1. AUGUSTITHIS, S. S. (1962). Non-eutectic, graphic, micrographic,
and graphic-like “myrmekitic” structures and textures. Beit.
Mineral. Petrog. 8, 491-498.
2. BARKER, D. C. (1970). Composition of granophyre, Myrmekite,
and graphic granite. Geol. Soc. Am. Bull. 81, 3339-33350.
3. BASTA, E. Z. and ZAKI, M. (1961). Geology and mineralization of
Wadi Sikait area, south-Eastern Desert, Egypt. J. Geol. UAR 15:
1-38.
4. CARSTENS, H. (1983). Simultaneous crystallization of quartz-
feldspar intergrowths from graniticmagmas. Geology 11, 339-341.
5. ČERNY, P. (1971). Graphic intergrowths of feldspars and quartz
in some Czechoslovak pegmatites. Contrib. Mineral. Petrol. 30,
343-355.
6. ČERNY, P. and MEINTZER, R. E. (1988). Fertile granites in the
Archean and Proterozoic fields of rare-element pegmatites: crustal
environment, geochemistry and petrogenetic relationships. In:

Taylor, R.P., Strong, D.F., (Eds.), Recent Advances in the
Geology of Granite-related Mineral Deposits. Can. Inst. Min.
Metall., Spec. Vol. 39, 170-207.
7. DUNN, T. (1986). Diffusion in silicate melts: An introduction and
literature review. In: Scarfe, C. M. (Ed.), Silicate Melts. Mineral.
Assoc. Can., Short Course Handbook 12, 57-92.
8. EL-SHARKAWY, M. F. and HARRAZ, H. Z. (2001). Progressive
boron metasomatism next to a pegmatite at Wadi Sikait area,
South Eastern Desert, Egypt. The 5th
Intern. Conf. Geochem.
Alex. Univ.1, 25-51.
9. ERDMANNSDORFER, O. R. (1942) studien im Gneisgebirge des
Schwarzwaldes. XII. Uber Granitstrukuren. Itzb. Heidelberg Akad.
Wiss. Math.-naturw. K1.2, Abh.
10.FENN, P. M. (1977). The nucleation and growth of alkali feldspars
from hydrous melts. Can. Mineral. 15, 135-161.
11.FENN, P. M. (1986).On the origin of graphic granite. Am. Mineral.
72, 25-33.
12.FENN, P. M. and LUTH, W.C.(1973). Harzrds in the interpretation
of primary fluid inclusions in magmatic minerals. Feol.Soc.Am.,
Abst. Progr. 5, 617p.
13.GRAY, P., and SCOTT, S. K. (1990). Chemical Oscillations and
Instabilities: Non-Linear Chemical Kinetics. Oxford Univ. Press,
Oxford, England.350p.
14.GREILING, R. O. (1990). Structural and geologic map of Wadi
Hafafit area. TFH. Berlin.
15.HOFMANN, A. W. (1980). Diffusion in natural silicate melt. In:
Hargraves, R.B. (Ed.), Physics of magmatic processes. Princeton
Univ. Press, New Jersey, pp. 387-417.
16.HUGHES, C. J. (1971). Anatomy of a granophyre intrusion.
Lithos. 4, 403-415.
17.KIRKHAM, R. V., and SINCLAIR, W. D. (1988). Comb quartz
layers in felsic intrusions and their relationship to porphyry
deposits. In: Taylor, R. P., Strong, D. F. (Eds.), Recent Advances
in the Geology of Granite-Related Mineral Deposits. Can. Inst.

Min. Metall., Spec. 39, 50-71.
18.LENTZ, D., and FOWLER, A. D. (1992). A dynamic model for
graphic quartz-feldspar intergrowths in granitic pegmatites in the
southwestern Grenville Province. Can. Mineral. 30, 571-585.
19.LOFGREN, G. E. (1974). An experimental study of plagioclase
morphology: isothermal crystallization. Am. J. Sci. 274, 243-273.
20.LONDON, D., MORGAN, G. B. V., and HERVIG, R. L. (1989).
Vapor undersaturated experiment. Macusani glass-H2O at 200
Mpa and the internal differential of granitic pegmatites. Contrib.
Mineral. Petrol. 102, 1-17.
21.LUTH, W. C., JAHNS, R. H., and TUTTLE, O. F. (1964). The
granite system at pressures of 4 to 10 kilobars. J. Geophys. Res.
69, 759-773.
22.MANNING, D. A. C. (1981). The effect of fluorine on liquids phase
relationships in the system Qz-Ab-Or with excess water at 1 Kb.
Contrib. Mineral. Petrol. 76, 206-215.
23.MEHNERT, K.R. (1968).Migmatites and the origin of granitic rocks.
J. Afr. Earth Sci. 17, 525-539.
24.MOHAMED, F. H. (1993). Rare metal-bearing and barren
granites, Eastern Desert of Egypt; geochemical characterization
and metallogenetic aspects. J. Afr. Earth Sci. 17, 525-539.
25.MOHAMED, F. H., and HASSANEN, M. A. (1997). Geochemistry
and petrogenesis of Sikait leucogranite, Egypt: an example of S-
type granite in a metapelitic sequence. Geol. Rundsch. 86, 81-92.
26.NITTMANN, J., and STANLEY, H. E. (1986). Tip splitting without
interfacial tension and dendritic growth patterns arising from
molecular anisotropy. Nature. 321, 663-668.
27.RASHWAN, A. A. (1991). Petrography, geochemistry and
petrogenesis of the Migif-Hafafit gneisses at Hafafit mine area,
South Eastern Desert, Egypt. Scientific Series International Barea
Volume 5, Forschangszentran Julich GmbH, 359p.
28.SCARFE, C. M. (1986). Viscosity and density of silicate melts. In:
Scarfe, C. M. (Ed.), Silicate Melts. Mineral Assoc. Can., Short
Course Handbook 12, 36-56.

29.SECLAMAN, M. and CONSTANTINESCU, E. (1972).
Metasomatic origin of some micrographic intergrowths. Am.
Mineral. 57, 932-940.
30.SIMPSON, D. R. (1962). Graphic granite from the Ramona
pegmatite district, California. Am. Mineral. 47, 1123-1138.
31.SMITH, J. V. (1974). Feldspar Minerals-2 chemical and Textural
properties. Springer-Verlag. Berlin, 690p.
32.STEL, H. (1992). Diagnostic microstructures for primary and
deformational quartz rods in graphic granite. Am. Mineral. 77, 329-
335.
33.STERN, R. J. and Hedge, C. E. (1985). Geochronologic and
isotopic constraints of late Precambrian crustal evolution in the
Eastern Desert of Egypt. Am. J. Sci. 285, 97-127.
34.TUTTLE, O. F. and BOWEN, N. L. (1958). Origin of granite in the
light of experimental studies in the system NaAlSi3O8 - KAlSi3O8 -
SiO2 - H2O, Geol. Soc. Am. .Mineral. 74, 153p
35.VOGT, J.H.L. (1931) .Die Genesis der Granite physikochemisch
Gedeutet.Z. Deutsche Geol. Ges. 83, 193-214.

GRAPHIC QUARTZ-FELDSPAR INTERGROWTHS IN PEGMATITES: DIFFUSION AND GROWTH KINETICS MIGIF-HAFAFIT AREA, SOUTH EASTERN DESERT EGYPT

GRAPHIC QUARTZ-FELDSPAR INTERGROWTHS IN PEGMATITES: DIFFUSION AND GROWTH KINETICS MIGIF-HAFAFIT AREA, SOUTH EASTERN DESERT EGYPT

More Related Content

What's hot

Similar to GRAPHIC QUARTZ-FELDSPAR INTERGROWTHS IN PEGMATITES: DIFFUSION AND GROWTH KINETICS MIGIF-HAFAFIT AREA, SOUTH EASTERN DESERT EGYPT

More from Geology Department, Faculty of Science, Tanta University

Recently uploaded

GRAPHIC QUARTZ-FELDSPAR INTERGROWTHS IN PEGMATITES: DIFFUSION AND GROWTH KINETICS MIGIF-HAFAFIT AREA, SOUTH EASTERN DESERT EGYPT