Phys Chem Minerals (1992) 19:176-184
PHYSICS []]CHIMISTRY
(]]I]MIHIRALS
9 Springer-Verlag 1992
Morphological Instabilities during Rapid Growth of Metamorphic
Garnets
Bjorn Jamtveit I and Torgeir B. Andersen /
x Department of Geology, Universityof Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK
2 Department of Geology, Universityof Oslo, P.O. Box 1047, Blindern, N-0316 Oslo 3, Norway
ReceivedMay 24, 1991 / Accepted March 18, 1992
Hydrothermal grossular-andradite garnets
from contact aureoles in the Oslo region show morphological transitions from planar via cellular to hopper-like
structures. Dodecahedral surfaces {110} dominate during the planar growth stage, whereas the stable crystal
faces, developed during the cellular and hopper stages
also includes the ikositetrahedron {211} and possibly
the hexoctahedron {321}. Faceted cells develope when
initially 'wavy' perturbations on the dodecahedral surfaces become tangential to lower-index planar surfaces.
Inclusion patterns and morphologies of almandinerich garnets from Mageroy (northernmost Norway) that
formed during a period of rapid heating, suggest an early
stage of cellular growth followed by planar growth. The
morphological transitions suggest that the hydrothermal
garnets experienced an increase in the overstepping of
the garnet precipitation reaction at some stage during
their growth whereas the opposite was the case during
growth of the Mageroy garnets. The present observations put constraints on the garnet growth rates and
emphasize the importance of growth kinetics during
metamorphic processes.
Abstract.
Introduction
The processes taking place during natural crystal growth
have recently gained increasing attention among earth
scientists. Most studies of natural crystal growth have
focused on either crystallization from a silicate melt (e.g.,
Kirkpatrick 1975, 1981 ; Tiller 1977) or mineral precipitation from aqueous solutions in sedimentary or hydrothermal environments (i.e. highly porous rocks) at low
temperatures (e.g., Komatsu and Sunagawa 1965; Sunagawa et al. 1975; Carstens 1986). However, minerals also
develop euhedral crystals in metamorphic environments.
An understanding of the processes that control the morphological characteristics of metamorphic minerals may
potentially give important information regarding such
essential parameters as the crystal growth rates and consequently the direction and rate of change in temperature and/or chemical potentials of mineral-forming components during metamorphism. Furthermore, the mechanisms and rate of crystal growth may control the content of trace constituents taken up by a growing crystal
(Paquette and Reeder 1990). This will have implications
for the large scale transport of trace components in hydrothermal systems in general. Finally, knowledge of
the kinetics of crystal growth may be critical in models
for fluid-rock interactions based on coupled transport
and dissolution/precipiation reactions (e.g. Steffel and
Van Cappelen 1990).
Most metamorphic reactions are believed to take
place close to the equilibrium conditions. This is a consequence of the fact that many metamorphic rocks evolve
as a response to relatively slow tectonothermal processes, where the rate of the metamorphic reactions are
controlled by large-scale conductive heat-transport. In
such cases, the growth mechanism that controls the formation of new metamorphic minerals will be the mechanism that dominates at very low overstepping (supersaturation) at the reaction boundaries. However, there are
also numerous situations where metamorphic systems
evolve rapidly due to rapid changes in temperature, pressure or the chemical environment. Systems that are
pushed far enough away from equilibrium may in some
cases generate 'self-organized' ordered patterns in space
or time through amplification of small fluctuations in
the systems. Morphological structures formed by selforganization processes may be more common in metamorphic systems than hitherto thought (e.g. Ortoleva
et al. 1987). In this paper, we describe and discuss the
geological significance of the morphologies and the morphological transitions taking place in metamorphic garnets from two different geological environments. Both
environments are characterized by rapid changes in the
physiochemical environment of the growing garnets;
caused by fluid-infiltration and rapid heating respectively.
Fig, 1 a-h. Hydrothermal garnets from the Oslo region, a Secondary electron (SE-) image of platinum coated andradite-rich garnets
showing a characteristic hopper morphology with breakdown of
the central {110} faces, ttopper crystals are invariable modified
by {211} thces, b Naturally etched cell showing a typical striation
of the {211} faces (S-faces) generated by pile up of growth steps
on {110}. e Planar {110} surface, showing well developed polygonal
spirals reflecting a BCF growth mechanism. The scale bar in
the lower right corner is 10 gin. d Back-scattered electron (BSE)
image of oscillatory zoned grossular (Ca3A12Si3Olz) - andradite
(Ca3Fe2Si3Olz) garnet crystals. The dark cores are relatively
Al-rich (35-40 moI% Ca3AlzSiaO12) whereas the light rims are
nearly pure andradite (reaching 98 mol% Ca3Fe2Si3012). e BSE
image showing a section of an andradite-rich rim growing into
an initially open void (fluid channel) that has later been filled
with calcite, f A garnet rim that, in addition to a distinct surfacenormal oscillatory zonation, shows a faint striation parallel to the
growth direction which starts even before any morphological transitions can be seen. The BSE image shows that the growth layers
are darker near the cell boundaries, due to an anomalously low
andradite content in this area. The latter zonation is reflected by
a chemical profile (along the line A-B in f seen in Fig. 2g) BSE
image of a garnet growing with its apex into an open void that
was later filled with quartz. In this case the hopper morphology
is more irregular, h Close-up of g showing that the hopper surface
apparently develop into a more complex, dendritic, morphology
by twinning
178
Fig. 1 g-h
Table 1. Representative electron microprobe analyses of garnets
Garnets from Oslo
Garnets from Mageroy
O1
M1
Rim
M2
Sample no
Core
Intermed
Core
Rim
SiO2
TiO2
AlzO3
FeO a
MnO
MgO
CaO
37.07
0.55
7.42
17.99
1.17
0.05
33.82
35.69
0.00
1.22
26.17
0.47
0.00
33.43
35.24
0.00
0.48
26.99
0.46
0.00
33.35
35.21
0.02
0.22
27.36
0.41
0.00
33.27
37.51
0.12
20.96
27.03
6.74
2.17
5.54
37.85
0.02
21.48
33.42
2.17
3.27
3.27
Total
98.07
96.98
96.52
96.27
100.07
100.67
Core
M3
Rim
Core
Rim
37.55
0.12
20.87
27.83
6.97
1.83
5.07
37.63
0.00
21.66
32.57
1.48
3.20
3.82
37.29
0.11
20.82
29.13
5.27
2.14
4.77
37.13
0.03
20.72
33.12
1.35
3.25
3.88
100.24
99.76
99.53
99.48
3.02
0.01
1.98
0.01
1.90
0.47
0.22
0.44
0.00
3.02
0.00
1.99
0.01
2.22
0.10
0.38
0.33
0.00
3.01
0.01
1.98
0.01
2.00
0.36
0.26
0.41
0.00
2.99
0.00
1.96
0.04
2.19
0.09
0.39
0.33
0.00
Structural formulae based on
N cations-Si = 5.00
Si
Ti
A1
Fe(III)
Fe(II)
Mn
Mg
Ca
OHb
3.01
0.03
0.71
1.22
0.00
0.08
0.00
2.94
0.00
3.00
0.00
0.12
1.84
0.00
0.03
0.00
3.01
0.00
N cations = 8.00
2.97
0.00
0.05
1.91
0.00
0.03
0.00
3.01
0.11
2.98
0.00
0.02
1.93
0.00
0.03
0.00
3.01
0.08
3.01
0.01
1.98
0.01
1.82
0.46
0.26
0.48
0.00
3.01
0.00
2.01
0.00
2.28
0.09
0.39
0.28
0.00
The electron microprobe analyses (EMP) were obtained by the C A M E C A C A M E B A X EMP at the Mineralogical-Geological Museum
in Oslo using various natural and synthetic standards.
a F e O = T o t a l Fe
b OH = 4 x (3.00-Si)
Garnet Morphology - Two E x a m p l e s
Fluid-Infiltration Induced Growth
Garnet group minerals grow under a wide range of physiochemical
conditions. Unless the garnet growth takes place during a period
of extensive deformation (cf. Rosenfeld 1970), the garnets frequently develop well defined crystal faces even in metamorphic rocks
with very low porosities. We shall proceed with a description of
euhedral garnets formed in two quite different metamorphic environments.
During the Permian continental rifting event in the Oslo region,
southern Norway, massive intrusion of acidic batholiths at shallow
crustal levels caused extensive convective fluid transport through
Lower Paleozoic metasedimentary rock-units (Goldschmidt 1911;
Jamveit et al. 1992a, b). Euhedral garnet crystals that precipitated
in carbonate-rich rocks during infiltration of aqueous fluids (skarn
garnets) frequently display spectacular, oscillatory chemical zona-
179
tions normal to the crystal surfaces (Jamtveit 1991) and occasionally, as will be described below, morphological changes from planar
to non-planar surfaces. The garnets of this study grew in layered
Silurian carbonate-shale sequences located some 50 meters away
from the contact to the Drammen granite in the central Oslo Rift
region. Garnet precipitation is believed to have occurred from
aqueous solutions at temperatures in the range 3500400~ C and
a hydrostatic pressure of 200-300 bar (Jamtveit in prep.). Other
stable phases were calcite, quartz, magnetite, and various sulphides.
Figure I show a series of electron microscope photographs of
skarn garnets. Figure I a-c show the surface topologies of garnets
at various scales, whereas the cross-sections shown in Figs. I d-h
display both chemical zonation patterns and growth morphologies
outlined by the zonation patterns. The composition of the garnets
shown in Figs. 1 (Table 1), can roughly be described as a binary
mixture of the end-members grossular (Ca3AI2Si~O12) and andradite (Ca3F%Si3Olz). The most important minor components are
spessartine
(Mn3A12Si3Olz) and hydrogarnet (Ca3(A1,Fe)z(OH)12). All garnet crystals have a relatively andradite-poor
(grossular-rich) anisotropic core (60-65 tool% Ca3F%SiaO~z) followed by an isotropic andradite-rich rim (reaching 98 mol%
Ca3Fe2Si3012).
Figure 1a shows the typical hopper-morphology of fresh skarn
garnets with breakdown of the central dodecahedral crystal surfaces to a cellular structure. Such hopper morphologies are typically developed during rapid growth of materials with relatively
high melting enthalpy (Berg 1938; Wilcox 1977). Garnet crystals
showing hopper morphologies are invariably modified by the ikositetrahedron {211} near crystal edges. Planar surface breakdown
is only observed in crystals with diameters larger than about 2 ram.
Within the cellular area of the crystals, each cell (100-200 gm in
diameter) has a euhedral morphology similar to the host crystal,
including both the dodecahedron and the ikositetrahedron. This
striking self-similarity is revealed in Fig. 1 b that shows a naturally
etched euhedral 'cell'. The etched {211} surfaces show a distinct
striation that reflects that these surfaces are S-faces, generated by
the pile up of steps developing on {110}. The structure of the
planar {110} faces is shown in Fig. lc. This figure reveals well
developed polygonal spirals with steps paralleling the crystal edges.
A cross-section through garnets similar to those described
above is shown in Fig. I d. This figure shows the sharp contact
between the rather grossular-rich core and the andradite-rich rim.
However, the andradite-rich rim does also contain thin grossularricher layers, giving rise to a complex, non-periodic, zonation pattern. Figure 1 e displays a section of an andradite-rich garnet rim
that grew into an open fluid-filled void that was later filled with
calcite. The figure clearly shows a transition from a planar to a
cellular morphology with a wavelength of 50-100 gin. One observes
that a faceted hopper morphology rapidly developes from a more
wavy cellular structure. The faceted cells are bounded by {110},
{21 i} and possibly also {321} surfaces. The transition from a planar to a faceted cellular surface seen in Fig. 1e, is very similar to
what has been observed during melt growth of some semiconductor
materials. Bardsley et al. (1962) found that for gallium doped germanium, the transition from a planar to a cellular crystal-melt
interface with faeeted {111} cell faces, occurred over a distance
of ~100 gin. Faceting of the cells were reported to occur when
the cellular surface became tangential to low index surfaces. Finally, the hopper morphology has been overgrown by a subsequent
garnet layer that has reestablished the planar morphology. Figure
I f shows details from a similar garnet rim. The evolution from
a wavy to a faceted structure is clearly revealed. The cellular morphology of the garnet rim is mimicked by the surface-parallel striation (caused by oscillations in the chemical composition) which
emphasizes that the cellular surface is a growth phenomenon rather
than a resorption/dissolution controlled feature. Furthermore, one
observes that the wave-fronts tend to be more andradite-rich (lighter in color) than the cell boundary regions. Careful examination
of this figure shows that this feature gives rise to a faint striation
parallel to the growth direction. A chemical profile along the line
A-B in Fig. 1f shows that this oscillatory pattern is reflected by
6.00
4.50
3.00
E
[., J l!t.
1.50
Illllll
0.00
120
240
360
B
Distance (pm)
Fig. 2. Chemical zonation profile parallel to the garnet surface (represented by a bar in Fig. 1f. The profile shows peaks in the grossular concentration with a spacing of ca. 100 gin. The profile was
obtained by a CAMECA CAMEBAX electron microprobe at the
Mineralogical-Geological Museum in Oslo using various natural
and synthetic standards
variations in the grossular concentration of the garnet, with significant peaks separated by 50-100 ~tm (Fig. 2). These peaks in the
grossular concentration correspond to the boundary regions between the cells. This faint zonation pattern seems to develop before
the onset of cellular growth.
The dodecahedral garnet shown in Fig. I g grew with an edge
oriented towards the fluid filled void (now mainly filled by quartz).
A detail of the rim area of the same garnet is shown in Fig. lh.
The first, and main growth stage was characterized by the stability
of planar crystal surfaces. The planar morphology is followed by
a narrow zone of cellular growth and subsequently by a hopper-like
morphology. However, in this case the hopper-surface is made
more complex by twinning (as is most clearly seen in Fig. lh).
Twinning is the mechanism by which an hopper crystal may develop into a dendritic morphology, that would represent the ultimate
breakdown of the euhedral crystal shape. This hopper crystal has
been partly covered by a thin (~10 p.m) layer of quartz (SiOz)
before a new planar growth-period finally 'repaired' the irregular
surface shape.
Thermally Induced Growth
The second type of garnets described here, comes from a sequence
of pelitic schists from the Caledonian Mageroy nappe, near North
Cape in Northern Norway (Ramsay and Sturt 1976; Andersen
1981). The rock sequence, in which garnet occur as part of a lowvariance mineral assemblage containing staurolite, biotite, kyanite,
quartz and plagioclase, probably experienced a period of rapid
heating during intrusion of nearby mafic/ultramafic intrusives (Andersen 1984). The isotropic garnets are extensively zoned from an
average core composition near Alm6oSpeslsGroSlsPyr7 to a rim
composition near AlmvsSpes3Gros10Pyr12 (see Table 1 and Andersen 1984). Geothermometry, based on a local equilibrium assumption, suggests that garnet growth terminated at a temperature in
the range 5500600~ C at a pressure of 6 8 kbar (Andersen 1981).
The Mageroy garnets show growth sectors with a very regular
arrangement of solid inclusions (Andersen 1984). Slender, optically
continuous, quartz fibers (~1-10 lain in diameter) form a radial
pattern from the garnet core towards the crystal faces (Fig. 3 a).
The quartz inclusions are not captured from the surrounding matrix, but precipitated as a co-product in the garnet forming reaction
(Burton 1986). This radial quartz pattern is furthermore divided
into six sectors separated by another radially distributed set of
very small, rounded, inclusions of which quartz, graphite and il-
180
menite can be identified. The latter set of inclusions defines another
set of sectors in the regions between the garnet cores and the crystal
apexes (Fig. 3b). These sectors seems to show a relative expansion
towards the garnet margin (Fig. 3 b). Harker (1932) interpreted a
very similar sector arrangement of inclusions to arise from polysynthetic twinning, which is a common and easily observable phenomenon in non-isotropic grandite garnets, but less so in isotropic
pyralspite garnets.
Unlike the grandite from the Oslo Region, the Mageroy garnets
do not have any well defined 'stratigraphic marker horizons' with
distinctly different chemical compositions (cf. Fig. 1 d). Thus the
garnet rims represent the only isochronous surface that displays
the true morphology of the garnet crystals. The surface morphology of the Mageroy garnets are most clearly shown on the backscattered electron (BSE) image in Figs. 3c. It is quite clear from
the BSE images that the garnet, during its terminal growth stage,
develop inward moving macrosteps from the crystal apex. This
garnet morphology strongly resembles the morphology of olivine
microphenocrysts from basalt pillows described by Kirkpatrick
(1981, Fig. 40) who ascribes this morphology to an enhanced
growth rate at crystal edges caused by anisotropic supersaturation
around the crystal surface
Growth Rates and Mechanisms
At low to moderate supersaturation, growth of materials
with high melting enthalpy (such as silicate-minerals)
from melts or aqueous solution is believed to occur by
either a screw dislocation controlled spiral growth mechanism (the Burton-Cabrera-Frank (BCF) mechanism) or
by a two-dimensional surface nucleation mechanism (the
Kossel-Stranski (KS) mechanism). C o m p u t e r simulations by Gilmer (1977) suggest that the BCF mechanism
dominates at low supersaturation. This is consistent with
numerous observations of growth spirals on the surface
of natural minerals (e.g. Sunagawa 1987). Analyses of
surface features on synthetic garnets (e.g. Lefever and
Chase 1962) show growth spirals on the faces of garnets
that were grown slowly by evaporation of flux. However,
at higher degrees of supersaturation the crystal surface
m a y undergo roughening transitions to non planar morphologies (e.g. Bennema and van der Eerden 1987).
Garnets from the Oslo Region
Fig. 3a-e. Garnets from Mager~y. a Photomicrograph showing a
sector grown garnet porphyroblast sectioned through the center.
The field of view is 2 x 3 mm. Six sectors with abundant rod-shaped
quartz inclusions can be seen radiating from the core towards the
central areas of each crystal surface. Another set of sectors, expanding towards the garnet apexes, contain numerous spherical inclusions of quartz, F e - T i oxide and graphite, b Detail of the inclusion
patter seen in a. Field of view is 0.9 x 1.3 mm. Note the external
morphology near the crystal apex. c BSE-image of the garnet in
a, showing inward moving macrosteps from the crystal apexes/
edges towards the central parts of the crystal surface, a and b
are from Andersen, 1984
The secondary electron (SE) image shown in Fig. I c
shows that stable planar growth of the grandite garnets
of the present study occurred by a screw dislocation
controlled spiral growth mechanism. As demonstrated
by Miiller-Krumbhaar et al. (1977), growth spirals become increasingly polygonalized (structurally controlled) with increasing bond-energy between nearest
neighbors in the crystal lattice and thus with decreasing
solubility of the mineral. Therefore the silicate garnets
of this study, with high latent heats, are expected to
develop highly polygonized spiral patterns during the
BCF m o d e of growth as is observed. In their analysis
of the stability of singular ('flat') interfaces Chernov
and Nishinaga (1987) suggest that perturbations of a
flat surface by growth hillocks formed during spiral
growth, m a y lead to instability and to the formation
of macrosteps or microfacets. The transition from planar
181
to cellular morphology during garnet growth may have
resulted from a similar mechanism.
In magmatic rocks, the formation of hopper crystals
is commonly regarded as a result of a surface nucleation
controlled growth with the highest constitutional supersaturation, and thus nucleation rate, near crystal apexes
and edges (e.g. Kirkpatrick 1981). However, the growth
surfaces of the garnet crystals during the ' hopper stage'
(Fig. 1 e, f) show that significant growth occurred in the
central parts (the cellular areas) of the crystal surfaces
without addition of material by spreading of layers initially nucleated near crystal apexes and edges. The reason why a cellular surface developed at crystal surfaces
simultaneously with nearly planar surfaces at the edges
may be that the transition from a dodecahedral crystal
to the more spherical ikositetrahedron would require
more rapid addition of material to (and thus much more
rapid growth rate of -) the central parts of the crystal
surfaces relative to edges and apexes. Consequently, the
central parts of the crystal faces were 'starving' relative
to the areas near the edges.
The well defined morphological changes, from planar
via cellular to a hopper-like crystal surface, suggest increasing growth rate with time. However, as shown by
Bardsley et al. (1962), the onset of cellular growth may
be significantly delayed relative to the change in crystal
growth rate, because the solute boundary layer at the
crystal surface needs a finite time periode to attain
steady state. Thus, the actual increase in growth rate
for these garnets may well have been associated with
the sudden change in composition from the grossularrich core to the andradite-rich rim (Fig. 1 d, Jamtveit
et al. in prep.).
It is interesting to note that the morphological evolution of these garnet crystals, from a planar to a hopper
stage via an intermediate stage of cellular growth, is
consistent with theoretical models for the morphological
evolution of materials with isotropic interface kinetics
(rough surfaces) based on a Mullins and Sekerka (1964)
type of approach. Weakly nonlinear analyses of planar
interface stability during directional solidification suggest that even for isotropic materials, oscillatory (cellular) behavior is expected over a small parameter range
(Jenkins 1990). Furthermore, a morphological stability
diagram recently presented by Billia et al. (1990) predicts
a transition from planar to dendritic growth via an intermediate stage of cellular growth with increasing growth
rate.
It would clearly be of great interest to be able to
relate morphological structures to variables like crystal
growth rate. In the following, we shall briefely examine
how, within the Mullins and Sekerka framework, the
wavelength of cellular structures is expected to vary with
growth rate. Assume that a planar crystal surface is perturbed by a sinusoidal ripple of infinitesimal amplitude,
given by
z=b(t) sino~x
(1)
where z and x are distances normal and parallel to the
initially flat surface respectively, 6(t=O) is the initial
amplitude of the ripple with wavelength. 2 = 2 z/co where
co represents the frequency. For a planar surface to be
stable, d rid t must be negative for any ~o (within a coordinate system attached to the planar interface). Mullins and Sekerka (1964, eq. 10) present a planar surface
stability criterion expressed in terms of growth parameters and system characteristics. Although this criterion
is generally not useful for mineral surfaces with anisotropic growth kinetics, its functional form may still
give interesting information. Assuming that thermal gradients are negligible during growth of metamorphic minerals from hydrothermal solutions, the marginal state
condition of Mullins and Sekerka (dr/dt>O) reduces
to
TcFcoZ+mGc[o)* - V/D]/[oa*--(1 - k ) V/D] > 0
(2)
where Tc is the temperature of crystallization; F is a
capillary constant that tends to stabilize planar surfaces,
Gc denotes concentration gradients of the precipitating
components in the fluid (e.g. the gradient in the solubility product as a function of distance) at the unperturbed
surface; m is the gradient in the temperature overstepping with changes in solute concentration of the fluid
(analogous to the slope of a liquidus surface in the case
of melt growth); V is the crystal growth velocity; D
is the diffusion coefficient of solute in the fluid phase;
k is the liquid/solid solute concentration ratio and o)*=
(V/2D)+[V/2D)2+co2] 1/2 (Mullins and Sekerka, eq.
9a). Solving (2) with respect to m and assuming k ~ 1
gives
(/)max =
[m Gc/TcF] 1 / 2
(3)
where COmax is the maximum unstable frequency. Thus,
the minimum wavelength of a stable perturbation is
'tmi. ( = 2 Z/C0max)= 2 ~ (To r i m Gc] 1/2
(4)
G~ is inversely proportional to the solute field scalelength (D/V) through
G~ = (Co (1 - k)/k)/(D/V)
(5)
where Co is the (equilibrium) solute concentration in
the fluid at the surface (z= 0). Combining (4) and (5)
gives
•mln
=
2n (Tr
Co V)1/2
(6)
Thus the minimum stable wavelength of the cellular
structure is expected to be inversely proportional to the
square root of the growth velocity. However, these calculations are based on the assumption of small amplitude
perturbations, and the wavelength may therefore change
as the amplitude increases. Such a change is seen in
Fig. 1 e, but this change may also be controlled by
changes in the growth velocity. According to (6) formation of cellular structures during mineral growth is favoured by high growth velocities and low temperatures
of crystallization. Such conditions are probably more
common in hydrothermal environments than in most
other geological environments. In theory eq. (6) may
allow calculation of the garnet growth rate from the
initial wavelength of the cellular structures, but as the
surface stabilizing effects (here accounted for by F) are
182
generally not known for silicate minerals, this has not
been attempted here. However,. a comparison with
growth of semi-conductor materials from solutions or
melts with similar solute diffusivities may indicate
growth velocities (perpendicular to the crystal faces) in
excess of 1 cm per year (D.T.J. Hurle, pers. comm.).
Garnets from Mageroy
The garnet porphyroblasts from Mageroy are interpreted to have grown by displacement of insoluble matrix
grains (Rice and Mitchell 1991). The detailed morphological evolution of these garnets is not easily inferred
due to a lack of suitable stratigraphic marker horizons
(represented by the surface-normal oscillatory zonation
patterns in the skarn garnets from the Oslo region).
Thus, the growth mechanism can only be interpret from
the inclusion patterns and the shape of the present garnet
surface.
The capture of foreign material on the surface of a
growing crystal is clearly favoured by an increasing
roughness of this surface and thus by an increasing
growth rate (e.g. O'Hara et al. 1968). Formation of rod
like radial inclusions of silicate melts in magmatic minerals is a well known phenomenon (Roedder 1984, Figs. 231 to 2-34). This process has been modelled by Zhdanov
et al. (1980) who concludes that the entrapment frequency and the diameter of the inclusions increase with
increasing growth rate. Chernov and Temkin (1976) in
their review on capture of inclusions during crystal
growth, suggest that rod-like inclusions most readily
form during a cellular growth mode. Entrapment of
small fluid inclusions between growth cells on the (0001)
surface of synthetic quartz has been reported by Brown
et al. (1952), and sectors of inclusions very similar to
those shown in Fig. 3 have been produced during cellular
growth of KDP (KH2PO4) crystal from aqueous solutions (Chernov and Temkin 1976). Thus, we suspect that
the inclusion patterns seen in the garnets from Mageroy
reflect a cellular (rough-) growth mechanism. By analogy
with the garnets from the Oslo region, this imply growth
at a higher degree of supersaturation (overstepping of
the garnet-producing reaction) than during 'planar'
growth.
A possible alternative explanation stems from the results of Lefever and Chase (1962). These authors reported line imperfections and hollow tubes with a diameter approaching 1 gm, that were oriented approximately
perpendicular to the growth surfaces. The formation of
hollow tubes along a dislocation line with a large
Burgers vector was originally predicted by Frank (1949),
and similar growth spirals with hollow cores at their
centers were observed on SiC by Verma (1953). More
recently, van der Hoek et al. (1982) have analysed the
thermodynamical stability conditions for the occurrence
of hollow tubes. It turns out that hollow tubes may be
stable at certain ranges of supersaturation (e.g. Sunagawa 1987, Fig. 3 a). However, precipitation of abundant
quartz rods, reaching a diameter of tens of microns,
in such tubes seems highly unlikely.
In contrast to the enigmatic status of growth morphology of the internal parts of the Mageroy garnets,
the shape of the garnet surface give more unequivocal
evidence for the growth mechanism during the final
growth stages of these garnets. As mentioned above, the
inward moving macrosteps from the crystal apexes resembles the morphology of magmatic minerals interpreted to have grown by surface nucleation at large undercoolings. However, as demonstrated by Maiwa et al.
(1990), such morphologies may also arise by a BCF
mechanism during anisotropic supersaturation. Although, the structures from the Mageroy garnets are
equivocal, we interpret the transition from cellular to
planar morphology to be as a result of decreasing growth
rate with times.
Discussion
The garnets from the Oslo region and those from Mageroy formed in quite different metamorphic environments. In the first case, rapid garnet growth occurred
in a system open to fluid flow and advective mass transport. In the second case, the low-variance mineral assemblage implies closed-system behavior with respect to the
major mineral-forming components. However, both garnet types probably grew during rapid changes in the
physiochemical environment.
The solubilities of Ca, Fe and Si-bearing ionic or molecular species in even a moderately saline aqueous solution at 400 ~ C may well have been high enough for the
hydrothermal garnets to precipitate from the infiltrating
solutions without the need for extensive local dissolution
of other minerals containing these components. This
conclusion is supported by trace element and isotope
data (Jamtveit et al. in prep.). On the other hand, due
to the low solubility of A1 at moderate pH, the planar
growth of the more grossular-rich (Al-rich) core and
the grossular rich layer defining the surface-normal oscillatory zonation would probably require local dissolution
of Al-bearing minerals. It is likely that the variations
in fluid composition that drives the oscillatory zonation
was a result of the competition between external and
internal (locally buffered) controls on the fluid composition. Yardley et al. (1991) suggest that variations in oxygen fugacity caused by periodic boiling in hydrothermal
systems may represent a major factor in controlling oscillatory zonation in Fe-bearing minerals.
The cause of the morphological changes seen in the
Mageroy garnets is less certain. Most models for porphyroblast growth during metamorphism assumes that
the growth rate is transport controlled (e.g. Fisher 1978).
During diffusion-controlled growth, the growth rate is
expected to decrease with time as the diffusion distance
increases. For diffusion-controlled growth, one would
expect to see expanding halos around the growing porphyroblast that is depleted with respect to the components that are enriched in the growing crystal (Foster
1981). However, such structures are absent around the
Mageroy garnets. A high initial growth rate in the Mageroy case could be explained by a significant overstep-
183
ping of the garnet-precipitation reaction before the onset
of garnet nucleation. However, at present, the understanding of heterogeneous nucleation is too limited for
a discussion o f whether this is a plausible explanation
(e.g. Rubie and T h o m p s o n 1985).
The results presented in this paper emphasize the importance of reaction kinetics in metamorphic processes.
Knowledge of the kinetics of crystal growth is essential
in models for fluid-rock interactions based on coupled
transport and dissolution/precipitation reactions (e.g.
Steffel and Van Cappelen 1990). A m o n g the consequences o f cellular or rough growth is a probable deviation from the parabolic growth kinetics c o m m o n l y assumed for silicate mineral reactions (e.g. Blum and Lasaga 1986).
In conclusion, the m o r p h o l o g y of garnets in metamorphic rocks may potentially represent a valuable tool
in trying to decipher the garnet growth rate and thus
the rate of change in the physiochemicat variables during
metamorphic processes. The observations presented here
demonstrate that garnet precipitation reactions may be
overstepped to an extent where planar growth becomes
unstable and a roughening o f the crystal surface occur.
In the future, it will be o f interest to calibrate m o r p h o logical instabilities with respect to supersaturation for
materials (minerals) with anisotropic growth kinetics
(i.e. layer spreading mechanisms) to derive the possible
extent of deviation from equilibrium in hydrothermal
and other metamorphic systems.
Acknowledgement. We thank Jorn Hurum for supplying some of
the hopper-shaped garnets of this study. Expert advice and assistance by Don Hurle; discussions with Bernie Wood, Odd Nilsen,
Per Aagaard and technical assistance by Haakon Austrheim and
"lurid Winje are gratefully acknowledged. Critical comments by
Klans Langer and Ichiro Sunagawa greately improved the manuscript. This study was supported by The Norwegian Research
Council (NAVF) grants no 440.91/023 and 440.92/025.
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