Page 182 - Microtectonics

6.2 · Veins 171
suggested to influence the shape of fibrous and elongate
veins. Crystal growth can be isotropic, i.e. independent
of crystallographic direction, producing approximately
equidimensional grains in the case of free growth; or
anisotropic, with more rapid growth in some crystallo-
graphic directions than in others; this would produce
euhedral crystal faces and a crystallographic preferred
orientation in the grains that survive growth competi-
tion (Figs. 6.11, 6.12; Bons 2001; Nollet et al. 2005). Al-
though most crystal growth is thought to be anisotropic,
the original model (Urai et al. (1991) was based on iso-
tropic growth. Numerical experiments have shown that
growth anisotropy can influence the shape of individual
elongate grains in the case of growth competition where
vein filling does not keep up with opening. However, it
has little effect on fibre shape and development where
the veins are filled completely upon opening (Bons 2001;
Nollet et al. 2005). In the case of anisotropic crystal
growth, it is also possible that fast growing crystals are
tracking, while slowly growing ones are not.
Initial grain size of the wall rock can be of influence
Fig. 6.15. Composition of the wall rock can have a strong influence
on the size of elongate crystals in syntaxial veins. Elongate grains in on the width of elongate grains or fibres in syntaxial veins.
a vein adjacent to a monocrystalline coarse grained wall rock a can The grain size of monomineralic wall rocks will influ-
be narrower than those in a fine-grained polymineralic wall rock b ence that in the veins if it exceeds the amplitude of
antiformal asperities in the contact. In polymineralic
2001; Hilgers and Urai 2002; Nollet et al. 2005; Fig. 6.15). rocks, however, the effect can be opposite; a schist with
No fibres form in this case. Since the number of grains few small quartz grains has few nuclei for quartz crys-
decreases with growth, the mean grain width increases tals in a vein, so it can produce much coarser veins than
in the growth direction, and this can in many cases be a wall rock with more or bigger quartz crystals (Fig. 6.15;
used to reconstruct the growth direction in aggregates Fisher and Brantley 1992).
of elongate crystals. Elongate grains may become fixed Another factor to consider is that the shape of the
to asperities in the contact, but may not track the open- growth surface may change in the course of vein growth;
ing direction as well as fibres (Fig. 6.14). If the growth fibrous “antithetic” calcite veins can nearly always be
surface is smooth and planar, e.g. because the original shown to be actually composite veins (Figs. 6.5, 6.6), with
crack was straight, elongate crystals or fibres will grow thin slowly growing quartz-chlorite selvages along the
at right angles to the growth surface irrespective of open- vein edge. The shape of the growth surface is therefore
ing direction of the vein, and are said to be face-control- constantly changing in this case. Besides growth, frac-
led. This may be the case when a crack opens along a turing of wall rock and fibres along the growth surface
planar crystal face such as that of a pyrite cube (Figs. 6.15, can also lead to considerable changes (Hilgers et al. 2001).
6.21, ×Photo 6.21). Domains of face-controlled fibres can Summarising, in the model of Urai et al. (1991), dis-
contain ghost fibres (Ramsay and Huber 1983), usually placement-controlled fibres or elongate crystals which
solid inclusions or fibres of a deviant mineral that cross track the opening trajectory of a vein can only form un-
the face-controlled fibres (Fig. 6.15). Ghost fibres are der special circumstances of an irregular growth surface,
thought to track the opening direction of the vein. Veins and small opening rate with respect to growth rate of
can therefore be subdivided into end members with dis- crystals in the vein. The crystals are forced to grow into a
placement- or face-controlled crystals, and mixed types shape that is unlike their low-index crystallographic di-
that contain both displacement and face controlled crys- rections. If grains cannot keep track of a moving bound-
tals, or crystals that switch behaviour along their length. ary, non-tracking elongate grains will form with crystal
If crystal growth of all grains is slower than the open- facets in the case of anisotropic growth. The threshold
ing rate of the vein, crystals grow freely in a fluid filled between such “tracking” and “non-tracking” can be sharp
space; in most cases, growth competition leads to for- (Hilgers et al. 2001) and may be a means to determine
mation of blocky, euhedral crystals with low-index crys- strain rate in rocks (Sect. 9.8; Fig. 6.14). In many geo-
tal faces (Figs. 6.11, 6.12, 6.14). metrical situations, the ‘free’ surfaces on both sides of an
Besides opening rate versus growth rate, and rough- opening crack will move apart in the direction of ISA,
ness of the growth surface, some other factors have been and in that case the vein will track the ISA direction. It