Abstract
Experiments on growth of fibers and fibrous veins using analog
materials demonstrated that fibers can develop by a distinctive
process of growth, called Taber growth, which is
characterized by accretionary growth of vein-like bodies of
fibrous crystals in confined space at the fiber-wall interface, by
drawing nutrient from pore solutions in the "wall-rock" on one
hand and pushing apart the enclosing walls on the other, through
the action of a "force of crystallization". Taber growth differs
markedly from the crack-seal model of Ramsay, in that no
long-range, wall-parallel cracking is involved, and the growth
itself plays an active role in opening a vein. It differs from all
models of vein development that involve passive growth of crystals
following and keeping pace with externally imposed vein dilation,
or involve delivery of solute along a fluid-filled vein crack or
fissure, rather than through the wall rocks. Based on extensive
experimental observations the detailed characteristics of Taber
growth were documented and its essential growth conditions were
studied. It was found that the ambient humidity, the pore fluid
pressure and the grain or pore size are the principal controlling
factors that determine the morphologies of fibers and whether
fibrous or non-fibrous blocky crystals grow. Detailed examination
of experimental Taber fibers revealed microstructural features
that are reminiscent of similar features in natural fibrous veins.
Fibrous veins with various types of fiber curvature patterns were
produced under different growth conditions. Examination of the
tracking behavior of some typical experimental veins showed that
fibers in Taber growth generally track the instantaneous
displacement as long as the growth interface remains cohesive and
there is no internal deformation within the fiber aggregate. The
concept of tracking was criticized and re-evaluated in light of
the experimental observations, and a method was developed by which
vein displacement histories can be reconstructed using fibers that
are known to track the wall-vein displacements.
The displacive characteristic of Taber growth was specially
investigated through experiments on growth of fibrous veins under
large compressive loading conditions. It was demonstrated that
fibrous veins could grow against virtually any pressures or
stresses externally imposed on the wall blocks as long as the
pressure was not so large as to cause the failure of the blocks
and fibers couldn't grow at any other sites against smaller
pressures. The crystallization force in Taber growth was analyzed
from a point of view of thermodynamics, and it was interpreted as
reflecting a crystallization pressure, which is defined as
the difference between the fluid pressure and the theoretical
maximum independent pressure that a crystal can grow against
without dissolving under the given supersaturation conditions.
Theoretical analysis suggests that the crystallization pressure in
Taber growth can attain values of about the same order as
geologically realistic values as long as a high supersaturation
level can be maintained and growth occurs in confined spaces in a
fine porous medium. Further analysis of the surface energy effects
on crystallization in a fine-grained medium suggests that the
conspicuous displacive crystallization of Taber growth is due to
the distinctive process of crystal growth in fine porous media.
Solution confined in such a porous medium can become highly
supersaturated without much crystallization in the pores, thus
producing a large crystallization pressure that is capable of
forcing or pushing open a "vein" in the "wallrock" wherever its
strength is weakest.
Taber Growth affords significant implications for some natural
veins. Fibrous veins formed by Taber growth could be non-tectonic
as well as synkinematic. Taber growth readily explains why the
instantaneous direction of new fiber segments should parallel the
instantaneous direction in which older segments of the same fibers
are moving away from the vein wall. The possible role of the
displacive crystallization of Taber growth in formation of fibrous
veins further suggests that some natural fibrous veins may have
been forced open by displacive growth.
Li T., 2000. Experimental growth of fibers and fibrous veins.
Unpublished PhD dissertation, State University of New York at
Albany. 420pp., +ix
University at Albany Science Library call number: SCIENCE
MIC Film QE 40 Z899 2000 L5
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