Page 183 - Petrology of Sedimentary Rocks

Recrystallization, Inversion and Neomorphism

Carbonates, more than most other rocks, are susceptible to alteration because of
the highly reactive and unstable nature of many of their original constituents,
particularly aragonite and High-magnesian calcite. Reaction may take the form of
(I) total removal of the constituent, leaving an open cavity which exists for consider-
able time, and later filling by another mineral. This process, recognized as early as
1879 by H. C. Sorby, is a common one for aragonitic fossils whose molds are later filled
by sparry calcite. Best evidence for this is partial collapse of the rim or “micrite-
envelope” of the original shell (Bathurst, 1963). (2) Replacement of the original
constituents by another mineral of grossly different composition, with no significant
cavity developed; the host and guest minerals always remain in close contact, of course
with a thin liquid film between to conduct the ions in and out of the system. This is the
metasomatism of Lindgren, and is illustrated by dolomitization of a calcitic or
aragonitic limestone, or replacement by chert, pyrite, etc. Replacement is usually
volume for volume, indicating some addition or subtraction of ions from the system,
(3) Inversion from one polymorph of a mineral to another, gross chemical composition
remaining essentially constant --e.g. change from orthorhombic aragonite to rhombohe-
dral calcite. This can happen either by (a) a migrating liquid-film causing simultaneous
solution of the old and precipitation of the new as in ordinary replacement, or (b) a
switching of the positions of the ions in the crystal lattices without the presence of
liquids or long-distance export-import of ions. The liquid film process is probably much
more important in ordinary carbonate rocks. Evidence is the preservation or original
structure, e.g. organic Iaminae of the original aragonitic shell being preserved in the
calcite pseudomorph (Hudson; Nelson; Sorby). (4) True recrystallization, where the
original and final phases are the same mineralogically but merely differ in crystal form,
size, or orientation. An example would be fibrous calcite converting to microcrystal-
line calcite, or microcrystalline calcite going to sparry calcite. Again, thin liquid films
are probably present between the older and newer generations of crystals. This may be
caused by strain (like the ordinary recrystallization--or as termed here “strain”
recrystallization, of the metallurgist); or may be driven by other forces such as surface
energy.

Other processes are sometimes encountered. For example, calcite in replacing an
aragonitic shell may have a thin “no-man’s land” of powdery carbonate, or even
microporosity between the original and the new minerals (Schlanger I963), a situation
intermediate between solution-cavity-fill and inversion. Or High-magnesian calcite
may expel Mg ions to leave relatively pure calcite of form and texture microscopically
identical with the original.
Processes (3) and (4) above --inversion and true recrystallization may be lumped
under the term “neomorphism” (Folk 1965) which simply means that a mineral has the
same gross composition (ignoring changes in trace elements, isotopes, etc.) but merely
has a new form --crystal size, shape or orientation --differing from the original. It is
useful when referring collectively to inversion and recrystallization, or when the exact
process is not known.

Neomorphism (and its more specific processes, inversion and recrystallization)
usually result in increase of crystal size (aggrading recrystallization), such as when a
carbonate mud goes to sparry calcite. Occasionally, the crystal size decreases
(degrading recrystallization), as when oolites or fossils degrade to micrite, but this
porcess is much more rare.

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