TEXTURES FORMED DUE TO REPLACEMENT
- Replacement of one mineral by another or by a mineral formed during weathering is common in many types of ores.
- Recognition of replacement when no vestige of the replaced phase remains is difficult.
- The most easily recognized replacement textures are those in which organic materials such as a wood fragment or fossil shell has been pseudomorphed by metal sulfides or oxides (eg hematite, goethite, limonite, U-minerals).
- Pyrite cubes and marcasite laths that have been replaced by iron oxides during weathering are easily identified.
- Replacement may result from one or more of the following processes:
a) Dissolution and subsequent reprecipitation.
b) Oxidation, and
c) Solid state diffusion.
- Variuos replacement geometries have been described by ore petrologists viz., rim, core, zonal vein, frontal etc., but they all appear to represent variations of the same process.
- For their development, replacement textures mainly depend on three features of the phase being replaced:
a) Fractures, cleavages and grain boundaries
b) Crystal structures
c) Chemical composition
Fractures, cleavages and grain boundaries:
- Replacement is the result of surface chemical reaction, hence any channel between grains or within grains is the prime site for replacement.
- Replacement grain boundaries or fractures often appears in the form of thin laths or equant crystals of the replacing phase projecting into the host.
- It may also appear as thin concentric coatings developed roughly parallel to the advancing fromt of replacement.
- In the early stages of the process, replacement may be readily identified because much of the original phase remains, and the original grain boundaries, fractures or cleavages are still visible.
- In more advanced stages, the original phase may be reduced to 'islands' floating in a matrix of the replacing phase.
- Complete replacement of one mineral by another is difficult to establish unless vestigal structure such as the typical morphology of the replaced phase (eg pyrite cubes now seen as goethite etc) are left behind.
- Replacement can be confused with fracture infilling resulting from precipitation of a later phase.
- Replacenent tends to round off irregularities in shape of the replaced phase, whereas infilling leaves the infilling fractured surface intact.
- After replacement, the surfaces on either side if a fracture do not match, whereas after infilling they still retain the 'match'.
- Replacement along fractures may also resemble some of the exsolution textures. However, replacement commonly results in an increase in the volume of the replacing phase at the intersection of fractures, whereas this is not common in exsolutions.
- In fact exsolution often produces the opposite effect in which intersecting exsolution lamellae show depletion of the exolved phase at intersections.
- The crystal structure of the phase being replaced may control replacement because this determines cleavage direction or else diffusion takes place readily along certain crystallographic directions.
- For example the oxidation of magnetite results in replacement by hematite along (111) planes.
- The chemical composition of the host may control the composition of the replacing phase.
- During weathering and hydrothermal replacement the secondary phase retains the same cation composition as the primary phase with merely a change in the oxidation state (eg hematite forming during the oxidation of magnetite) or a change in anion (eg
hematite replacing pyrite or anglesite replacing galena).
- Replacement may also selectively remove one cation while leaving another, this is frequently seen in the replacement of chalcopyrite by bornite or covellite.
- Replacement may occur selectively, affecting one phase of an intergrowth or particular zones in a compositionally zoned crystal, producing an atoll structure.
- The reasons for such selective replacement may be extremely subtle and are poorly understood.
- An example of replacement is the open void "boxwork" texture composed of cellular criss-cross laths of goethite, hematite and sometimes pyrite, which is found in gossans.
- Pseudomorphs of one mineral after another and preservation of original textures are good evidence of replacement.
- In galena, replacement commonly develops along cleavage planes, resulting finally in the formation of pseudomorphs after it.
- Chalcopyrite and related sulfides may form pseudomorphs after pyrite crystals that have developed fracture cleavage.
- In some cases traces of the original cleavage are retained, despite the replacement of the primary mineral.
- Replacement of the areas of the host mineral between segments of the cleavage lattice or between replacement veins gives rise to concentrically banded texture.
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