Textures Formed due to Cooling -- Inversion and Exsolution Textures
- Most ores form at elevated temperatures and have undergone cooling over a range of temperatures.
- In case of Pb-Zn ores in carbonates cooling is less than 100oC, whereas in the case of Fe-Ni-Cu deposits in ultramafic rocks cooling is as much as 1000oC.
- Refractory minerals eg magnetite, chromite, pyrite, sphalerite and some arsenides retain their original composition & texture.
- On the other hand many sulfides, sulfosalts and native metals reequilibrate compositionally and texturally during cooling.
- Reequilibration of ores on cooling is accompanied to some degree by recrystallization of primary minerals, an effect that may or may not leave any vestige of the original texture.
- The textural effects resulting from cooling are varied:
- Many ore minerals undergo compositional and textural readjustments in the form of exsolution or inversion as they cool.
- In exsolution one phase is expelled from another, often in a characteristic pattern.
- The form of the exsolved phase varies with the minerals involved, their relative proportions, and the postdepositional cooling history of the ore.
- The exsolution process results from diffusion (usually of metal ions through a sulfur or oxygen lattice), the nucleation of crystallites, and the growth of crystallites to crystals.
- Similarities of srystal structure and chemical bonding between host and exsolved phase (particularly the matching of atomic arrangements in specific layers resulting in a shared plane of atoms) dictates that exsolution is crystallographically controlled.
- Such exsolutions are called Coherent Exsolutions.
- For example, pentlandite exsolves such that the (111), (110), and )112) planes are parallel to the (001), (110), and (100) planes respectively, of the host pyrrhotite.
- Ulv*p 153*spinel exsolves parallel to the (111) planes of host magnetite.
- If the parent and exsolved phase have completely different structures, or if there is no crystallographic continuity between phases, the exsolution is called a Noncoherent Exsolution.
- For example pyrite exsolved from pyrrhotite occurrs as individual grains (commonly euhedral cubes) rather than as recognizable exsolution lamellae.
- The distinction between crystallographically controlled exsolution and similar replacement textures can often be made because intersecting lamellae show depletion at the junction in the case of exsolutions and greater concentration in the case of replacements.
- The depletion of exsolved material around a large bleb (known as seriate distribution) is a distinctive feature of exsolution.
Types of exsolutions:
a) Simple exsolutions - in which a previously simple solid solution is chemically conceivable eg perthite (90% Or, 10% Ab), and Hematite-ilmenite.
b) Complex exsolutions - involve a disturbance of the stochiometric amount eg a feldspar yielding hematite plates besides orthoclase (these are 'anomalous solid solutions').
c) Sub-exsolutions - exsolved bodies separated at high temperatures are still solid solutions and re-exsolve at lower temperature eg. chalcopyrite in sphalerite exsolves into pyrite + cubanite.
Mechanism of Exsolution:
- dissimilarities of the lattices of the guest and the host.
- if one direction of the lattice is similar, strong directional preferences appear.
- rate of cooling (determines the size of exsolved bodies).
- differences in contraction rates with cooling.
- compositional zoning in minerals leads to uneven distribution of exsolved bodies. (Similar effects can occur by migration of the guest towards the grain boundaries of the host).
Various forms of exsolution textures:
- The large variety of exsolution textures is difficult to classify using a simple terminology.
- However certain terms are widely used to describe the textures:
- Myrmekitic - very irregular forms of exsolution bodies.
- Dispersions (emulsions) - rounded, oval or regular shaped bodies dispersed uniformly through the host.
- Lamellar textures - exsolved bodies occurring as parallel to subparallel lamellae.
- Netlike textures - networks formed by segregation of exsolved material along two or more sets of intersecting cleavages.
- Skeletal, feathery of feligree textures -
- Snake and garland shapes -
- Flames of pentlandite in pyrrhotite.
- Stars of sphalerite in chalcopyrite.
-Exsolution itself is a form of decomposition because the original high temperature composition no longer exists as a single homogeneous phase.
- Most products of decomposition cannot be distinguished from exsolution textures.
- In most cases the products of "Exsolution Textures" are quite unrelated crystallographically to the original material.
- These are to be interpreted as products of decomposition.
- Examples: Galena + bismuth from cosalite(Pb2Bi2S5).
Galena with arsenic inclusions from fahlore,
Chalcopyrite + Gudmundite + pyrrhotite from fahlore,
Chalcopyrite + pyrrhotite from cubanite (CuFe2S3).
- Digenite Cu9S5 is not stable below 70o C unless it contains 1% Fe and decomposes to forma complex mixture of anilite and djurleite.
- Study of the Cobalt, Ontario ores has revealed that a complex intergrowth of galena and chalcocote has apparently formed from decomposition of a Cu-Pb sulfide which is only stable at a high temperature.
- Lathlike matildite-galena intergrowths and the myrmekitic arsenic-stibarsen intergrowths are also products of decomposition.
Bird's eye textures - decomposition texture formed by the breakdown of pyrrhotite to a fine mixture of pyrite and marcasite.
- The term decomposition is more commonly applied when a phase undergoes an abrupt change into two phases of distinctly different compositions as in a eutectoid breakdown.
- The term is also applied to the breakdown of the central portion of a complete solid solution series with the resulting development of an intimate intergrowth of compositionally distinct phases.
- Transformations of a compound into a stable form from a form that is becoming unstable eg marcasite to pyrite, High quartz (β-quartz - hexagonal trapezohedral)) to Low quartz (α-quartz - trigonal trapezohedral) (occurring at 573oC) are referred to as inversions.
- These also include structural changes in contact aureoles or those outside aureoles which are brought about by hot solutions.
- Inversion from one polymorph to another may be introduced by temperature of pressure or both.
- High temperature polymorphism is found in lavas and certain high temperature ores.
- Inversion from one crystal lattice to another involves distortion and shear, which may produce characteristic structures.
- Most common of these is the inversion twinning induced by the cubic -> tetragonal inversion in certain substances of the diamond structure group.
- High temperature disordering of the metal components of chalcopyrite and stannite causes the symmetries of these to change from tetragonal to cubic.
- Fall in temperature leads to reordering, and the shearing involved induces the formation of inversion twins.
- These are, in general, somewhat patchy, producing a mosaic of areas with differently oriented sets of twins.
- Such twinning often provides a useful geothermometer.
Chalcopyrite -> 550o C, Stannite -> 600o C.
TEXTURES FORMED DUE TO ANNEALING:
- These textures include effects of slow cooling of ores after deposition or slow heating during metamorphism.
- Since cooling and metamorphism are both prolonged annealing processes, the effects discussed here must include those discussed under transformation textures.
- Reequilibration of ores on cooling is accompanied to some degree by recrystallization of primary minerals - an effect that may or may not leave any vestige of the original texture.
- Recrystallization is an effort to minimize the areas of grain surfaces and interfacial tension by development of equant grains with 120o interfacial (dihedral) angles.
- The interfacial angles in annealed monominerallic aggregates tend to approach 120o, whereas those of polyminerallic aggregates vary as a function of mineral composition.
- The interfacial angles of some annealed (equilibrated) pairs of sulfide minerals are:
Galena-sphalerite 103 and 134o
- in polished sections, which are cut at random, the apparent angles range from 0-180o.
- It is, therefore, necessary to measure many interfacial angles in a given section in order to determine the true interfacial angles.
- If a large number of angles are measured, the most frequently observed shall represent the true interfacial angle.
- Porphyroblastic growth or overgrowth makes paragenetic interpretations difficult because there is no way of distinguishing between porphyroblasts and early formed euhedral crystals.
- However, porphyroblasts contain different amounts and types of inclusions as compared to primary minerals in an ore.
- During the annealing process, small grains are resorbed at the expense of larger ones. However small grains of minor phases may remain trapped as lenslike bodies along grain boundaries of larger ones.
ZONED OVERGROWTHS & HOMOGENIZATION OF PRIMARY ZONING:
- Reequilibration during annealing can also produce zoned overgrowths on grains, or homogenization of grains containing primary zoning.
- For example, overgrowths of pyrite on primary pyrite may be visible in polished sections (may be only after etching).
- Residual primary growth zoning is rarely visible in sphalerite or tetrahedrite (minerals that are frequently strongly zoned), due to its homogenization during annealing.
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