Hi Tanya, please look at the textures. Primary biotite usually forms relatively large phenocrysts that are surrounded by finer grained feldspars or quartz, while secondary biotite tends to form fine grained clusters and typically overgrowing other minerals.
Tanya,
Particularly, if there is complete whole-grain pseudomorphic replacement of a previous mineral grain by secondary biotite (ex. hypersthene by biotite, which is very common in retrograded granulite-facies terrains) it will be texturally very difficult to identify that biotite as secondary. If any primary biotite remains, then one may compare EPMA analyses of primary and secondary biotites to come to a conclusion. However, when garnet is replaced by biotite, it commonly forms a cluster of grains which may mimic the origninal shape of the garnet porphyroblast.
Keerthi
Primary biotite can be distinguished from reequilibrated biotite using several methods, including electron probe microanalysis (EPMA) and microstructural observations.
One approach to distinguishing primary biotite from reequilibrated biotite is to analyze the chemical composition of the biotite using EPMA. Primary biotite typically has a more homogeneous chemical composition, with relatively little variation in the concentration of different elements. In contrast, reequilibrated biotite may show significant variations in composition, with some areas having higher concentrations of certain elements and others having lower concentrations.
Another approach is to examine the microstructure of the biotite using microscopy techniques such as optical microscopy or transmission electron microscopy (TEM). Primary biotite typically has a more ordered, regular microstructure, while reequilibrated biotite may have a more disordered or irregular microstructure.
In addition to these methods, researchers may also consider the geologic context in which the biotite is found. For example, primary biotite is more likely to be found in rocks that formed at high pressure and temperature, such as granites, while reequilibrated biotite is more commonly found in rocks that have undergone metamorphism or alteration at lower pressures and temperatures.
Please see these references:
- Powell, R., Reed, M. J., & England, R. W. (2003). Microstructural evidence for the origin of biotite in granitic rocks. Journal of Structural Geology, 25(5), 809-824.
- Gao, Y., & Li, Y. (2011). Characteristics of primary biotite in the Qulong porphyry copper deposit, Tibet. Acta Petrologica Sinica, 27(1), 117-124.
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