plagioclase thin section

Sushi Authors

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14-10-2024

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Plagioclase in Thin Section: A Window into Igneous Rocks

Plagioclase feldspar, a common mineral found in igneous and metamorphic rocks, is a fascinating subject for petrologists. When studied under a microscope in thin section, plagioclase reveals intricate details about its formation, composition, and the geological history of the rock it is found in.

What is Plagioclase?

Plagioclase is a solid solution series of minerals, meaning it is a continuous mixture of two end members: Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8). The ratio of these two end members dictates the specific type of plagioclase, with various names assigned depending on the proportion of albite to anorthite.

Identifying Plagioclase in Thin Section

• Cleavage: Plagioclase exhibits two distinct cleavages at approximately 90 degrees, a characteristic shared by other feldspars. However, the angle of these cleavages can vary slightly depending on the composition.

• Twinning: Plagioclase often displays twinning, which occurs when two or more mineral crystals intergrow with a specific orientation. This can be observed as straight, parallel lines within the mineral.

• Relief: Plagioclase exhibits moderate relief in thin section, meaning it is slightly raised above the surrounding minerals. This is due to its higher refractive index compared to other minerals in the rock.

• Color: Plagioclase can range in color from colorless to white, grey, or even light green, depending on the presence of trace elements.

Understanding Composition from Thin Section

• Albite-Anorthite Ratio: By observing the twinning patterns and the refractive index (measured using a petrographic microscope), petrologists can determine the relative proportions of albite and anorthite in the plagioclase. This information is crucial for classifying the rock type and understanding its formation.

• Zoning: Plagioclase crystals often display zoning, which refers to variations in composition and thus refractive index within the crystal. This can be observed as concentric bands or irregular patches with different colors or brightness. Zoning indicates changes in the chemical environment during the crystallization of the magma or lava.

Example: Examining a Basalt Thin Section

Let's consider a thin section of a basalt, a common volcanic rock. Under the microscope, we observe plagioclase crystals with well-defined cleavage and twinning. The crystals display moderate relief and range in color from colorless to grey. Some crystals exhibit zoning, showing a gradual transition from a more calcium-rich core to a more sodium-rich rim. This zoning suggests that the magma cooled slowly, allowing the composition of the plagioclase to change as crystallization progressed.

Beyond Thin Section: Plagioclase's Role in Igneous Rocks

The composition and abundance of plagioclase in igneous rocks provide valuable insights into the:

• Cooling History: The presence of zoned plagioclase suggests that the magma cooled relatively slowly, allowing for compositional changes during crystallization. Conversely, unzoned crystals suggest rapid cooling.
• Chemistry of the Magma: The albite-anorthite ratio in plagioclase can be used to infer the overall chemical composition of the magma from which the rock formed.
• Geochemical Evolution: By studying the changes in plagioclase composition across different layers of an igneous intrusion, petrologists can trace the evolution of the magma and the processes that led to the formation of the rock.

Further Exploration:

• Online Resources: Explore websites like the University of California Berkeley's "Mineral Identification Key" for detailed information on plagioclase and other minerals.
• Petrographic Microscopy: If you are interested in examining thin sections yourself, consider taking a course in petrography or visiting a geological museum with a mineral collection.

By understanding the complexities of plagioclase in thin section, we gain a deeper appreciation for the processes that shape our planet and the wealth of information locked within its rocks.