Model Answer
0 min readIntroduction
Plagioclase feldspars are ubiquitous rock-forming minerals exhibiting a continuous solid solution series between albite (NaAlSi<sub>3</sub>O<sub>8</sub>) and anorthite (CaAl<sub>2</sub>Si<sub>2</sub>O<sub>8</sub>). Zoning, a common feature in many igneous rocks, refers to the systematic variation in chemical composition within a single crystal. This compositional variation is often visible as distinct bands or sectors within the crystal. Understanding zoning in plagioclase, particularly through the lens of the albite-anorthite system, provides crucial insights into the crystallization history and magmatic processes that shaped the rock. This answer will explain the concept of zoning and how it is manifested in plagioclase using the albite-anorthite system as a model.
Understanding Solid Solutions and Zoning
A solid solution is a homogeneous mixture of two or more crystalline solids dissolved in each other. The albite-anorthite series represents a complete solid solution, meaning that any proportion of albite and anorthite can be mixed to form a stable plagioclase feldspar. Zoning occurs when the conditions of crystallization change during crystal growth, leading to variations in the composition of the solid solution incorporated into the crystal structure. These changes can be driven by factors like temperature, pressure, or changes in the composition of the surrounding magma.
The Albite-Anorthite System
The albite (Ab) – anorthite (An) system is a binary solid solution series. Plagioclase compositions are expressed as a percentage of albite and anorthite (e.g., An50Ab50 represents a plagioclase with 50% anorthite and 50% albite). At high temperatures, the solid solution is more readily mixed, allowing for greater miscibility of Na+ and Ca2+ ions. As temperature decreases, the solubility of Ca2+ in the albite structure decreases, leading to exsolution or zoning. The system exhibits a solvus, a temperature range below which complete solid solution is no longer possible.
Mechanisms of Zoning in Plagioclase
Several mechanisms contribute to zoning in plagioclase:
- Disequilibrium Crystallization: Rapid changes in magma composition or temperature can cause the crystal to grow faster than it can equilibrate with its surroundings. This results in a core composition that differs from the rim.
- Diffusion: The diffusion rate of Na+ and Ca2+ ions within the plagioclase structure is slow. Therefore, even small changes in magma composition can lead to compositional gradients within the crystal.
- Magma Mixing: The mixing of magmas with different compositions can introduce compositional variations that are recorded in the growing plagioclase crystals.
- Crystal-Melt Boundary Layer: A boundary layer exists between the growing crystal and the surrounding melt. The composition of this boundary layer can differ from the bulk melt, influencing the composition of the crystal surface.
Manifestation of Zoning in Plagioclase
Zoning in plagioclase can be observed through various techniques:
- Optical Microscopy: Different plagioclase compositions exhibit different refractive indices. This can be observed under a polarized light microscope as variations in extinction angles and interference colors. Normal zoning appears as alternating light and dark bands.
- Electron Microprobe Analysis (EMPA): EMPA allows for precise determination of the chemical composition of different zones within the crystal. This provides quantitative data on the variations in albite and anorthite content.
- Scanning Electron Microscopy (SEM): SEM can reveal fine-scale zoning patterns that are not visible with optical microscopy.
A typical zoning pattern in plagioclase might show a calcium-rich core (An80-90) surrounded by progressively more sodium-rich rims (An50-60). This indicates that the crystal began to grow in a magma relatively rich in calcium, but as the magma evolved (e.g., through fractional crystallization), the calcium content decreased, leading to the formation of a more sodium-rich rim.
| Zone | Composition (Approximate) | Formation Condition |
|---|---|---|
| Core | An80-90Ab10-20 | Early stage crystallization, Ca-rich magma |
| Intermediate | An60-80Ab20-40 | Magma evolving towards Na-rich composition |
| Rim | An40-60Ab40-60 | Late stage crystallization, Na-rich magma |
Conclusion
Zoning in plagioclase, exemplified by the albite-anorthite system, is a powerful tool for deciphering the crystallization history of igneous rocks. The variations in composition within a single crystal reflect changes in the magmatic environment, providing insights into processes like fractional crystallization, magma mixing, and temperature gradients. Analyzing zoning patterns, using techniques like optical microscopy and EMPA, allows geologists to reconstruct the evolution of magmatic systems and understand the formation of diverse igneous rocks. Further research into the kinetics of diffusion and the influence of trace elements will continue to refine our understanding of this important petrological phenomenon.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.