Model Answer
0 min readIntroduction
Igneous rocks, formed from the cooling and solidification of magma or lava, exhibit a diverse range of mineral compositions. The stability of minerals within these rocks is governed by thermodynamic principles, specifically the phase rule and the chemical environment of the magma. Forsterite (Mg<sub>2</sub>SiO<sub>4</sub>), an end-member of the olivine series, and quartz (SiO<sub>2</sub>), a pure silica mineral, represent contrasting compositions within silicate systems. Their simultaneous stable presence in an igneous rock is highly improbable due to fundamental chemical and physical constraints related to magma evolution and mineral crystallization. This answer will explain the reasons behind this incompatibility, drawing upon principles of petrology and phase equilibria.
Understanding Forsterite and Quartz
Forsterite is the magnesium-rich end member of the olivine solid solution series (Mg2SiO4 – Fe2SiO4). It crystallizes from relatively high-temperature, magnesium-rich, silica-undersaturated magmas. Its formation is favored in mantle-derived melts. It has a relatively low silica content compared to quartz.
Quartz, on the other hand, is pure silicon dioxide (SiO2). It is a felsic mineral, typically crystallizing from silica-rich, relatively low-temperature magmas. Quartz is stable in silica-saturated or silica-oversaturated conditions. Its formation is associated with granitic and rhyolitic magmas.
Bowen’s Reaction Series and Mineral Compatibility
Norman Bowen’s Reaction Series describes the order in which minerals crystallize from a cooling magma. This series is divided into two branches: the discontinuous series (olivine, pyroxene, amphibole, biotite) and the continuous series (plagioclase feldspar). Forsterite belongs to the discontinuous series, crystallizing at high temperatures. As the magma cools, forsterite reacts with the remaining melt to form other minerals like pyroxenes and amphiboles, consuming silica in the process. Quartz, however, crystallizes at the very end of the series, from a silica-rich residual melt.
The key incompatibility lies in the differing silica requirements for their formation. Forsterite formation *consumes* silica, while quartz formation *requires* an abundance of silica. A magma cannot simultaneously provide the conditions necessary for both minerals to be stable.
Phase Rule and Thermodynamic Constraints
The phase rule (F = C - P + 2) governs the number of phases (F) that can coexist in equilibrium in a system with C components and P external variables (typically pressure and temperature). In a simple system like the SiO2-MgO system, the coexistence of forsterite and quartz would require specific temperature and pressure conditions. However, natural magmas are far more complex, containing numerous components (Na, K, Ca, Fe, Al, etc.). This increases the dimensionality of the phase diagram and significantly reduces the stability field where both minerals can coexist.
Furthermore, the formation of forsterite typically occurs in a reducing environment (low oxygen fugacity), while quartz formation is less sensitive to redox conditions. These differing environmental requirements further hinder their simultaneous stability.
Magmatic Differentiation and Fractional Crystallization
Magmatic differentiation, including fractional crystallization, plays a crucial role. As a magma cools and minerals crystallize, the remaining melt changes in composition. If forsterite crystallizes early, it removes magnesium and silica from the melt, driving the remaining magma towards a more silica-rich composition. This process eventually favors the crystallization of quartz. Conversely, if a magma is initially silica-rich and quartz crystallizes, it depletes the melt of silica, making forsterite formation less likely. The two minerals represent opposite ends of a differentiation trend.
Examples from Igneous Rocks
In nature, we rarely find igneous rocks containing both forsterite and quartz in stable equilibrium. Ultramafic rocks, like peridotites, are rich in forsterite but contain virtually no quartz. Conversely, felsic rocks like granites are rich in quartz but contain little to no forsterite. Rocks like diorites and gabbros may contain olivine (forsterite-rich) and quartz, but these are typically found as xenocrysts (crystals incorporated from another source) or represent disequilibrium textures resulting from rapid cooling or magma mixing, rather than stable coexistence.
Conclusion
In conclusion, the stable coexistence of primary forsterite and quartz in an igneous rock is thermodynamically improbable due to their contrasting chemical compositions, differing crystallization conditions dictated by Bowen’s Reaction Series, and the constraints imposed by the phase rule in complex magmatic systems. Magmatic differentiation processes further reinforce this incompatibility, driving magmas towards either a forsterite-rich or a quartz-rich composition, but rarely both simultaneously. The absence of these minerals in stable equilibrium is a fundamental principle in understanding igneous petrology and magma evolution.
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.