Examining cordierite in thin section reveals a mineral with a unique textural and optical personality that makes it a frequent and diagnostic feature in metamorphic petrology. This magnesium aluminum silicate typically forms during medium to high-grade regional metamorphism of pelitic or psammitic sediments, and its presence in a thin section immediately indicates specific pressure-temperature conditions. Under the petrographic microscope, cordierite is instantly recognizable by its distinctive brown to yellow-brown color, low relief, and characteristic inclusions of sillimanite, quartz, and sometimes biotite or plagioclase. Its anisotropic nature, displayed through a shifting palette of interference colors, provides the critical clues needed to distinguish it from look-alike minerals like hypersthene or even altered glass.
Optical Properties and Identification
The optical behavior of cordierite in thin section is fundamental to its identification and is governed by its monoclinic crystal structure, although it often appears isotropic due to its low birefringence. When aligned with the stage compensator, it displays a low first-order gray to white interference color, which can be confused with other low birefringence minerals. A definitive test is its inclined extinction, typically between 30° and 45° relative to the cleavage direction, which is rarely seen in isotropic materials like quartz. The mineral is uniaxial negative, and while the 2V angle is usually small, the pleochroism is weak to distinct, shifting from a pale yellow-brown to a deeper brownish-orange when the stage is rotated 90 degrees.
Distinguishing Features from Similar Minerals
Differentiating cordierite from other common thin section constituents requires a systematic approach to avoid misidentification. Unlike hypersthene, which shares a similar brown color, cordierite exhibits lower relief and lacks the high surface relief and distinct cleavage of pyroxene. It is also softer than quartz, meaning its grain boundaries often appear slightly indented or sutured where it has been fractured. Furthermore, while it may resemble some forms of scapolite, cordierite does not exhibit the high relief or the distinct blue fluorescence sometimes seen in scapolite under a petrographic microscope, making careful comparison of optical properties essential.
Textural Relationships and Inclusions
The textural context of cordierite provides a window into the geological history of the rock, revealing the sequence of metamorphic events it has endured. It is common to find cordierite porphyroblasts that have grown porphyroblastically around earlier grains of biotite or plagioclase, creating a poikiloblastic texture that wraps inclusion minerals. Within these porphyroblasts, one frequently observes fine-grained inclusions of sillimanite aligned along the elongation axis, which is a classic mineralogical assemblage indicating high-grade metamorphism. The presence of quartz rims or intergranular films between cordierite and surrounding minerals further helps to define the equilibrium mineral assemblage and the mobility of elements during metamorphism.
