Final answer:
Minerals grow as euhedral crystals in conditions that involve slow cooling, space availability, and appropriate geothermal gradients, usually deep below the Earth's surface where the crystals have time and space to develop uninterrupted, defined structures.
Step-by-step explanation:
The geologic conditions that allow minerals to grow as euhedral crystals include a combination of factors such as slow cooling, the availability of space, and a suitable geothermal gradient. Specifically, euhedral crystal growth is favorable when molten rock intrudes into other rocks below the Earth's surface and cools gradually and slowly. This slow cooling process provides the necessary time for crystal lattices to form uninterrupted, leading to the development of well-defined, straight and flat crystal edges that are characteristic of euhedral crystals. The presence of sufficient space allows individual crystals to grow large without being constrained by other crystals, contributing to their well-formed nature. Additionally, the compositional context, like that in a terrane undergoing metamorphism at medium-pressure or low-pressure conditions, influences mineral growth and the presence of specific minerals such as kyanite, sillimanite, and andalusite.
Porphyritic textures in igneous rocks also demonstrate the distinction between different cooling rates, where larger crystals known as phenocrysts are embedded in a fine-grained matrix formed due to quicker cooling. The contrast between the coarse-grained texture of slowly cooled intrusive igneous rocks and the fine-grained texture of quickly cooled extrusive rocks further illustrates the importance of cooling rate in mineral crystal growth. Hence, understanding the geologic setting and the conditions present during rock formation is crucial for identifying and interpreting crystal growth patterns in minerals.