3.4" Calcite Crystal on Mordenite - India

About Calcite Crystals

Calcite crystals are a form of calcium carbonate (CaCO₃) known for their diverse shapes, transparency, and vibrant range of colors. They typically form in rhombohedral, scalenohedral, or prismatic shapes, often with well-defined, sharp edges and glossy surfaces. Calcite crystals are often translucent or transparent, sometimes displaying a double refraction effect where objects viewed through the crystal appear doubled. They can appear in various colors—white, clear, yellow, pink, blue, green, and orange—depending on impurities or trace minerals.

A notable characteristic of calcite is its reaction with weak acids like vinegar, which causes it to effervesce, or fizz, as it releases carbon dioxide. This property makes calcite crystals a key tool in geological identification and studies. Calcite forms in many environments, from sedimentary rocks like limestone and marble to hydrothermal veins.

This specimen comes from the Deccan Traps, a large igneous province and one of the largest volcanic features on Earth, in the Pune Province of western India. In the course of quarrying for other materials or digging wells, sometimes large pockets of zeolites and other minerals are unearthed in the volcanic rock.

Apophyllite, while not a zeolite itself, is almost always found associated with zeolites in the same pockets. It has two crystal habits: a rectangular prism capped by a steep four-sided pyramid, or a pseudo-cubic structure. While not a well-known mineral to the general public, it is popular among mineral collectors due to its pastel colors and beautiful crystal formations.