Onyx is a mineral known for its distinct parallel bands of color. Its formation involves slow, meticulous geological processes that unfold over vast stretches of time. Its creation involves an intricate interplay of chemistry, water, and specific environmental conditions within Earth’s crust. This article explores the geological pathways that create both silica-based and calcite-based onyx.
Defining Onyx
Onyx is characterized by its layered structure and parallel bands. Geologically, “onyx” refers to two chemically distinct mineral compositions. One form is a variety of chalcedony, which is microcrystalline quartz, composed primarily of silicon dioxide (SiO2). The other type, often called “calcite onyx” or “Mexican onyx,” consists mainly of calcium carbonate (CaCO3). Despite their differing chemical makeups, both share the defining visual characteristic of distinct, parallel banding, which arises from their depositional environments.
Formation of Silica-Based Onyx
Silica-based onyx, a form of chalcedony, commonly develops within gas cavities (vesicles) found in volcanic rocks like basalt or rhyolite. After eruptions, these rocks cool, trapping gas bubbles that later become open spaces. Over long periods, silica-rich water, percolating through the surrounding rock, seeps into these vacant cavities. As the water slowly evaporates or its silica concentration reaches saturation, microscopic crystals of silica begin to precipitate and accumulate on the cavity walls.
This deposition occurs in successive layers, with each layer representing a period of silica accumulation. The characteristic parallel banding of onyx results from subtle changes in the silica solution’s composition, temperature, or the presence of trace impurities during different depositional phases. For instance, variations in iron oxides can introduce reddish or brownish hues, while carbonaceous material might create black bands. The continuous, even flow of silica-rich fluids and the stable environmental conditions within these sealed cavities are crucial for forming the uniform, parallel layers seen in silica-based onyx.
Formation of Calcite-Based Onyx
Calcite-based onyx is primarily composed of calcium carbonate. This variety forms in environments where calcium-rich water interacts with the atmosphere, leading to the precipitation of calcite. One common setting for its formation is within limestone caves, where it contributes to speleothems such as stalactites, stalagmites, and flowstone. As groundwater saturated with dissolved calcium carbonate flows through the cave, exposure to the atmosphere causes carbon dioxide to degas from the water, reducing the water’s acidity and leading to calcite precipitation.
Another significant environment for calcite onyx formation is around hot springs, forming travertine deposits. Here, geothermal waters, rich in dissolved calcium carbonate, emerge at the surface. As the hot water cools and experiences a sudden drop in pressure, carbon dioxide escapes, and calcium carbonate precipitates rapidly. In both cave and hot spring environments, the parallel banding of calcite onyx arises from sequential layers of calcite deposition. These layers often vary in color and translucency due to slight changes in the water’s mineral content, flow rates, or the presence of impurities like iron oxides or organic matter during the depositional process.
Characteristics Resulting from Formation
The defining characteristic of both silica-based and calcite-based onyx is their distinctive parallel banding, a direct consequence of their layered depositional processes. Each band represents a distinct episode of mineral precipitation, influenced by subtle shifts in the chemical environment or the presence of impurities during its formation.
The varied colors observed in onyx—ranging from white and black to brown, red, and green—are also a direct result of these formation conditions. Trace elements and mineral impurities incorporated into the crystalline structure during layering impart specific hues. For example, iron oxides can produce warm reddish-brown tones, while manganese oxides might lead to darker shades.