OceanJasper.org ~ Preservation Appreciation Research

Ocean Jasper ~ Spherulitic Chalcedony

Spherulite Tails in Ocean Jasper


A Whale with a Tail, Oh My!
Let me tell you a Tale
Of the Whale with a Tail
To let your mind Sail!

Whale's Tail: Two whimsically matched specimens (cut from the same rough) of classic old stock Ocean Jasper from Marovato region, Madagascar. The characteristic "spherulite tail" of overlapping spherulites in a wavy pattern occurs for many different OJ varieties, with varying spherulite size and complexity. *How does it occur?* In my model, Ocean Jasper is formed from a large-scale ferrous-silica solution. Spherulites form randomly in the fluid from a phase transition process as a result of density fluctuations and/or impurities in the supercooled (and soon supersaturated) solution. This process is called nucleation, which leads to a first order phase transition. The spherulite starts out as a microcrystalline quartz phase point, and over time experiences outward growth in a fibrous crystal habit of chalcedony quartz. In the large-scale pre-cursor fluid solution, these spherulite nucleation points can occur from random density fluctuations or impurities, which may be randomly distributed along a line ("tail") in the viscous supercooled, supersaturating fluid. Seemingly linked spherulites occur along the line, as they overlap in their outward growth from random nucleation sites, as seen in the pictured specimens at a macroscopic scale. The viscous supersaturating fluid is responsible for the wavy line, and supersaturation eventually leads to another (continuous) phase transition, that of gelation or formation of a gel-sol in the matrix surrounding the spherulites, which later hardens into an opaque or translucent chalcedony, or macrocrystalline quartz, depending on water content. The time scale of these transition and growth events is correlated, otherwise we would not see the heterogenous patterns and structures we see in Ocean Jasper. Spherulite banding and overall self-organization of macro patterns and color is predominantly driven by a process called reaction-diffusion which I describe in another post on Pattern Formation in Ocean Jasper and in the book.
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