Deciphering the Origin of Plume-Textured Geodes
Year: 1990
Type: article
Abstract: Readily available plume-textured geodes from southern Brazil serve as excellent materials with which to challenge the observational and interpretive abilities of students in mineralogy. The most obvious puzzle is that major portions of these geodes appear to have grown outward rather than inward! We hereby present our interpretations of these fascinating geodes. Typical plume-textured geodes exhibit a convoluted outer surface confined within a streamlined envelope. Such geodes are composed of interlaminated chalcedonic and quartzine agate that exhibits plume-like growth patterns. The plumes occur in quartzine-rich zones containing sub-micron inclusions that scatter white light and transmit brown light. In places, the inclusions are arranged in regular fine bands that cause Bragg-diffraction of the spectrum. The surface morphology and growth pattern are attributed to the incomplete crystallization of a precursor layer of presumed silica gel lining a vesicle in lava. Although crystallization nucleated at th...
Source: Journal of Geological Education
Institution: Cal Poly Humboldt
Language: English
Subfield: Geochemistry and Petrology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: hybrid
OpenAlex ID: W89364335