quartz - cavity filling - hand/display specimen of a quartz filled geode from Morocco
Geodes form when a vesicular basalt flow is permeated by silica-rich hydrothermal solutions. In the solidified flow, quartz crystallizes into any available space, typically into large gas bubbles frozen into the lava. Formation of crystal-lined cavities can take thousands of years.
The basaltic rind eventually weathers and erodes and the mineral-filled cavities weather out. When broken open, if lucky, they reveal attractive crystal linings like these.
Banded agate-filled geodes from Brazil are often cut into slices and polished, with agate completely filling the cavity.
These open-centered crystal-filled examples are some of the best that we have seen from Morocco. As with many Moroccan minerals, the precise locality information is lost, as these passed from the hands of the miners to middlemen with minimal information, if any. We were given the Marrakech Region of Morocco as an uncertain locality.
This example has inclusions of what appear to be brown goethite or hematite near the surface of the quartz crystals, an example of a change in the chemistry of the geothermal fluids.
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