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Definition of geode structure

Geodes are mineralogical structures which occur in certain sedimentary and volcanic rocks. They are essentially hollow, spherical to oblate masses of mineral matter that may form from either the filling of vesicles (gas bubbles) in volcanic to sub-volcanic rocks by minerals deposited from hydrothermal fluids, or by the post - depositional dissolution of sedimentary nodules or concretions (that were deposited syngenetically within the rock formations they are found in) and partial filling by the same or other minerals precipitated from diagenetic or hydrothermal fluids.

Geodes differ from vugs in that the former were formed as early, rounded, structures within the surrounding rock, whereas vugs are irregular voids or cavities within a cross-cutting formation, usually a vein or breccia. Geodes also differ from "nodules" in that a nodule is a mass of mineral matter that has accreted around the nodule nucleus. Both structures had the minerals contained within, deposited from groundwater or hydrothermal processes.

Geodes commonly have a chalcedony (cryptocrystalline quartz) shell lined internally by various minerals, often as crystals, particularly calcite, pyrite, kaolinite, sphalerite, millerite, barite, dolomite, limonite, smithsonite and quartz, which is by far the most common and abundant mineral found in geodes. Geodes are found mostly in basaltic lavas and limestones.


 
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