Amygdaloidal

Amygdaloidal is a texture in igneous rocks where vesicles formed by escaping gases during cooling are later filled with secondary minerals, creating small, rounded cavities known as amygdules. The name derives from the Greek amygdale, meaning almond, a reference to the shape of many fillings.

Formation and composition: After the lava solidifies, circulating hydrothermal fluids or groundwater deposit minerals into the open vesicles. Fillings are typically silica minerals such as chalcedony, quartz, or opal, but calcite, zeolites, and iron-oxide minerals are also common. The resulting rock presents a mosaic of filled and unfilled vesicles, which can weather to reveal the rounded amygdules on the surface.

Occurrence and significance: Amygdaloidal textures are especially associated with basaltic rocks, including basalt, andesite, and rhyolite, though they occur in a range of volcanic rocks. The texture records post-emplacement alteration and fluid activity that filled the original vesicles. In some cases, amygdaloidal basalts host decorative or semi-precious fillings such as agate or chalcedony, contributing to their economic and aesthetic interest.

Related terms: The term amygdaloid can refer to the rock possessing this texture, while amygdaloid or amygdaloidal basalt specifically denotes basaltic rocks with amygdaloidal filling. The texture is distinct from simple vesicular textures, which lack the mineral infill of the vesicles.