Adinole

Adinole is a fine-grained, dense metasomatic rock produced where argillaceous sediments (shale or slate) are chemically altered along the contact of a sodium-rich igneous intrusion, typically a diabase or spilite. The process, called albitization, floods the country rock with sodium, converting it into a hard rock rich in albite and quartz.

The rock is usually pale grey to greyish-white and very fine-grained, sometimes banded where it preserves original sedimentary layering. It is a textbook example of contact metasomatism, where chemistry as well as heat transforms the rock.

Adinole is mostly of scientific and academic interest rather than economic value.

Adinole forms by sodium metasomatism at the contact between a shale or slate and an intruding basic igneous body, classically the spilite-keratophyre association. Hot, sodium-bearing fluids derived from the cooling magma penetrate the adjacent sediment and replace its original minerals.

Clay minerals and other components are converted to albite (sodic plagioclase) plus quartz, producing a hard, fine-grained, often hornfels-like rock. This chemical addition of sodium, rather than simple thermal baking, distinguishes adinole from ordinary contact hornfels.

Classic occurrences are described from the Harz Mountains of Germany and from spilitic terrains in Cornwall and elsewhere.

Adinole is a very fine-grained, hard (Mohs ~6-7), dense rock, typically pale grey to greyish-white, occurring as a narrow altered zone against a diabase or spilite intrusion. It may preserve faint sedimentary banding and has a tough, splintery to flinty fracture.

Its streak is white and luster dull to slightly waxy. Because it is so fine-grained, mineral identification usually requires thin section, which shows abundant albite and quartz. Distinguish it from ordinary hornfels by its association with sodic intrusions and its high albite content, and from chert by its geologic setting and feldspar content.

Adinole has little economic or ornamental use. It is too fine-grained and unremarkable in appearance to serve as a gemstone or decorative stone, and it is not mined as a resource.

Its significance is almost entirely scientific: adinole is a classic illustration of contact metasomatism and sodium metasomatism, important in petrology for understanding fluid-rock interaction around igneous intrusions and the spilite-keratophyre problem.

Geologists study it to reconstruct the chemistry of magmatic fluids and the alteration history of igneous contacts.