Adinole Identification Guide

What Adinole Looks Like

Adinole is a dense, very fine-grained metasomatic rock formed when sodium-rich fluids alter shale or slate at the contact with a dolerite (diabase) intrusion. It is essentially an albite-rich (sodium feldspar) hornfels.

• Color: typically pale gray, grayish-white, cream, or pinkish; sometimes greenish or banded with darker layers inherited from the original sediment.
• Luster: dull to slightly waxy or flinty on fresh fracture.
• Texture: aphanitic (grains too small to see); compact and homogeneous, sometimes preserving faint sedimentary banding.
• Form: occurs as zones a few centimeters to meters thick along the margins of basic intrusions in slate/shale country rock.

Step-by-Step Field ID Checklist

1. Find the geologic setting. Adinole only forms at the contact of a dolerite/diabase dike or sill with fine-grained sediments. Setting is half the identification.
2. Look for hardness and toughness. It is hard and tough, breaking with a splintery to conchoidal fracture.
3. Check grain size. Featureless and aphanitic — you cannot see individual crystals with a hand lens.
4. Note color and banding. Pale, flinty, often with relict bedding.
5. Test with acid. No fizz (it is silicate/feldspathic, not carbonate).
6. Look at the contact zone. Adinole grades into unaltered slate away from the intrusion.

Key Diagnostic Tests

• Hardness: about 6–7 overall (albite is ~6, plus quartz); scratches glass with difficulty.
• Fracture: splintery to conchoidal, flint-like.
• Acid: no reaction.
• Magnetism: none significant.
• Density: moderate (~2.6–2.7).
• Composition clue: dominated by albite; thin section or chemical test confirms the high sodium content that defines it.

Common Look-Alikes and How to Tell Them Apart

• Chert/flint: also flinty and aphanitic, but chert is essentially pure quartz (H 7, no feldspar) and is not tied to an igneous contact. Adinole's albite content and contact setting distinguish it.
• Hornfels (ordinary): adinole is a specific sodium-metasomatized hornfels; generic hornfels lacks the albite enrichment and may show spotting (cordierite/andalusite porphyroblasts) rather than uniform pale albite.
• Quartzite: quartzite is recrystallized sandstone, granular under a lens and not associated with a dike margin.
• Felsite/rhyolite: these are igneous; adinole is metasomatized sediment and usually shows relict bedding and a clear sedimentary protolith nearby.

Where Adinole Is Found

Adinole is found wherever dolerite/diabase intrudes pelitic sediments and sodic fluids drive contact metasomatism. The classic localities are in the Harz Mountains (Germany), where the rock and the associated "spilite-keratophyre" reactions were first described, and in Cornwall and the Welsh Borderland (UK). Look for it as bleached, hardened selvages along the chilled margins of basic dikes and sills cutting slate.