Appinite Identification Guide

What Appinite Looks Like

Appinite is a field term for a group of dark, coarse intrusive (plutonic) igneous rocks characterized by large, prominent crystals of hornblende (amphibole) set in a groundmass of plagioclase feldspar. The defining look is big, black to dark-green, shiny hornblende prisms, sometimes needle- or lath-shaped, standing out against a lighter gray to white feldspar background. The texture is often porphyritic, giving a strongly speckled or spotted appearance.

• Color: overall dark gray to greenish-black, salt-and-pepper
• Texture: coarse, often porphyritic with conspicuous hornblende phenocrysts
• Hornblende: black, prismatic, glassy, sometimes radiating
• Feldspar: white to gray plagioclase groundmass

Step-by-Step Field ID Checklist

1. Confirm it is a coarse intrusive rock: interlocking visible crystals, not glassy or fine-grained.
2. Identify the dark mineral as hornblende: look for shiny black prisms with two cleavages meeting at about 56 and 124 degrees (amphibole cleavage), not the near-90-degree cleavage of pyroxene.
3. Check the crystal size contrast: large hornblende crystals in a finer feldspar matrix (porphyritic) is characteristic of appinite.
4. Estimate dark/light ratio: appinite is mafic-rich but still has clear feldspar; abundant hornblende dominates.
5. Look at the geologic setting: appinites occur as small intrusions, dikes, and pipes associated with granitic complexes.

Key Diagnostic Tests

• Mohs hardness: hornblende about 5 to 6, feldspar about 6; the rock scratches glass
• Streak: white to pale gray (hornblende streak is gray-green to white)
• Cleavage: hornblende shows two cleavages at ~56/124 degrees (the amphibole signature); plagioclase shows twinning striations
• Fracture: rock breaks across interlocking grains
• Specific gravity: moderately high, roughly 2.8 to 3.0, due to abundant amphibole
• Acid/magnetism: no acid reaction; generally non-magnetic

Common Look-Alikes and How to Tell Them Apart

• Diorite: also a hornblende-plagioclase intrusive, but appinite is distinguished by its conspicuously large, often vertically prominent hornblende crystals and porphyritic habit; appinite is essentially a hornblende-rich, large-crystal relative.
• Gabbro: dominated by pyroxene rather than hornblende; pyroxene cleavage is near 90 degrees, and gabbro is typically darker and more even-grained.
• Lamprophyre: also dark and hornblende- or biotite-rich and porphyritic, but lamprophyres lack feldspar phenocrysts (feldspar is confined to the groundmass); the two grade into each other and require care.
• Amphibolite: a metamorphic rock with foliated/aligned hornblende, whereas appinite is igneous with randomly oriented crystals.

Where Appinite Is Typically Found

Appinite is named for Appin, in the Scottish Highlands, its type area. It occurs in the Caledonian intrusions of Scotland and Ireland, and similar appinitic rocks are found in many orogenic granite belts worldwide, typically as small plugs, dikes, and marginal facies around larger granitoid plutons.