Shonkinite Identification Guide

What Shonkinite Looks Like

Shonkinite is an uncommon, dark (melanocratic) plutonic igneous rock — essentially a mafic relative of syenite. It is dominated by black, stubby pyroxene (augite) crystals set with potassium feldspar (orthoclase), and it often carries a feldspathoid such as nepheline plus biotite, olivine, and accessory magnetite.

• Color: dark gray to black, sometimes greenish-black
• Texture: phaneritic (visibly crystalline) and often porphyritic, with chunky pyroxene crystals; can look almost like a coarse gabbro
• Luster: dull to glassy on individual mineral faces
• Feel/weight: dense and heavy for its size

Step-by-Step Field-ID Checklist

1. Confirm it is coarse-grained. You should see interlocking crystals with the naked eye or a hand lens — this rules out lava (basalt).
2. Estimate the dark-mineral percentage. Shonkinite is mineral-rich in dark pyroxene (typically 60%+ mafics), much darker than ordinary syenite.
3. Identify the augite. Stubby black prisms with two cleavages meeting near 90 degrees indicate pyroxene, not amphibole.
4. Find the feldspar. Pale orthoclase fills between the dark grains; if you can detect nepheline (greasy luster, no quartz present), that confirms an alkaline, silica-undersaturated rock.
5. Confirm no quartz. Shonkinite is silica-undersaturated; glassy quartz grains should be absent.

Key Diagnostic Tests

• Mohs hardness: mixed — pyroxene ~5.5–6, feldspar ~6; the rock scratches glass overall.
• Density: high (~2.9–3.1) owing to abundant pyroxene and magnetite.
• Magnetism: often weakly magnetic from accessory magnetite — a small magnet may drag.
• Acid: no fizz with dilute HCl (silicate rock, not carbonate).
• Cleavage clue: pyroxene cleavage angles near 90 degrees distinguish augite from amphibole (~120 degrees).

Common Look-Alikes and How to Tell Them Apart

• Gabbro: gabbro is dominated by calcium plagioclase plus pyroxene and lacks alkali feldspar and feldspathoids; shonkinite's pale feldspar is potassium-rich orthoclase, and nepheline may be present.
• Pyroxenite: an ultramafic rock made almost entirely of pyroxene with little feldspar — shonkinite always has notable feldspar.
• Syenite: ordinary syenite is light-colored and feldspar-dominated; shonkinite is the dark, mafic-rich end member.
• Basalt: basalt is fine-grained (aphanitic) lava; if you cannot see individual crystals, it is not shonkinite.
• Lamprophyre/minette: these dark dike rocks are typically porphyritic with mica or amphibole phenocrysts in a fine groundmass, not the coarse, even plutonic texture of shonkinite.

The defining combination is dark, coarse, augite-rich + potassium feldspar + possible nepheline + no quartz.

Where Shonkinite Is Found

Shonkinite is named for the Shonkin Sag laccolith in the Highwood Mountains of central Montana, USA, its classic locality. It occurs in alkaline intrusive complexes worldwide, typically as sills, dikes, and the dark lower portions of differentiated laccoliths, associated with syenites and other alkaline rocks. Search alkaline igneous provinces and the chilled, mafic margins of layered intrusions.