Luxullianite Identification Guide

What Luxullianite Looks Like

Luxullianite is a striking and rare tourmaline-bearing granite, named for the village of Luxulyan in Cornwall, England. It is best known for radiating sprays of black tourmaline set in a matrix of pink orthoclase feldspar and quartz.

• Color: pink to flesh-colored feldspar groundmass with black tourmaline and gray quartz
• Luster: vitreous quartz and feldspar; black tourmaline has a vitreous to resinous luster
• Texture: porphyritic granite; large pink feldspar phenocrysts plus radiating acicular (needle/sun-burst) tourmaline clusters
• Key minerals: orthoclase (pink), quartz, schorl tourmaline (black), minor mica

Step-by-Step Field ID Checklist

1. Look for radiating black needles — sunburst sprays of acicular schorl tourmaline are the signature feature.
2. Note the pink feldspar matrix enclosing the tourmaline.
3. Confirm it is a granite — interlocking quartz, feldspar, and dark minerals, coarse-grained.
4. Check hardness — quartz and tourmaline scratch glass easily (7+).
5. Examine for a "shot-through" look where fine tourmaline replaces parts of the rock.

Key Diagnostic Tests

• Mohs hardness: quartz 7, tourmaline 7-7.5, feldspar 6 — all scratch glass.
• Cleavage: feldspar shows two good cleavages; tourmaline shows none (it has poor cleavage and breaks unevenly).
• Streak: tourmaline streak is white/colorless despite the black color.
• Density: typical granite range (~2.6-2.7), tourmaline slightly denser.
• Acid: no reaction.

Common Look-Alikes and How to Tell Them Apart

• Ordinary granite: lacks the radiating tourmaline sprays; luxullianite is defined by its acicular schorl.
• Tourmalinated quartz: dominated by clear/gray quartz with scattered tourmaline needles, not a pink feldspar granite matrix.
• Schorl in pegmatite: pegmatites are much coarser with blocky giant crystals rather than radiating fine sprays in granite.
• Black mica granite: the dark mineral is biotite (perfect single cleavage, peels in sheets), not prismatic tourmaline.

Where It Is Typically Found

The type and classic locality is Luxulyan, Cornwall, UK, within the Cornubian granite batholith. Similar tourmaline-replacement granites occur in other Cornish granite bodies and a few other tin-tourmaline provinces.