Tinguaite Identification Guide

What Tinguaite Looks Like

Tinguaite is a fine-grained, dike-forming variety of phonolite — an alkaline, silica-undersaturated igneous rock. It is famous for its distinctive coloration.

• Color: characteristic dull grass-green to gray-green, sometimes greenish-gray, owing to aegirine and feldspathoids
• Luster: dull to slightly greasy on fresh surfaces
• Transparency: opaque
• Form: fine-grained (aphanitic) groundmass, commonly porphyritic with small phenocrysts of alkali feldspar and nepheline; occurs in dikes and sills

Step-by-Step Field ID Checklist

1. Note the overall green tint — unusual for a fine-grained igneous rock and a strong first clue.
2. Look with a lens for tiny needle-like green aegirine crystals threaded through the groundmass (the 'tinguaitic texture').
3. Identify small light phenocrysts of feldspar and glassy nepheline set in the fine matrix.
4. Confirm it is an intrusive/dike rock, often cutting alkaline complexes.
5. Tap it — phonolites tend to ring with a clinking sound when struck.

Key Diagnostic Tests

• Hardness: ~5.5–6 overall (feldspar/feldspathoid dominated).
• Acid test: no reaction (not a carbonate).
• Texture: the felted mat of slender green aegirine microlites is diagnostic under a hand lens or microscope.
• Density: moderate (~2.6).
• Mineralogy: nepheline + alkali feldspar + aegirine, no quartz (silica-undersaturated).

Common Look-Alikes

• Phonolite (extrusive): same chemistry but typically gray and lava-flow textured; tinguaite is the green, dike-facies version with aegirine needles.
• Greenstone / metabasalt: green from chlorite/actinolite, but these are metamorphosed basalts lacking nepheline phenocrysts and the clink test.
• Trachyte: lighter, contains quartz or is silica-saturated and lacks feldspathoids and the green aegirine.
• Aplite / fine granite: pale and quartz-bearing, never grass-green.

The combination of green color, aegirine microlites, nepheline, and a dike setting separates tinguaite from these.

Where It Is Found

The type locality is the Tinguá massif, Rio de Janeiro, Brazil. It also occurs in alkaline igneous provinces such as the Kola Peninsula (Russia), the Magnet Cove complex (Arkansas, USA), Portugal, and other nepheline-syenite associated intrusions.