Comendite Identification Guide

How to recognize comendite, a peralkaline rhyolite, by its pale color, sparse sodic minerals, and silica-rich glassy or flow-banded groundmass in the field.

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What Comendite Looks Like

Comendite is a peralkaline rhyolite (a volcanic, fine-grained felsic rock) defined chemically rather than by a single appearance. In hand sample it is typically pale: light gray, greenish-gray, tan, pinkish, or cream. The groundmass is aphanitic (too fine to see grains) and often glassy, silky, or faintly flow-banded. Luster ranges from dull to slightly waxy. It is opaque. Where phenocrysts occur they are sparse and small: clear-to-glassy alkali feldspar (sanidine/anorthoclase), smoky quartz, and thin dark needles or specks of sodic amphibole (arfvedsonite) or sodic pyroxene (aegirine), which give it a subtle peralkaline signature.

Step-by-Step Field ID Checklist

Confirm it is volcanic and felsic. Look for a very fine, light-colored groundmass, possibly with flow banding, spherulites, or a glassy sheen.
Scan for sparse phenocrysts. A few small feldspar and quartz crystals in a pale matrix point to rhyolite family.
Hunt for sodic mafics. Tiny dark-green to black needles or specks of aegirine/arfvedsonite suggest a peralkaline (comendite/pantellerite) rock rather than ordinary rhyolite.
Assess overall darkness. Comendite is more silicic and generally paler/less iron-rich than its sister pantellerite, which tends to be darker green-brown.
Note the setting. Comendite forms in within-plate and rift volcanic provinces, oceanic islands, and continental rift zones.

Key Diagnostic Tests

Hardness: Bulk rock scratches glass (~6.5–7) because of high quartz/feldspar; quartz phenocrysts reach 7.
Streak: White to pale gray.
Fracture: Conchoidal to splintery on glassy varieties; no mineral cleavage at rock scale.
Magnetism: Generally weak to none (low iron oxides).
Acid: No reaction to dilute HCl (no carbonate).
Density: Low to moderate (~2.4–2.6 g/cm³), lighter in pumiceous varieties.
Lab note: True confirmation is chemical (peralkaline index, molar Na+K > Al), so field ID is provisional.

Common Look-Alikes and How to Tell Them Apart

Ordinary rhyolite: Visually near-identical; comendite differs by its sodic amphibole/pyroxene and peralkaline chemistry. Without the dark needles, lab analysis is needed.
Pantellerite: The other peralkaline lava; darker, greener-brown, more iron-rich and less silicic. Comendite is paler and more quartz-rich.
Dacite/felsite: Dacite is usually more crystal-rich and slightly darker; lacks sodic mafics.
Obsidian: Fully glassy and dark; comendite is typically devitrified, paler, and may carry visible phenocrysts.
Chert: Sedimentary, waxier, lacks any phenocrysts or flow banding.

Where It Is Typically Found

Comendite occurs in peralkaline volcanic provinces: the East African Rift, Sardinia (Comende/San Pietro, the type area), Atlantic and Pacific oceanic islands, Ascension, and continental rift settings worldwide. Look in ash-flow tuffs, lava domes, and flows associated with alkaline magmatism.

Frequently asked questions

How can you tell if it's real comendite?

A confident answer is chemical: comendite is a peralkaline rhyolite with molar (Na+K) exceeding Al and relatively high silica. In the field, a pale, fine-grained, quartz- and feldspar-bearing volcanic rock carrying tiny sodic amphibole (arfvedsonite) or pyroxene (aegirine) needles is a strong indicator, but lab analysis confirms it.

What is the difference between comendite and pantellerite?

Both are peralkaline lavas. Comendite is more silica-rich, paler, and more quartz-bearing, while pantellerite is darker, greener-brown, more iron-rich, and less silicic. The distinction is formally drawn on chemical (alumina, iron, silica) grounds.

Is comendite the same as rhyolite?

It is a special variety of rhyolite. All comendite is rhyolite, but only peralkaline rhyolite with the right sodium-potassium-to-aluminum ratio and sodic mafic minerals qualifies as comendite.

Where is comendite found?

In peralkaline volcanic provinces such as the East African Rift, Sardinia (its type locality), and various oceanic islands and continental rift zones, occurring as lava flows, domes, and ash-flow tuffs.