Latite Identification Guide

What Latite Looks Like

Latite is a fine-grained, intermediate volcanic rock, essentially the extrusive equivalent of monzonite. It is typically gray, brownish-gray, or pinkish-gray, often with a slightly mottled or speckled appearance. It commonly shows a porphyritic texture: visible phenocrysts of feldspar (both alkali feldspar and plagioclase in roughly equal amounts) and sometimes dark biotite, hornblende, or augite set in a dense, aphanitic groundmass. Luster is dull to slightly waxy on fresh breaks. The rock is opaque and lacks the glassy sheen of obsidian or the gas holes typical of frothy lavas (though some latites are vesicular).

Step-by-Step Field ID Checklist

1. Confirm it is volcanic (fine-grained). The groundmass should be too fine to resolve individual crystals with the naked eye, unlike granite or monzonite.
2. Look for phenocrysts. Latite usually carries blocky feldspar crystals; a hand lens helps reveal both white/gray plagioclase (often striated) and pinkish alkali feldspar.
3. Check the color index. Latite is intermediate, so it should be medium gray, not the near-black of basalt nor the pale cream of rhyolite.
4. Test hardness. The feldspar-dominated rock will not be scratched by a steel knife on its hard grains (Mohs ~6).
5. Note the absence of quartz. Unlike dacite or rhyolite, latite is essentially quartz-poor; you should not see glassy gray quartz eyes.

Key Diagnostic Tests

• Hardness: Bulk hardness ~6 from feldspars; quartz-free so no abundant 7-hardness grains.
• Streak: Gray to whitish, not useful diagnostically; rely on texture instead.
• Fracture: Breaks with a rough, somewhat blocky to subconchoidal fracture; no cleavage at the rock scale.
• Acid: No reaction to dilute HCl (silicate, not carbonate). Any fizz means calcite cement or alteration, not latite.
• Density: Moderate (~2.6-2.7 g/cm3), lighter in the hand than basalt.

Common Look-Alikes and How to Tell Them Apart

• Andesite: Very similar gray porphyritic look, but andesite is plagioclase-dominated with little alkali feldspar. Distinguishing the two reliably often needs thin-section or chemistry; latite trends pinker and more alkali-rich.
• Trachyte: Trachyte is alkali-feldspar dominated and usually paler; latite has roughly equal plagioclase, making it look more salt-and-pepper.
• Dacite/Rhyolite: Both contain visible quartz and are paler; latite lacks quartz eyes.
• Basalt: Darker, denser, plagioclase-and-pyroxene rich; latite is lighter in color and weight.

Where Latite Is Typically Found

Latite occurs in continental volcanic arcs and rift settings as lava flows, dikes, and dome rocks. Classic occurrences include the Highwood Mountains and other Montana volcanics, the western United States Basin and Range, and parts of the Andes and Mediterranean volcanic provinces. Look for it among intermediate lava sequences interbedded with andesite and trachyte.