Arkose Identification Guide

What Arkose Looks Like

Arkose is a coarse- to medium-grained sandstone distinguished by a high content of feldspar (by definition at least 25% of the framework grains), alongside quartz and rock fragments. It typically has a pink, reddish, or pale gray color, the pink coming from potassium feldspar (orthoclase/microcline). Grains are usually angular to subangular, reflecting short transport from a granitic source, and the rock often looks like "crumbled granite." Luster is dull to slightly glassy on quartz grains; the rock is opaque and granular overall.

Step-by-Step Field ID Checklist

1. Confirm it is a sandstone — sand-sized grains (0.06–2 mm) cemented together, gritty to the touch, you can usually see individual grains with a hand lens.
2. Look for feldspar. Search for blocky, opaque, often pink or white grains that show flat reflective cleavage faces when you tilt the rock to light. Quartz grains are glassy and lack cleavage.
3. Estimate the feldspar percentage. If feldspar makes up roughly a quarter or more of the visible grains, it qualifies as arkose.
4. Check grain angularity. Angular grains and a generally immature, poorly sorted texture support a nearby granitic source.
5. Note the color — pink/red tints favor arkose over clean quartz sandstone.

Key Diagnostic Tests

• Hardness: Mixed. Quartz grains are 7; feldspar grains are ~6. A steel knife will not scratch quartz grains but the cement may crumble.
• Cleavage vs. fracture: The single best test. Feldspar grains have two good cleavages producing flat, mirror-like flashes; quartz fractures conchoidally with no flat faces. Seeing abundant flat cleavage flashes = feldspar = arkose.
• Acid test: A drop of dilute HCl tells you the cement type. Fizzing means calcite cement; no reaction suggests silica or iron-oxide cement. Arkose is silicate, so the grains themselves do not effervesce.
• Streak: Not diagnostic; the rock is a mixture.

Common Look-Alikes and How to Tell Them Apart

• Quartz sandstone / quartz arenite: Lacks feldspar; grains are uniformly glassy and well-rounded, usually buff or white. Tilt to light — no cleavage flashes means it is not arkose.
• Granite: Granite is a crystalline igneous rock with interlocking, not cemented, crystals; it shows no sedimentary bedding and grains do not separate as loose sand. Arkose is derived from granite but is clastic and often bedded.
• Greywacke (lithic sandstone): Darker, dirtier, with a clay-rich matrix and more rock fragments than feldspar.
• Feldspathic sandstone: Same family but with 10–25% feldspar — below the arkose threshold.

Where Arkose Is Found

Arkose forms where granite or gneiss weathers rapidly and is buried before feldspar can decompose to clay — typically in arid climates, rapid uplift settings, alluvial fans, and rift basins close to a crystalline source. Classic occurrences include the Triassic redbeds of the eastern U.S. (Connecticut Valley), the Fountain Formation flanking the Colorado Rockies (Red Rocks, Flatirons), and many fault-bounded basins worldwide.