Feldspathic Sandstone Identification Guide

What Feldspathic Sandstone Looks Like

Feldspathic sandstone — often called arkose when feldspar exceeds about 25% of the grains — is a clastic sedimentary rock made of sand-sized particles dominated by quartz and a high proportion of feldspar. The abundant feldspar gives it a characteristic pink, salmon, or reddish cast (from potassium feldspar) and a slightly "dirty" or grainy appearance compared with clean white quartz sandstone.

• Color: pink, salmon, reddish-brown, buff, or grey
• Texture: granular, sandy, with visible individual grains
• Luster: dull overall; tiny grains may glint
• Grains: angular to sub-angular (immature, short transport)

Step-by-Step Field ID Checklist

1. Confirm it is a sandstone — sand-sized grains (0.06–2 mm) visible with a hand lens, gritty to the touch.
2. Look for pink/cloudy grains among the glassy clear quartz; these are feldspar.
3. Scratch a single grain: feldspar grains stop at Mohs 6, quartz grains at 7 — use a hand lens to watch which grains scratch.
4. Check grain angularity — angular feldspar indicates short transport, supporting an arkosic origin.
5. Note the cement and color to gauge iron content.

Key Diagnostic Tests

• Hand-lens mineralogy: The defining test is identifying abundant feldspar (cloudy, pink, with flat cleavage flashes) mixed with clear quartz. If feldspar is conspicuous, it is feldspathic.
• Hardness: Overall the rock scratches glass because of its quartz/feldspar framework; loose grains test individually at 6–7.
• Acid: Usually inert; light fizzing only if carbonate cement is present.
• Grain shape: Angular grains and poor sorting point to arkose; well-rounded grains suggest a more mature, less feldspathic sandstone.

Common Look-Alikes and How to Tell Them Apart

• Quartz sandstone (quartz arenite): Cleaner, whiter, and made almost entirely of glassy quartz with little or no pink feldspar. Lack of feldspar grains is the key difference.
• Greywacke: Darker, grey to greenish, with a muddy matrix and abundant rock fragments rather than clean feldspar grains; arkose is paler and more feldspar-rich.
• Granite: A crystalline igneous rock with interlocking crystals, not loose cemented grains. Arkose has clastic, rounded-to-angular grains held by cement, often derived from weathered granite.
• Red siltstone/mudstone: Finer grained — grains not visible to the eye; arkose is gritty and sand-sized.

Where It Is Typically Found

Feldspathic sandstone forms where granitic or gneissic source rocks weather rapidly and the sediment is buried before feldspar can decompose — typically in arid climates, alluvial fans, and rift basins near uplifted basement rock. Classic examples include the Triassic arkoses of the Newark Basin (eastern USA), the Fountain Formation of Colorado (the red rocks of the Front Range), and many Old Red Sandstone units.