Arkose
Feldspar-rich sandstone (quartz + >25% feldspar)
A coarse, feldspar-rich sandstone, often pink, that records rapid erosion of granitic source rock under arid conditions.
- Mohs hardness
- 6-7
- Color
- Pink, reddish, tan, gray
- Type
- sedimentary
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Overview
Arkose is a type of sandstone that contains a high proportion of feldspar (by definition at least 25%) alongside quartz. The abundant feldspar, especially pink potassium feldspar, often gives arkose a characteristic pink to reddish color.
It is typically coarse-grained and poorly to moderately sorted, with angular grains, reflecting rapid deposition close to its source. Arkose looks almost like a sedimentary version of granite, because it is essentially eroded granite that has not been chemically broken down.
The survival of feldspar is significant: feldspar weathers readily to clay, so abundant fresh feldspar implies fast erosion, short transport, and/or a dry or cold climate.
Formation & geology
Arkose forms from the rapid mechanical breakdown and deposition of granitic or gneissic source rocks before the feldspar has time to weather chemically into clay.
This requires conditions that limit chemical weathering: arid or semi-arid climates, cold climates, steep relief with rapid erosion, or short transport distances such as alluvial fans flanking uplifted granite mountains.
The feldspar-rich sand is then buried and cemented (commonly by silica, calcite, or iron oxide) into rock. Arkose often occurs as "granite wash" near basement highs and in rift basins where mountains shed coarse debris quickly into adjacent lows.
How to identify it
Look for a coarse, often pinkish sandstone with abundant visible feldspar grains mixed with glassy quartz. The pink color and granular, granite-like appearance are strong clues.
Feldspar grains are typically blocky and may show cleavage flashes; quartz grains are glassy and lack cleavage. Hardness is high (~6-7) because both quartz and feldspar are hard. It does not fizz in acid unless calcite-cemented.
Look-alikes: Ordinary (quartz) sandstone has little feldspar and is often paler; granite is an interlocking igneous crystalline rock, not made of separate rounded/angular sand grains; greywacke is darker, muddier, and lithic-rich.
Uses & significance
Arkose is used as building and paving stone and as crushed aggregate. Its warm pink and red tones have made it a popular dimension stone for walls, flagstones, and decorative masonry in many regions.
Geologically, arkose is a valuable provenance indicator: its abundant feldspar tells geologists about the source rock (granitic basement), the climate (arid or cold, limiting weathering), and the tectonic setting (rapid uplift and erosion, often in rift basins).
It is not a gemstone or carving material, but well-cemented arkose is a durable and attractive construction stone.
Frequently asked questions
What makes a sandstone an arkose?
An arkose is a sandstone containing at least about 25% feldspar in addition to quartz, often giving it a pink or reddish color.
Why is arkose often pink?
Its abundant potassium feldspar grains are commonly pink, and iron oxide cement can add red tones, giving arkose its characteristic color.
What does arkose tell geologists?
Abundant fresh feldspar means rapid erosion of granitic rock with little chemical weathering, indicating arid or cold climates and high relief near the source.
How is arkose different from regular sandstone?
Regular sandstone is dominated by quartz with little feldspar, while arkose is feldspar-rich, coarser, and looks much like eroded granite.
Arkose guides
In-depth guides for identifying, valuing, and understanding Arkose.
Arkose identified by the community
Real specimens identified with Rock Identifier.
