This is a sandstone containing 25% or more of feldspars, usually derived from silicic igneous rocks.

This is a sandstone containing 25% or more of feldspars, usually derived from silicic igneous rocks.

• A. Quartz sandstone
• B. Arkose
• C. Greywacke
• D. Siltstone

Answer: (B)

Feldspar-rich composition in sandstones signals a type called arkose, where a substantial portion of the framework is feldspar—commonly 25% or more. This high feldspar content points to a source rich in silicic igneous rocks, like granites, and also to relatively rapid erosion with limited chemical weathering, so the sediment remains immature. Arkose typically has coarse to medium sand grains that are often angular to subangular and poorly sorted, reflecting the short transport distance from a nearby granitic source. This helps distinguish it from quartz sandstone, which is dominated by quartz and indicates a very mature, well-weathered, long-transport sediment with little feldspar. Greywacke, on the other hand, contains more clay matrix and abundant rock fragments, with less emphasis on feldspar, and tends to be poorly sorted and darker. Siltstone is finer than sandstone, consisting mainly of silt-sized particles, so it isn’t the proper category for a feldspar-rich sandstone.

Feldspar-rich composition in sandstones signals a type called arkose, where a substantial portion of the framework is feldspar—commonly 25% or more. This high feldspar content points to a source rich in silicic igneous rocks, like granites, and also to relatively rapid erosion with limited chemical weathering, so the sediment remains immature. Arkose typically has coarse to medium sand grains that are often angular to subangular and poorly sorted, reflecting the short transport distance from a nearby granitic source.

This helps distinguish it from quartz sandstone, which is dominated by quartz and indicates a very mature, well-weathered, long-transport sediment with little feldspar. Greywacke, on the other hand, contains more clay matrix and abundant rock fragments, with less emphasis on feldspar, and tends to be poorly sorted and darker. Siltstone is finer than sandstone, consisting mainly of silt-sized particles, so it isn’t the proper category for a feldspar-rich sandstone.