A rock described as a well-sorted sandstone with less than 10% argillaceous matrix and relatively simple composition is called?

• A. Arkose
• B. Siltstone
• C. Arenite
• D. Wacke

Answer: (C)

Naming sandstones relies on two ideas: what the grain framework is made of (mostly quartz versus other minerals) and how much clay-rich matrix fills the spaces. A well-sorted sandstone with very little argillaceous matrix—less than about 10%—has a clean, simple mineral makeup dominated by quartz, so it is classified as an arenite. When the quartz content is especially high and the rock is essentially quartz with minimal other minerals, you’re looking at a quartz arenite, but the general term arenite captures this clean, low-matrix sandstone.

That simple, quartz-dominated, low-matrix description wouldn’t fit a feldspar-rich or lithic-rich rock (which would be arkose or litharenite, respectively). It also doesn’t fit a rock with noticeable matrix, as in a wacke, or a rock that’s finer than sandstone, like a siltstone.