Metasandstone Identification Guide

What Metasandstone Looks Like

Metasandstone is sandstone that has begun to metamorphose but has not fully recrystallized into quartzite. It sits at the transition between sedimentary sandstone and metamorphic quartzite: the sand grains are partly welded, original bedding is often still visible, and new metamorphic minerals (sericite, chlorite, fresh quartz overgrowths) start to appear. It is harder and more compact than sandstone but may still show grain outlines.

• Color: grey, buff, tan, pink, greenish (with chlorite/sericite)
• Luster: dull, with possible silky sheen from fine mica on partings
• Transparency: opaque
• Texture: clastic grains partly recrystallized and welded; relict bedding/cross-bedding common; incipient foliation possible

Step-by-Step Field-ID Checklist

1. Confirm sandy, clastic origin: visible sand-sized grains and relict bedding.
2. Test cohesion: harder and more welded than ordinary sandstone; it does not shed grains as easily.
3. Examine fracture: breaks are intermediate — partly through grains, partly around them (full through-grain fracture means it has become quartzite).
4. Look for new minerals: silky sericite or greenish chlorite on partings indicates metamorphism has started.
5. Note any weak foliation overprinting the original bedding.

Key Diagnostic Tests

• Hardness: quartz grains 7; overall the rock is hard but may have weaker matrix zones.
• Fracture: intermediate between sandstone (around grains) and quartzite (through grains).
• Acid: inert unless a calcareous cement is present (then it fizzes).
• Streak: white.
• Density: ~2.6.

Common Look-Alikes and How to Tell Them Apart

• Sandstone (unmetamorphosed): softer, more porous, sheds loose grains, breaks cleanly around grains; lacks sericite/chlorite sheen. Greater hardness and incipient metamorphic minerals indicate metasandstone.
• Metaquartzite: fully recrystallized, fractures entirely through grains with a sugary glassy surface and usually no relict bedding visible. Metasandstone still preserves grain outlines and bedding.
• Metagreywacke: derived from muddy, poorly sorted sandstone; darker, with more matrix and mica, and grades to schist. Metasandstone is cleaner and quartz-rich.
• Hornfels: contact-metamorphosed fine rock, very tough and splintery, but lacks the sandy clastic texture of metasandstone.

Where It Is Typically Found

Metasandstone occurs at the margins of metamorphic belts and in the lower-grade portions of orogens, where sandstone units were heated and squeezed but not intensely altered. It is common in foreland fold-and-thrust belts, around contact aureoles, and in shield-margin sedimentary sequences that experienced mild regional metamorphism.