Metasandstone

Metasandstone is sandstone that has been metamorphosed but not fully converted into quartzite. The original sand grains, mostly quartz with feldspar and rock fragments, begin to recrystallize and interlock, hardening the rock while sometimes preserving recognizable sedimentary features.

Geologists also use the term metapsammite for metamorphosed sandy sediments. Metasandstone sits on a continuum: with increasing metamorphism it grades into quartzite, where original grain boundaries are erased.

It is a common rock in regionally metamorphosed sedimentary sequences, recording the burial and mild-to-moderate metamorphism of ancient sandy deposits.

Metasandstone forms when quartz-rich sandstones are buried and subjected to increased temperature and pressure during regional or contact metamorphism.

The heat and pressure cause quartz grains to recrystallize and fuse along their margins, reducing porosity and producing a tougher, more interlocked rock. Accessory minerals like mica, feldspar, and chlorite may grow, sometimes imparting a faint foliation. With further metamorphism the rock becomes quartzite.

Metasandstone occurs in fold belts, mountain roots, and metamorphosed sedimentary basins worldwide, often interbedded with slate, phyllite, and schist derived from the muddier layers of the same sequence.

Identify metasandstone by its sandy, granular look combined with extra hardness and interlocked grains compared with ordinary sandstone. It is harder to scratch and may ring when struck.

Relict sedimentary features such as bedding, cross-bedding, or graded layers often survive, distinguishing it from fully recrystallized rocks. A hand lens may reveal partly fused quartz grains.

Key look-alikes are sandstone (softer, more porous, grains easily rubbed off) and quartzite (fully recrystallized, breaks across grains, glassy fracture). Metasandstone is transitional between the two.

Metasandstone is used as a durable construction material, including crushed aggregate, paving, and dimension stone, valued for being harder and more resistant than ordinary sandstone.

Its study helps geologists reconstruct the burial and deformation history of sedimentary basins and mountain belts, and it serves as a marker for metamorphic grade alongside associated slates and schists.

Metasandstone is not a gem or ornamental specialty stone, though attractively colored or banded varieties are sometimes used decoratively.