from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. The fine-grained crystalline base of porphyritic rock in which larger crystals are embedded.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. The matrix of fine-grained crystalline material in which larger crystals are embedded.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. (geology) the matrix of fine-grained crystalline material in which larger crystals are embedded


ground +‎ mass (Wiktionary)


  • The rock here is a porphyritic granite (porhyritic meaning that the stone has large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass).

    Of Books and the Sea

  • [Small, separated mineral crystals in a glassy groundmass]

    Boing Boing

  • Immersed in this groundmass are a wide variety of rock shapes, sizes, textures—round and angular—many with varying consistencies.

    First Man

  • The groundmass throughout the area is a very fine sand to a silt.

    First Man

  • Liquid (l) is seen at the bottom right, the rest being vapor (v). (g) Primary vapor-rich inclusion at the tip of a quartz stringer (outlined for clarity) in the granophyric groundmass.


  • Radiating fringe of feldspar in groundmass is in optical continuity with phenocryst, from which it nucleated. (e) Quartz phenocryst is surrounded by feldspar groundmass.


  • Quartz in groundmass does not appear to have nucleated directly on phenocryst, but is in optical continuity with it. (f) Primary vapor-rich fluid inclusion in phenocrystic quartz.


  • Figure parts a-e with crossed nichols. (a) Micrographic groundmass surrounds feldspar phenocryst fragment. (b) Micrographic groundmass around clinopyroxene phenocryst.


  • K/Na of the groundmass feldspar equals K/Na in pumiceous glass of the host pumice, hinting that the granophyric minerals grew by closed-system crystallization of the rhyolitic melt.


  • Granophyric intergrowths of quartz and feldspar in the groundmass contain abundant, primary, vapor-rich fluid inclusions, 1 to 10 µm in diameter.



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