hyalopilitic

from The Century Dictionary.

• In petrography, noting the structure of some lavas in which microscopic prisms are crowded in a glass matrix.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective A textural term used in petrographic classification of volcanic rocks, referring to a volcanic rock groundmass, which is visible only under magnification with a petrographic microscope, that contains a mixture of very fine-grained mineral crystals either mixed with natural volcanic glass, or surrounded by thin bands of volcanic glass.