tach·y·lyte also tach·y·lite 
(tăkə-līt′)
(tăkə-līt′)n.
A brown, black, or green volcanic glass formed from basaltic magma and often found along the edges of dikes, sills or volcanic flows, where rock usually forms first by contact with the cooler surrounding rock.
[German Tachylyt : Greek takhu-, tachy- + Greek lutos, soluble (from lūein, to loosen; see leu- in the Appendix of Indo-European roots).]
tach′y·lytic (-lĭtĭk) adj.