Definition

Noun: A dark, opaque, or nearly opaque volcanic glass that forms from rapidly cooled basaltic or basic lava. It is characterized by its lack of visible crystals due to extremely fast solidification.

Usage

Tachylite is a geological term used to describe a specific type of natural glass. It is typically found as crusts on lava flows, in the margins of dikes, or as fragments in volcanic ash. - The geologist identified the shiny black crust on the lava flow as tachylite. - Tachylite often forms when molten lava comes into rapid contact with water or air.

Advanced Usage

• Petrographic Context: In thin-section analysis under a microscope, tachylite may show microlites (tiny crystals) but remains predominantly glassy.
  • Under polarized light, the sample revealed the characteristic isotropic nature of tachylite.

Variants and Related Words

• Tachylyte: An alternative, less common spelling for tachylite.
• Sideromelane: A related, often paler brown basaltic glass. Tachylite and sideromelane can occur together, with tachylite being the darker variety.

Synonyms

• Basaltic glass
• Volcanic glass (specific to basic/basaltic composition)

Notes on Meaning

The term specifically denotes glass formed from basic (low silica) magma, such as basalt. It is distinct from obsidian, which is a volcanic glass formed from high-silica (acidic) magma like rhyolite.

Noun

1. a basic or basalt glass