tachylite - tachylite "frosting" on a section of the 2018 Kilauea flow that destroyed Kapoho, Hawaii - display specimen

Tachylite is a light weight and fragile basaltic volcanic glass, low in silica and with a pitchlike or resinous luster. In thin sections, tachylite is brown and translucent, with numerous crystals of magnetite.

In Hawaii, an entire lava flow surface can be tachylite, where the rapid cooling of basaltic flows has inhibited crystal formation. It rapidly weathers to palagonite, an orange to yellow isotropic mineraloid, forming a crust on lava flows. 

Near the top of an andesitic or basaltic flow, gas that was dissolved in the magma comes out of solution to form bubbles because of the reduction in pressure. Deeper in the flow the basalt is more dense, with smaller bubbles, or even none.

This example is a section from the top of the flow, showing the tachylite surface with vesicular basalt below, and with the transition from large vesicles near the surface to smaller ones below. Cross section detail is illustrated.

This tachylite was collected from the edge of a flow channel near the Puna Geothermal Plant. Lava began to flow on May 3, 2018 from a vent in Kilauea's East Rift Zone. By May 17 lava had reached the ocean and by May 27 had overrun two geothermal wells of the Puna Geothermal Project that was producing 25% of the island's electricity. A second flow overran Kapoho on June 4 and completely filled in Kapoho Bay the next day.

The pencil is 5 1/2"long, for scale.