tachylite - tachylite from the 2018 Kilauea flow that destroyed Kapoho, Hawaii - display specimen

Tachylite is a lightweight 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, entire lava flows 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, or simply crumbles as it weathers. 

The term tachylite was first used by James Dwight Dana in 1868. Dana led the first geologic expedition to Hawaii in 1880-1881, followed by another look after Charles Dutton's 1884 Hawaii expedition. The publication of his observations in 1890 became the definitive source of information on Hawaiian volcanics for many years.

Dana's Manual of Mineralogy, first published in 1848, became a much revised college text. My copy, the 1961 reprint of the 17th edition, revised by Harvard professor of mineralogy Cornelius Hurlbut Jr, is getting a bit raggedy after six and a half decades of use, but is often in hand. Dana taught geology at Yale from 1850 to 1892.

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.

Because this material is fragile, large specimens like these are unusual. Excellent for display and careful handling. An iridescent glassy surface, formed when the flow overtopped the channel and dropped back down, leaving a thin layer of tachylite draped over the edge.

The pencil is 5 1/2 inches long, for scale. Both sides of each specimen are shown. The example with a pencil and coin has some maroon vesicular basalt embedded in the glass.

Dana's photo separates the specimens.