Lipari obsidian

Rocche Rosse, Lipari, Aeolian Islands
Erupted in the 13th century AD

The youngest rock in the tray: a flow of volcanic glass that ran down the north-east of Lipari some eight hundred years ago.

Glass has no crystal structure, so between crossed polars it is black. The spheres are spherulites: fans of feldspar and silica needles that crystallised radially in the cooling glass, strung along its flow bands. Like the ooids of Portland, each shows a fixed dark cross. In plane light, look for the curved perlitic cracks and the trains of tiny microlites that trace how the lava moved.

Under the microscope

What to look for

01 · crossed polars · ×4Spherulite crossesRadiating fibres: the cross stays aligned with the polarisers while the stage turns.
02 · plane light · ×10Perlitic cracksOnion-skin cracks in the glass, from contraction as it cooled and hydrated.
03 · tint plate · ×4Tint plateWith the gypsum plate in, opposite quadrants of each spherulite turn blue and yellow.

The whole section

Plane light | crossed polars

The same slice of rock with the analyser out and in. Drag the line.

PPLXPL 07 · LIPARI · 30 µm

Mode, by eye

What it is made of

Mineral	Share	δ	Γ max at 30 µm	Strongest colour
volcanic glass	70%	—	0 nm	dark at every angle
spherulites	27%	0.008	240 nm	first-order grey–white
sanidine, microlites	3%	0.007	210 nm	first-order grey–white

Γ (retardation) = thickness × birefringence δ. Swatches are computed by the same optics that draw the microscope.