Shock and weathering grades

Classification codes often end with an S and a W number, as in L6 S4 W1. These record what impacts did to the rock in space, and what Earth did to it afterwards.

Shock in space

Asteroids collide at speeds of several kilometres per second. Such impacts send shock waves through rock, creating pressures of many gigapascals for a fraction of a second. One gigapascal is about ten thousand times normal atmospheric pressure.

Shock leaves fingerprints in minerals. Under a polarising microscope, unshocked olivine goes dark evenly when the stage is rotated. With increasing shock, the darkening becomes wavy, then the grains crack along planar fractures, then they break into a mosaic of tiny misaligned domains.

At high pressures, feldspar can turn into a glass called maskelynite without melting in the normal sense. Pockets and veins of melted rock form, and in the most extreme cases high-pressure minerals such as ringwoodite, a dense form of olivine, crystallise in the melt veins.

The S1 to S6 scale

In 1991, Dieter Stöffler, Klaus Keil and Edward Scott published a shock classification for ordinary chondrites that is still widely used. It runs from S1 to S6.

S1, unshocked, shows sharp optical extinction in olivine. S2, very weakly shocked, shows undulatory, or wavy, extinction. S3, weakly shocked, adds planar fractures. S4, moderately shocked, shows mosaic extinction. S5, strongly shocked, adds maskelynite and planar deformation features. S6, very strongly shocked, shows local melting and high-pressure minerals in and near melt zones.

The original paper gave estimated pressures rising from below about 5 gigapascals for S1 to roughly 75 to 90 gigapascals for S6. Later work has revised some of these calibrations, so the stage is best used as a descriptive category rather than a precise barometer. Rock that was totally melted is described as an impact melt rock instead.

Weathering on Earth

Once a meteorite lands, Earth starts to change it. Metal and troilite oxidise to rust-coloured iron oxides and hydroxides, which spread along cracks and stain the rock. Later, silicate minerals begin to alter.

For ordinary chondrites, the most widely used scale was introduced by Frank Wlotzka in 1993. It is based on polished sections. W0 shows no visible oxidation of metal or sulphides. W1 has minor oxide rims and veins. W2 shows moderate oxidation, W3 heavy oxidation and W4 complete oxidation of metal and troilite. W5 shows beginning alteration of mafic silicates, mainly along cracks, and W6 massive replacement of silicates.

Antarctic meteorites use a different hand-specimen scale, A, B and C, for minor, moderate and severe rusting, with an added e when evaporite minerals, salts formed by evaporation, are visible.

Why these grades matter

Shock and weathering grades are not just extra detail. They tell researchers which samples suit which studies.

Shock can reset some radiometric clocks, especially the argon-argon system, so heavily shocked meteorites often record the date of a major collision rather than their formation. Shock darkening, caused by fine metal and sulphide spread through cracks, can make a meteorite look much darker, which matters when comparing meteorites with the colour and brightness of asteroids.

Weathering changes chemistry. Rusting adds oxygen and water, and soils can add elements such as barium and strontium, especially in hot deserts. A W4 desert find is a poor choice for studying trace elements, but it may still be perfect for studying chondrule textures or cosmic-ray exposure.

Knowing the grades helps scientists choose the right sample and interpret results correctly.

Did you know?