Halloysite

It's just clay

Clay minerals don’t get much respect. They seldom make big colorful crystals, and what they do make is often soft, white, and massive. Clays are important, though, in floor and wall tiles, toilets and other fixtures, kitty litter, oil well drilling mud, cement and concrete, bricks, paper coatings, fillers in paint, plastic, and rubber products, refractories like certain ceramics, and other uses, adding up to a $1.7-billion-dollar-a-year industry in the U.S., more than 2½ times the value of the lead mined in the United States (data from USGS).

Sometimes, clays make pretty (or at least interesting) specimens, like the three here, especially when they deposit on a contrasting substrate. Halloysite is a common clay, hydrous aluminum silicate, Al₂(Si₂O₅)(OH)₄. The one here from Chuquicamata is tiny white balls about a tenth of a millimeter across on green chrysocolla. Below, the single ball in a dark cavity in a volcanic rock from the Laacher See volcano in Germany is about a third of a millimeter across. Laacher See was formed in a massive eruption about 12,900 years ago.

You may not think of Germany as a volcanic area, but the East Eifel Volcanic Field erupted beginning about 500,000 years ago, and the most recent activity, the Laacher See eruption, was only about 12,900 years ago. The Laacher See crater lake, just west of Koblenz, Germany, is 2 km across, and the eruption that formed it is estimated to have been comparable to the Mt. Pinatubo eruption in the Philippines in 1991. The timing, 12,900 years ago, is very close to the onset of the Younger Dryas, a return to glacial conditions following a period of post-glacial warming. The Laacher See eruption was probably intense enough to have affected climate, but acting as a major trigger for the Younger Dryas is unlikely. Because it is so recent, the timing of the Laacher See eruption has been dated with considerable accuracy: besides conventional radiometric age dates, the eruption can be correlated with varves (annual layers in lake sediments) and tree rings, so we are really quite confident that the eruption was within a few years of 12,926 years ago.

The caldera lake, Laacher See.  Photo by Df1paw, licensed under the Creative Commons Attribution-Share Alike 4.0 International license.

There’s been persistent volcanism there going back at least 15 million years, and the area is considered to be volcanically active today, although there is probably no imminent threat. There are more than 200 scoria cones, flows, and maars in the region from the relatively recent activity. A maar is a relatively small volcanic crater that results from the violent explosive event that occurs when water interacts with molten rock near the surface. The word maar is a Moselle Franconian [a Germanic language] word for the circular lakes that occupy these craters in the Eifel Volcano area.

The volcanic activity is related to the active Rhine Graben, an extensional fault system that runs from Basel, Switzerland, through Alsace-Lorraine and on to Frankfurt, Germany, with additional expressions further north. Ultimately, the rift is caused by the oblique collision of pieces of the old continent of Gondwana with southern Europe. Those pieces, especially blocks we know today as Italy and Iberia, are being pushed northward by Africa and adjacent terranes, with the Alps, Pyrenees, and other mountains the most obvious results. But the Rhine Graben and the volcanoes in Germany are also products of that collision.

The third specimen (16 mm wide) is a pretty shade of blue because it has chromium impurities. It’s from an old iron mining district near Rudnjak, Serbia, where similar material has been called chrome-kaolin. Kaolin is a related clay mineral, but this one has, according to its label, been analyzed as halloysite-10Å, renamed hydrohalloysite in 2022, where the 10Å means it is a specific mineral in the halloysite family that has molecular water in its structure, Al₂Si₂O₅(OH)₄ · 2H₂O, and a molecular layer thickness of 10 Ångströms (an ångström unit is one hundred-millionth of a centimeter). The other varieties of halloysite contain OH groups, not H₂O, and the mineral now called halloysite (renamed from halloysite-7Å also in 2022) has a 7-Ångström layer thickness. Mineral nomenclature can sometimes be a pain, even when it increases accuracy and knowledge.

Halloy circa 1850, painter unknown.

This mineral was named for Belgian geologist Jean-Baptiste Julien d'Omalius d'Halloy (1783-1875), who discovered it in Liège Province, Belgium. Halloy is considered to be one of the founders of geology; he defined the Cretaceous Period in 1822 and produced the first geologic map of France (below). In 1831 he promoted the idea of “descent with modification,” a concept acknowledged by Darwin as a predecessor to his ideas about evolution. Halloy served in various government positions in Belgium for much of his life and died as a consequence of a solitary scientific expedition he undertook in the winter of 1874-75, when he was 91 years old.

Halloy’s Geologic Map of France