Flexible Sandstone

It’s not a slab of cornbread

“Flexible sandstone” isn’t really a contradiction, but it’s not as if you can fold it in a right angle. Laths and thin slabs of this material do flex visibly. Such rocks are called itacolumites, named for a famous locality at Itacolumi Mountain in Minas Gerais, Brazil, but they are also known from India, Madagascar, North Carolina, and elsewhere.

Quartz, the main constituent of these sandstones, isn’t flexible. It’s notoriously brittle, and it doesn’t have good cleavage planes that could give the quartz grains, and possibly the whole rock, a preferential orientation for flexing or anything else; quartz and rocks made mostly of quartz usually just break.

The itacolumites contain abundant pore spaces between the quartz grains that contribute to (or create) the flexibility, but most sandstones have some porosity yet few are flexible. One study (Siegesmund and others, 2002, The anisotropy of itacolumite flexibility: Geological Society, London, Special Publications, v. 205, p. 137 – 147) suggests that these sandstones have undergone a rather special dissolution and rounding of the grains (and enlargement of the surrounding pore space) allowing individual grains to rotate enough to allow the entire rock to flex – but just enough to flex and not to break.

Another overview (Kerbey, 2011, Itacolumite, flexible sandstone and flexible quartzite – a review: Proceedings of the Geologists' Association, v. 122:1, p. 16-24) suggests that the quartz grains may not be completely floating in the surrounding pore space but may have some degree of interlocking between grains, creating a hinge-like arrangement among grains allowing them to flex and rebound without breaking or really even separating. Most analyses seem to agree that the flexibility depends mostly on the pore space with or without preferential textures developed on the grains’ surfaces.

In some itacolumites mica is present that does not seem to be required for the flexibility but may contribute to it and may guide the anisotropy in the rocks. Anisotropy is from Greek words meaning “not equal turns,” or not having the same properties in all directions. Anisotropy is a reflection of the flexibility being greater, or simply present, in one direction and less, or absent in others. The mica also would suggest that the rocks might be at least somewhat metamorphosed, which would contribute to giving the rocks preferential fabrics in the original material and perhaps also in the later dissolution processes that produced the flexibility.

Original map by Woudloper, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license.

My specimen (Cat. No. 1895) is yellowish, like most of the itacolumites from Brazil, because of traces of iron oxides. Although I don’t know for sure exactly where it is from, it’s most likely from the well-known rocks in Minas Gerais, where the sandstone is probably part of the Moeda Formation, a package of sedimentary rocks deposited about 2.5 to 2.7 billion years ago (Madureia and others, 2021, Depositional setting and U-Pb detrital record of rift-related deposits in the Moeda Formation (Minas Supergroup) at the Gandarela and Ouro Fino synclines, Quadrilátero Ferrífero, Brazil: Brazilian J. Geology 51:3) . The Moeda includes a conglomerate that contains gold mineralization interpreted to be of detrital origin, that is, it was originally a placer deposit of gold in the conglomerate. It’s quite similar to the much larger gold deposits in the Witwatersrand of South Africa (Minter and others, 1990, Early Proterozoic gold placers of the Moeda Formation within the Gandarela Syncline, Minas Gerais, Brazil: Economic Geology 85:5, p. 943). The original sedimentary rocks were probably deposited in an early rift stage of the development of the São Francisco Craton, one of the ancient cores of South America.

These rocks have been metamorphosed repeatedly, though not really that intensely given how old they are. The São Francisco Craton grew during the Trans-Amazonian orogeny 2.0-2.26 billion years ago as a result of at least four collisions of relatively small terranes, possibly island-arc-like zones. The rift sediments of the Moeda Formation were incorporated into the enlarged craton by those amalgamations.

My specimen is only barely flexible now, mostly from too much flexing over time so that it finally broke. It’s still a pretty cool rock. It’s rather fine-grained (quartz grains about 0.2 to 0.5 mm) and there’s nothing really obvious to visually give away its flexible nature, although in the photomicrograph above (field of view about 20 mm) you can perhaps see the thin platy zones (micas and clays) that impart a planar fabric to the rock. The slab was originally about 10 cm x 15 cm.

The name “itacolumite” was first introduced in 1822, so it’s a well-established term. Understanding how these rocks work has potential for inventing earthquake-resistant building materials (Yamaguchi et al., 2007, A new structural system: Friction-resistant dry-masonry: Building Research and Information 35:6, p. 616), flexible ceramics, and other applications.

In my History of the Earth Perpetual Calendar, if the book had a proper chronological scale, tomorrow November 14 would be the end of the Precambrian, and the book would have been pretty boring. Instead, I chose to assign each month to the major divisions of geologic time, but mentioning that true scale tells you, I hope, something about the scope of deep geologic time.