Spec Tech: Sandstone… It’s a Living

Geologically speaking, “sandstone” has a very precise meaning. A sandstone is a sedimentary rock that is composed of less than 50% mud-sized particles and less than 30% gravel, with the remaining rock or mineral grains being sand-sized. Sand-sized grains are between 62 microns and 2 millimeters in size. To a non-geologist, this can easily translate out to, “a sedimentary rock that is mostly made out of sand.”

There’s a lot of detail that we could get in to with how sandstones are classified – how well the sand is sorted, how big the sand is, what this tells us about the environment of formation, etc – but we can save that for a later date if people are interested. I’m a sedimentologist, which means I could just go on and on about those topics for days while the eyes of my readers’ eyes glaze over.

Sandstones are a diverse group of rocks because their composition can vary wildly. “Sand” is merely a way to classify size and implies nothing about what minerals might be present. Since sand is made from pieces of other rocks that have been mechanically broken into smaller pieces (and possibly chemically altered) any mineral that is present in a rock can be made into sand.

Most people are familiar with pale tan sand that’s made mostly of quartz. Quartz is the most common mineral in any sand because it’s extremely resistant to chemical weathering. However, you can easily find places where there is green sand (made mostly of Olivine), black sand (pieces of basalt, black minerals), red sand (composed mostly of garnet), and pure white sand (calcite). The list goes on and on, and imagination is really the limit. If there’s a common rock-forming mineral, somewhere there is a sand made out of it.

If conditions are right, the sand mass will eventually be buried. This means that the sand grains are squashed together with the pressure of more sediments being deposited on top of it. Fluids moving through the sand deposit secondary minerals that act as cement to hold the sand grains together. With sufficient pressure, this can cause what is called pressure solution, where sand grains (particularly quartz grains) will fuse together. These processes transform the loose sediment into the rock.

The resulting sandstone mirrors the composition of the sand that formed it. So you can have black sandstones, white sandstones, green sandstones. You can get beautiful salmon pink sandstones, with that color normally caused by iron oxide (rust) staining.

The above figure is called a QRF (Quartz, Rock fragments, Feldspar) plot. It’s used to classify sandstones by their mineral composition. Anything that isn’t quartz or feldspar (such as a grain of olivine) is considered a rock fragment. Sandstones that have very little mud-sized particles lurking between the sand grains are called arenites. The point I want to make with this plot is that it takes a composition of 90% quartz or more for a sandstone to be considered to be considered a quartz arenite. Almost any sandstone will be mostly quartz, but what gives the sandstone its character are the other minerals it contains.

So what implications does any of this have on sandstone being used as a building material?

Historically, sandstone has been used for tens of thousands of years as a building and crafting material because it’s relatively easy to work. There’s a lovely example of a lamp made out of sandstone that’s 17,000 years old. Sandstone could be worked even in low technology times using harder rocks (such as flint/chert or pieces of volcanic rocks) or using abrasion techniques. Humanity has a long and productive relationship with this particular rock.

Part of the reason sandstone is easy to work is because the sand grains are normally held together with some form of cement. Depending upon the cement type, this can provide a point of weakness for crafters to exploit, allowing them to scrape away successive layers of sand grains for fine work, or easily split off chips of stone. Sandstones often have sedimentary structures (internal geometry created during their deposition as sand) that provide other points of weakness that can be used. For example, the Lyons Sandstone in Colorado is used extensively for paving stones and building bricks because the sand is in thin, regular layers (beds and laminations) that can be easily split apart. These regular laminations exist in the Lyons sandstone because it was formed in a desert with massive sand dunes; the dunes are literally built in layers caused by successive waves of wind-blown ripples migrating across them.

Sandstone also makes a useful building material because it’s normally quite resistant to weathering. Quartz as a mineral is extremely resistant to chemical weathering and is difficult to break down mechanically once it’s been rounded to a spherical shape. A quartz arenite (more than 90% quartz!) that is cemented by even more quartz is a stone for the ages, so to speak.

However, not all sandstones are made out of all quartz, or cemented with quartz. The composition of the sandstone and its cement can have a profound effect on how well the rock holds up against the elements. Sandstones containing high temperature minerals such as olivine would be extremely susceptible to weathering, since those minerals aren’t very stable on the surface of the earth. Sandstones with a lot of feldspar are also more susceptible to chemical weathering; feldspar is chemically altered to clay with the application of enough rainwater.

The minerals cementing the sandstone will also determine how well the minerals are held together. An example of this in the natural world can be seen in my home state of Colorado. Near Boulder, the feldspathic arenite Fountain Formation forms a very dramatic feature called the Flatirons. Elsewhere in the state, the Fountain Formation is much less prominent and can’t stand up in such beautiful, highly tilted structures. This is because near Boulder the formation is cemented with feldspar – which is mechanically very strong – and elsewhere in the state it’s cemented with calcite. Calcite cement also tends to dissolve with prolonged exposure to water. Other sandstones can be only weakly held together by layers of clay and can be easily crumbled by hand – not the sort of thing you’d want to build with!

Any builder wanting to work with sandstone would of course want to assess the strength of the stone first. It’s easy enough to tell if a sandstone is so weakly cemented that it can be crumbled, but other problems are less obvious unless you live in the modern world and have access to technology like a petrographic microscope. This is why some older buildings constructed of sandstone actually have to have their stones replaced after a relatively short (for a rock) period of a few hundred years.

This means, for example, you could build your fantasy cathedral from beautiful green sandstone made of olivine, but as long as chemistry works the same, the stonemasons will be kept in regular employment to replace the stone facing as it weathers. But hey, it’s a living.


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