Laterite (not lattes)

Author: Ten Para

Science often reveals a rather uncanny knack to leap out from unexpected places at unexpected times.  Breaking the shackles that try to confine it to tomes of words, floating out of closed rooms and lab spaces, tubes, filters and maybe some illustrations.  A gentle reminder that you cannot escape the lovely interactions between science and art, wherever you go and wherever you look around in this world.

Recently, I came across a set of stones that immediately took me on a nostalgic trip to my childhood. These were stones that I absolutely loved as a child (and still do) primarily because I used them as weights to place on my science textbooks (in a vain attempt to keep science confined within the books) every year. The trick was to place them on the books and then pour just a little water on the blocks. The balance was in pouring just enough to allow the water to percolate and stain the books and not too much to let others (your parents and snitching siblings) know you poured water. This was followed by the practice of a look of injured innocence as you went to your parents and whined “I can’t study science, my books are spoilt”. Needless to say, it didn’t always work the way I wanted as parents seem to grow just that little bit wiser with age (I call it the AAW (anticipatory aging wisdom) as it seems to develop mainly to anticipate all the stuff we want to do but apparently should not do but almost never agrees with the things we really want to do).  I loved these stones for many other reasons as well, watching the rain drops break into many patterns as monsoon broke over the land, rivulets, streams, rivers and dams and rain drops that just seemed to disappear, all in a stone. Colors that change from a dark red, to brown, black, blood and milk, and green as the monsoon really set in. Ferns and mosses, plants that came from crevices you never even knew existed and if you were really lucky, insects that crawled out and termites that crawled away. Lean against the wall, smelling the marriage of the rain with the earth, just becoming wet, plucking a termite out of your hair and a fern from your nose!  Oh!

Allow me to introduce you to the world of laterites. I do credit them with introducing me to chemistry, biology, history, geography, earth science and architecture.

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Laterites are a soil and rock type that has a color that varies between shades of red. It is rich in iron and aluminium with a high iron oxide content that gives it a rusty red color. The darker stones are harder than the lighter stones. The laterites are most often found in the tropics, between the tropics of Cancer and Capricorn.  

The laterites are often found under residual soils and undergo a prolonged process of chemical weathering (laterization) that lead to a wide variety in the thickness, grade, and chemistry of the resulting soils.  Laterites are formed by the leaching of parent sedimentary rocks, which leaves the more insoluble ions such as iron and aluminium. The mechanism of leaching involves acid dissolving the host mineral lattice, followed by hydrolysis and precipitation of insoluble oxides and sulfates of iron, aluminium and silica under the high temperature conditions of a humid tropical monsoon. The repetition of wet and dry seasons is an essential feature for the formation of laterite. Rocks are leached by percolating rain water during the wet season; the resulting solution containing the leached ions is brought to the surface by a capillary action in the dry season. These ions form soluble salt compounds that dry on the surface; these salts are washed away during the next wet season.

The laterites cover  about one-third of the Earth’s continental land area. They may form the subsoils of the equatorial forests, of the Savannahs of the humid tropical regions, and of the Sahelian steppes. They cover most of the land area between the tropics of Cancer and Capricorn; areas not covered within these latitudes include the extreme western portion of South America, the southwestern portion of Africa, the desert regions of north-central Africa, the Arabian peninsula and the interior of Australia.

Some of the oldest laterites are found in the complex precambrian shields in Brazil and Australia. Smaller highly deformed Alpine type intrusives have formed laterite profiles in Guatemala, Colombia, Central Europe, India and Burma. Present-day laterite occurring outside the humid tropics are considered to be indicators of climatic change, continental drift or a combination of both.

When moist, laterites can easily be cut with a spade into regular-sized blocks. Laterite is mined while it is below the water table, so it is wet and soft. Upon exposure to air it gradually hardens as the moisture between the flat clay particles evaporates and the larger iron salts lock into a rigid lattice structure and become resistant to atmospheric conditions.

There are several examples of construction with laterites. An advantage of laterite was the ability to place blocks against each other without the use of mortar in between.

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Angkor Wat, located in present-day Cambodia, is among the largest religious structure built by Suryavarman II, who ruled the Khmer Empire from 1112 to 1152. This World Heritage site is built on a mix of sandstone and laterites. The foundations and internal parts of the temple contain laterite blocks behind the sandstone surface. The masonry was laid without joint mortar.

You can see laterites quite often as you traverse India, especially southern and central India. Keep your eyes open as you travel next! However, the use of laterites in masonry seems to have declined considerably with concerns related to longevity and a potential need for repeated chemical treatment to keep out pests. Sad, considering that the art of masonry with laterites is considered an original contribution from the Indian subcontinent to the larger world.

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