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I was reading about kimberlite on Wikipedia and it mentioned eluvium. I've heard of alluvium, alluvial fans, alluvial deposits, etc. However, when I looked up eluvium, it sounded exactly like alluvium.

Indeed, looking at the Wikipedia articles, the definitions sound identical to my (expertly untrained) ears:

Alluvium: loose, unconsolidated (not cemented together into a solid rock) soil or sediments, which has been eroded, reshaped by water in some form, and redeposited in a non-marine setting.

Eluvium: geological deposits and soils that are derived by in situ weathering or weathering plus gravitational movement or accumulation.

So, both of these sound like soil and sediments, eroded and deposited. But obviously, they can't be the same thing.

What's the difference? How can I tell the difference if I see it "in the wild"?

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Both are very similar, as they are based on minerals undergoing erosion and the presence of water in the erosion and deposition cycles, the difference is quite subtle, an example is defined in the Gold Prospecting in Western Australia website Alluvial Gold and Eluvial Gold as being

Alluvial deposits are mixed with other deposits and are washed downstream in rivers or transported in among other sediments with water as its medium. An example is that gold deposits this way are smoothed and are ground down the further from the source.

Eluvium deposits are in situ or very near its source, rather than been washed away by water. For example, eluvial gold are often large nuggets nearer to its source.

A much more succinct definition is provided by the Australian Museum:

Alluvium:

Detrital material which is transported by a river and usually deposited along the river's pathway, either in the riverbed itself or on its floodplain.

Eluvial:

Weathered material still at or near its point of formation.

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  • $\begingroup$ Another major difference is that in Alluvial deposits Alluvial fans are formed and alluvial deposits are often found in mountainous regions. $\endgroup$ – user2849 Apr 27 '15 at 23:08
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Also keep in mind that eluvium and eluvial have a different meaning in soil science. There, it means leaching of soil matter or chemical substances by water. (Usually, after some downward transport, the substances precipitate in an iluvial horizon.)

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  • $\begingroup$ and in spodosols one gets an eluvial depleted horizon, often pure white quartz, and an illuvial precipitated horizon below it, often dark red or even purple. Very striking in cross-section. $\endgroup$ – cphlewis Apr 23 '15 at 23:40
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It's also worth noting that Eluvial soil is moved by water, but not necessarily river water. The geotech reporting on my hillside property which has no river on it, but experiences significant shift of soil when it rains due to the property's location in a trough (basically a topography-scale gutter on a hillside). Basically, there's a lot of run-off down the mountainside through the property which shifts the soil on the property over time due to the impact of rain storms. He referred to the displaced earth as el

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