The imported substrates from Japan, Kanuma and Akadama ARE clay. They are clay derived from volcanic rock.
I was a bit annoyed by the fact that I do not know if kanuma and akadama are technically clays, clay minerals, or pumice so an igneous extrusive type of rock.
Turns out, it is not so simple.
Clay refers to very finely particles. Particles smaller than 4 micrometers. They are produced by eroding & weathering of rocks.
When clay particles sediment, they can form sedimentary rock. Often, clay minerals are formed from other weathered sedimentary rocks, which originally came from magma or lava. So there can be many cycles of weathering and re-sedimentation.
Akadama and kanuma are said to be pumice from volcanic eruptions. Several sources clearly say they are both weathered pumice. And that their main chemical component is allophane. Which is not a mineral but a mineraloid because it is amorphous and therefore not a crystal. In general, it is said that allophane is formed from either weathered rock of volcanic ash.
So it seems that indeed akadama and kanuma are not forms of pumice. The allophane particles inside are way smaller than 4 micrometer but nanometer tubes. They are porous because they are formed from tiny clay-like minerals. Not because they are igneous extrusive glass containing air bubbles. Volcanic ash contains 2mm particles of rock and volcanic glass. So it is either a weathered volcanic ash soil that then formed into sedimentary rock. Or it is a combination of weathered pumice plus sedimentation of a vulcanic ash soil. The high rainfall is important for the formation.
And with weathered pumice I mean the pumice was eroded down to small particles, then sedimentated. But it may even be the case that the pumice rock is largely intact, and that sub 4 micrometer particles of soil form new clay mineraloids inside the pumice.
It is also said that allophane can also be formed through hydrothermal alteration. But this seems more unlikely for kanuma&akadama, as the volcanic eruption is known and the layers are not way deep inside the earth crust, or very old and deep sedimentary layers then exposed to the surface much later.
I don't wan to go into a deeper dive, am not a geologist, skipped most of the inorganic chemistry, but this seems to be a correct explanation. Or, we may not even know in what way akadama and kanuma are both clay and pumice.
As for stubmle comment & bump, yep indeed one should check what cat litter is actually made of. You are buying a certain type of mineral. Many materials can function as cat litter. And even if they are the right type of material, say diatomaceous earth, it may not be horticulture grade. Some type of sands, perlite, DEs might be contaminated with stuff. Making them unsuitable. This can be true even for sand in children's playgrounds. You'd have to wash & test to make sure they do not contain any compounds you don't want. Which then makes them a horticulture-grade version of said product. Some times of coarse sand may be made by grinding down bricks after breaking down some building. Which may theoretically even include bricks that were part of a highly polluting factory. Same with stuff that comes from the beach or coastal areas. Too much table salt is very bad for plants. Coco coir had this reputation, but I think now the sea salts are properly washed out. Diatomaceous earth high in sea salt is perfectly fine for cat litter. But not for plants. I think that practically sodium chloride in DE is never an issue. But especially if you put expensive plants in non-horticultural grade soils or substrates, be aware of the potential risks.
