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What is Vermiculite?

Mary McMahon
Updated Jun 04, 2024
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Vermiculite is a hydrated basaltic mineral with some unusual properties which have caused it to be popular in industrial manufacturing since the early 1900s. Many consumers probably interact with vermiculite, since it is used as a filler in a wide range of products, and it can also be found in things like insulation, absorbent materials, and soil mixes. Direct contact with vermiculite is relatively rare, since it is used as a constituent rather than a primary ingredient, although people who work with some forms of insulation may handle vermiculite.

There are two interesting things about vermiculite which make the mineral desirable to manufacturers. The first is its physical structure, which takes the form of crystalline layers like mica. The second is the water trapped inside the vermiculite. If the mineral is heated, the water turns to steam, forcing the mineral to expand, and the layers fold out like an accordion, creating strands of very lightweight, porous material. When vermiculite is used, it is usually heated to expand it, in a process known as exfoliation.

The name “vermiculite” is derived from the Latin word for worm, a reference to the wormy threads of material which form when vermiculite is exfoliated. This property was well known to people in the 19th century, although it was treated as a novelty rather than a potentially useful mineral until people realized its potential for things like insulation and concrete mixing. The mineral was named, incidentally, by Thomas Webb in 1924. It is more formally known as hydrated laminar magnesium-aluminum-iron silicate, which is rather a mouthful.

Some people have raised health concerns about vermiculite. The mineral itself is not harmful, but it often contains impurities which are not healthy, such as asbestos. In addition, the exfoliation process can generate silicate fibers which could be dangerous to inhale. Because removing impurities from the mineral is not really feasible, people should handle vermiculite carefully to reduce the potential exposure to health risks. When handling substances like insulation, nose and mouth protection should be worn so that people do not inhale small shards of silicates which could damage their lungs.

One of the largest sources of vermiculite historically was a mine owned by W.R. Grace in Libby, Montana. Although this mine is now closed, vermiculite can be found in other regions of the United States such as Virginia. There are also large mines in South Africa, China, and Australia. Depending on where it is mined, the mineral may have greater or fewer impurities.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a All Things Nature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By gimbell — On Jun 17, 2011

@seHiro - Real vermiculite and perlite also look different if you bother to examine them closely enough. As nextcorrea and whiteplane said here, they are minerals that naturally look a bit crystalline, and are actually very pretty to examine up close.

Foamy old styrofoam can't compare to crystals, so people should learn about vermiculite and perlite before they go using it in their potting soil, and then they'll be in the know enough to tell the difference just by looking.

I know after reading this article, I'm never going to be duped by any cheap company into planting things with styrofoam in the soil.

By seHiro — On Jun 14, 2011

@TheGraham - Perlite and vermiculite are almost the same thing -- I like to think of vermiculite as temporary perlite. Did you know that both of these minerals are volcanic? I'll bet that's why they're so full of holes and air pockets. Volcanic rock always seems to be very porous.

Not all of those "styrofoam spots" in potting soil are vermiculite or perlite. It sounds low of them, but a few companies who don't want to pay for actual vermiculite or perlite do in fact sneak bits of styrofoam into their potting soil mixes.

Most people don't know enough about vermiculite to know the difference -- after reading this article, that won't be anybody from this discussion. Just a word of warning for anybody out there who decides to start dumping potting soil from the plants they have bought into their garden beds.

To tell the difference between vermiculite, perlite and styrofoam, try crushing a piece of the material between your fingers.

Perlite will crunch up or not break at all. Vermiculite tends to mush or squish, especially if you get it wet first, and not return to ordinary shape. Last but not least, styrofoam will of course have a foamy texture that makes it spring back to its original shape the moment you stop squeezing it.

By TheGraham — On Jun 12, 2011

I first read about horticultural vermiculite in my adventures in growing venus fly trap plants, which require a very humid environment and lots of watering.

I wanted to add a potting soil mix with vermiculite to the glass container that I grow my venus fly traps in, but carnivorous plants get their nutrients from bugs, and actually require poor soil, so I was afraid I would fertilize them and harm them. Determined to try vermiculite, I bought some of it plain in a bag and added it to the soil.

It worked out great -- until the vermiculite turned to mush and stopped holding all of that moisture. As it turns out, vermiculite collapses once it becomes too wet, and the liquid-holding properties disappear.

I went to the local plant nursery where I bought the vermiculite and asked what had gone wrong, and they explained that vermiculite does that normally -- it's perlite that lasts forever. Apparently sometimes people prefer vermiculite to mush away after awhile because they feel it is more biodegradable.

Needless to say, I got some perlite and repeated the process of adding it to the venus fly traps' soil. That was a couple of years ago, and the perlite is still there.

Conclusion: perlite is better than vermiculite in my book.

By hanley79 — On Jun 10, 2011

@jonrss - That sounds like one of the better vermiculite uses out there! I always thought that growing mushrooms was tough and messy, but I might be able to grow some using vermiculite.

I've read before coming to WiseGEEK here that you have to grow mushrooms on old rotting wood, like stumps, or that you have to use straw packed in around them base they grow on to keep them moist.

Are you saying your friend didn't use anything except the spores and the vermiculite to grow them, then? No straw or other big mess like that? I just might go out and get myself some vermiculite if that's the case.

Thanks for explaining about this. It's not a wonder that your friend had tons of vermiculite on hand -- sounds like a wonder material for mushroom growing, and he was using it for fancy ones, too. I just want to grow the little common white kind!

By SkittisH — On Jun 10, 2011

Wow, so that white styrofoam looking stuff mixed in with my potting soil is a natural mineral? I never would have called that! It looks just like styrofoam, and kind of feels like it, too. I always wondered why they put styrofoam in potting soil!

It's kind of strange that there is vermiculite insulation too, though. You would think something found in house insulation, especially something that might have asbestos in it, wouldn't be healthy to have near your plants.

If there is asbestos in your potting soil vermiculite, will it leak that asbestos into any vegetables or other edible plants that you grow in that soil? I noticed the white vermiculite spots that look like styrofoam only seem to show up in potting soil for things like flowers, not veggie starts.

Is it because the vermiculite would make the vegetables dangerous to eat? I need to know this stuff before I choose what kind of potting soil to use for the little backyard garden in planting pots that I'm starting up this summer.

By whiteplane — On Jun 09, 2011

@nextcorrea - I agree, vermiculite looks incredible. From a distance it just looks like little dots, but when you go up to examine it you realize how many tunnels and ridges run through the surface. It is like a labyrinth or something that has been carved out by craftsman. I feel like I could stare at one of those little pieces for hours.

By jonrss — On Jun 09, 2011

I had a friend once who grew a variety of exotic mushrooms for fancy restaurants around town. He had literally 10 different kinds of mushrooms that I had never heard of. Vermiculite was a big part of the growing process.

When the mushrooms are actually sprouting out of their spore clusters they have to be kept very moist. My friend would make a large bed of wet vermiculite and place the spore clusters on top. The vermiculite would stay wet for days and would provide the necessary moisture to sprout the mushrooms.

It seemed like at any given time he would have at least 10 bags of vermiculite lying around. he probably got a great deal buying it in bulk.

By nextcorrea — On Jun 09, 2011

Many people may not have heard of vermiculite but they have probably come into contact with it in common soil mixes.

Whenever you get a potted plant and you notice the little white dots that are everywhere in the soil, that is vermiculite. It helps the soil to both retain water and to increase drainage.

It is really a cool looking substance. If you find some around, wash it off and take a look. Its like a round diamond painted white.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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