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GeneralSilica from plants

29th Jun 2010 03:52 UTCBarry Miller

Is it true that the burning of certain vegetation occasionally results in accumulations of silicon dioxide that comes from the silica that occurs naturally in certain grasses? I heard this once when I was in college and always wondered if it's true.

29th Jun 2010 10:15 UTCAmir C. Akhavan Manager

Since the source of the silica is the soil, you would not expect to see an increase of total silica in the ground. Some of the silica might even be carried away by the wind during fires.

There might be a redistribution in the layers of the soil, for example, an otherwise mostly organic top soil might be enriched in silica. If there is a fire, a large part of the carbon in the plants will not go into the top soil as it normally does during decay, so the ash layer will be enriched in silica relative to regular top soil (just as it is enriched in calcite, phosphates, pottash, etc.).

The amount of opal-AG and opal-AN (and possibly tridymite, cristobalite and lechatelierite because of the heat) should be increased, though, as the silica is deposited in the plants as amorphous, opaline silica.

If the grass grows on a rock that is poor in free silica, like a foidite, the acids released by the plant roots might speed up the weathering of the rock, the plants might take up silica released during the decomposition of silicates, and when the grass burns down you might find a silica-rich layer on top of a silica free rock. Just an idea, I got no data on this.

29th Jun 2010 12:28 UTCPavel Kartashov Manager

Nettle and tobacco , for example, are rich by silica. Some gem cutters use tobacco ashes as polishing reagent due its silica content and its fine state.

29th Jun 2010 13:09 UTCAlfredo Petrov Manager

Grasses, bamboo and horsetails are all silica-rich. Barry may be referring to lumps of "ash glass", a silica-rich glass that can form when , for example, a haystack burns?

29th Jun 2010 13:39 UTCAnonymous User

Most plants absorb quantities of silica; most store or use tiny amounts that are not physically noticeable. Even those that have small amounts can be chemically detected, and can form concentrations when enough plant biomass is burned or decomposed under the right conditions. The surface soil is often rich in silica to begin with, as more soluble materials are dissolved and translocated down the soil profile in many soil types. This makes the extra silica taken in by plants hard to detect, and chemically insignificant in many cases. But as Amir said, if the surface soil is silica-poor, and the plant biomass rich enough in silica and/or concentrated enough, then increases in silica may be noticeable.

Plants that take up and use silica on a physically noticeable level include diatoms (single cells with cell walls composed largely of silica, and fossil deposits of which form beds of diatomaceous earth), Equisetum (horsetail as Alfredo mentioned, with enough silica in the outer cell walls to make most stems feel gritty), bamboo (which uses some in the outer cell walls, and stores more in the hollow stem in a form called tabashir), and many tropical timbers (where it is noticed as a dulling effect on saw blades). My plant physiology book here at the office also mentions that rice requires and takes up silica. It does not go into detail, but I think in general silica is used to strengthen cell walls and deter herbivory.

29th Jun 2010 15:33 UTCjacques jedwab

You should look after phytoliths: an immense subject. Many ways of access. Try google.

30th Jun 2010 00:12 UTCBarry Miller

Thank you all for your responses. Alfredo Petrov's reply is exactly what I was wondering about - "ash glass" from burning haystacks. Are these pictured anywhere? I Googled but did not find such a picture. Also, if these silica lumps are the result of a natural fire (i.e., not caused by man), would you consider them to be a mineral (or possibly a mineraloid if they have no crystalline structure - like opal)?

30th Jun 2010 00:42 UTCAlfredo Petrov Manager

Barry, some crystallized minerals have also been described from ash from trees burned in forest fires: see references for Fairchildite and Buetschliite here on Mindat. What mixtures of crystallized carbonates and crystallized silica, or carbonate glass and/or silica glass you get after a forest fire or grassland fire would depend on the composition of the plants in the mix, and how hot the fire got, I suppose. Also on what is left over after lixiviation of soluble Na and K carbonates after a rain. But yes, these substances could certainly fit the definition of mineral, unless you started the fire deliberately :)

(I can picture the headlines now: "Young Irish mineralogist arrested for attempting to create fairchildite by arson.")

On a tangential note, trees are sometimes burned in lava flows, and interesting products can be produced, which indubitably fit the definition of "mineral" - for examply Native Iron in a lava flow from Mt Fuji, the result of magnetite grains smelted by charcoal from the trees embedded in the lava. Don't know what would happen when lava runs through a silica-rich bamboo forest... probably doesn't get hot enough for pure silica glass.

30th Jun 2010 11:06 UTCRay Hill Expert

Hi Barry...just a little response to your question....first ..horsetail plants which usually grow in high silica sites, were used for scrubbing pots in the days of prairie and hinterland settling in Canada

I went on a trip to Jamaica for collecting Jamaican gem material,many moons ago...and my host, who owned a little shop in the International village just outside of Montego Bay, called Blue Mountain Gems...had cut some gems from the slag that formed on the inside of sugar boilers...What the practice used to be, before oil fired boilers for sugar cane became the norm, was that the cane was cut and the debris was fed into the boiler to fire it's rendering down of the cane juice, the result was an accummulation of layered and nicely coloured silica slag on the walls and bottoms of these old mill boilers. The old mills are pretty well all gone, and now the canes are burned right on the fields to concentrate the sugar in the cane before transport to the mills...which sadly, concentrates the silica in these horrible iron rich pre-laterite soils , pretty well guaranteeing that they will only have a future of cane harvests, because cane is hardy and can survive and grow where a lot of other crops cannot.

So much for progress. But it does illustrate that silica can be gathered from the burning of these stalks of silica rich sugar cane grass.

30th Jun 2010 12:20 UTCChris Stanley Expert

A good article on the quantification of silica in sugar cane and the possible dangers of silicosis on burning it can be found here



Chris S

30th Jun 2010 14:07 UTCGord Howe

I have pieces of ash from the wood bark combuster at a pulp mill I work in that look almost like obsidian. They are hard enough to be obsidian,too. The bark is from spruce and western hemlock and there are also microorganisms from the effluent treatment plant mixed in that may also contain silica. If the mixing chamber plugs up and the draught is right the silica will fuse, and if left long enough the whole mixing chamber will eventually fill up with fused ash that end up having to be jack hammered out.


30th Jun 2010 16:09 UTCAlfredo Petrov Manager

Fabulous, Gord! Bring some of that stuff to a mineral show... I'm sure lots of collectors would like to see it.

30th Jun 2010 22:24 UTCAmir C. Akhavan Manager


> http://www.rsc.org/publishing/journals/EM/article.

> asp?doi=c0em00020e


I've only read the accessible abstract, but one thing they might have got wrong and at least could be misunderstood: cristobalite will not only form from amorphous silica when temperatures reach its stability field. This also happens at much lower temperatures. At least when you start with pure silica glass at 800 deg C, for example, cristobalite is the first to form, not (high) quartz. This is also true under diagenetic conditions (formation of tridymite and cristobalite from opal in ageing diatomites) or in hot springs (sequence: opal-AG -> cristobalite -> tridymite -> quartz). Generally the transition from an amorphous state to a stable crystalline polymorph will take place in steps from the least stable to the most stable polymorph.

The fact that quartz is present in higher concentrations than cristobalite is still in accordance with this, as the transition to quartz (possibly via tridymite) might be kinetically promoted by the other elements in the ash (or quartz was already present in the plants).

BTW, laterites are actually deprived of silica and other soluble components and enriched in iron and aluminum.

1st Jul 2010 01:40 UTCIbrahim Jameel Expert

I haven't read all the above posts, but I would guess it is true. Case in point would be rice.

Silica concentrations in the dry hull can be as high as 21.5 wt%1 and 12 wt% in the dry leaves2. The total concentration of other inorganic oxides in relation to the silica content is close to 5 wt%. Hulls are almost 20 wt% of the dry seeds; thus, the total amount of silica in rice hulls can be as high as 4 wt% of a rice crop2. The annual worldwide output of rice hulls is approximately 80 million tons, which corresponds to 3.2 million tons of silica3

Source: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-14392000000200005

I don't know how exactly accurate these numbers are, but those for the silica content are within the ballpark of what I have seen in the past.

Most of the husks get burned, resulting in the release of massive amount of sillica, in addition to other pollution from the burning organics...

One of my professors mentioned hat there was a start up that was trying to use the rice hull ash to produce micro or nano (cant remember which) sillica particles for use in other applications.
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