Silica is also a name for a grade of sand employed in water filtration systems, and has a different structure than a “silicate” (SiO3= Silicon Trioxide) or (SiO4) which has an extra Oxygen atom or two attached to its Silicon atom. Silicates (silicon trioxide) form the basis for clays (Silicon along with Aluminum--that “silica” does not employ). Diatomite is often referred to as a diatomaceous earth, but this appellation may be inappropriate, especially for clays, even though they also are the result of trillions of defunct diatoms piling up. The reason for this has more to do with the endurance of the tiny shells of certain species of diatoms upon handling by machinery. Different proportions of other elements joining with Silicon to form either a clay (e.g., Bentonite, ) or a non-clay, such as diatomite influence this. Thus, endurance of a particular mineral crystal is dependent upon the unique elements comprising its make-up, as well as, the geologic processes leading to its formation.
Furthermore, there is a distinction between diatomite and actual diatomaceous earth. When aquatic diatoms died and dropped to the bottom of the seabed, the shells in old deposits, not being subject to decay, collected in the primordial ooze and eventually formed the classical material known as diatomaceous earth (sometimes called “kieselguhr”). When the “ooze” began to solidify from pressure after being buried, it became a more compact substance. In this new form as a light-weight rock, soft and chalky, it then may be termed diatomite technically. http://www.mii.org/Minerals/photodiatom.html
The typical chemical composition of diatomaceous earth, a non-clay, is 86% silicon, 5% sodium, 3% magnesium and 2% iron. http://en.wikipedia.org/wiki/Diatomite Clays by definition are termed aluminum silicates and their proportions of Aluminum are generally more like 4/10 of the Silicon content, but with a lot of trace elements and additional minerals imbedded in humic matter included in their bulk.
For the most part diatomite (originates as diatomaceous earth) and calcite, for example, were formed in shallow marine environments, and are therefore, much older than many clays (such as Montmorillonite, a very soft phyllosilicate) that were formed in more recent times, and in fresh water lakes. The fact that the fossilized remains of algae forming calcite, diatomaceous earths and diatomite, were buried for long periods under the ocean and protected from certain organic impurities infiltrating their strata, not only left them a more homogeneous material, but the added pressure, over time, often increased their hardness.
Consequently, the crystalline structures formed from silica are more glass-like than clays.
A thorough quotation from a USGS pamphlet explains:
Once deposited, siliceous diatoms are more resistant to dissolution than other types of biogenic matter; however, the deposits are subject to diagenesis, burial, and exposure, all of which can have significant effects. Diagenesis may preserve a deposit, enhance its quality, or completely destroy it, depending on the type and severity of the process. A sediment cover can preserve the deposit by protecting it from erosional processes; in addition, a sediment cover can compact the diatomite and remove excessive water from the diatomaceous materials. Leaching may remove organic matter and other biogenic materials from the diatoms, and enhance the deposit quality… [Barron 1987], however, observed that deeper burial and related increasing temperature can cause recrystallization of frustules and destruction of some of their useful physical properties. He described a progression of rock-property changes, associated with burial depths ranging from 600 to 1,500 m, that transition from biogenic silica to opal-CT at 50°C and then to quartz at about 80°C. The lithified final products of this diagenetic process, porcellanite and chert, have significantly lower porosity and (even) higher density than those associated with commercial diatomite. http://pubs.usgs.gov/bul/b2209-d/b2209d.pdf
When diatomite crystals rupture, the result is that sharp shards are produced that can puncture the integument of insect pests--also a characteristic of true diatomaceous earth. Pulverized diatomite and diatomaceous earth are both rough, powdery substances that have a textured feeling similar to ground pumice. Especially diatomaceous earth is well known as an insecticide due to its abrasive properties. The fine powder scratches and wears away (abrades) the waxy outer layer of insects' integument, or exoskeletons, and when adhered to the often-perforated cuticula, its absorbency causes the insects to dehydrate. This also works against garden snails and slugs (gastropods). However, the absorbent qualities of diatomite can result in a significant drying of human hands if handled without gloves.
Furthermore, the sharpness of its crystals makes saltwater diatomite dangerous to breathe and a dust mask should be required when working with it. The precise hazard posed by inhalation depends on the form of the silica. Crystalline silica (silicon dioxide) poses a serious inhalation hazard because it can cause silicosis and can eventually lead to cancer. Amorphous silica can cause dusty lungs, but does not carry the same degree of risk as crystalline silica. http://en.wikipedia.org/wiki/Diatomite
An extreme example of the dangers associated with breathing air-borne particles of silica dust comes to us from the history of a now-defunct mining operation in Lincoln County, Nevada, known in former times as, the “Widow-Maker”.