Many types of earthy and dispersed forms of opaline material come into this category. They may form large deposits of white to off-white, sometimes powdery, sometimes compact, material. The best known of these types of deposits are those known as Diatomite, or diatomaceous earth. They may contain a proportion of kaolinitic clay, when they are termed opal claystone. The latter occurs in large amounts in south eastern United States, where it occurs in extensive bedded deposits.

Opaline silica is mined on a large scale for use, for example:

	in the ceramic industry
	as a filtration agent
	as an absorbent
	as a filler
	for the manufacture of dog and cat litter

Similar deposits occur in southern Russia, where the material is called 'opoka', and in western Europe, where it is variously termed tripoli, gaize, kieselguhr or moler depending on its nature and country of origin footnote.

Soft earthy deposits are often composed of the skeletons of minute marine or fresh water organisms called diatoms. Diatomite, which is soft and of low density, is a good insulating material, and is widely used for this purpose. It is used as well for the manufacture of lightweight refractories, as an absorbent, filler and filtration agent. Large deposits occur in eastern Australia, and are described by Irene Crespin (R1548).

In parts of the deep oceans extensive deposits of siliceous sediments occur. These are often composed of radiolaria and diatoms which are minute organisms which have silica skeletons; these fall to the ocean floor on the death of the organisms, and, over long periods of time, build up extensive layers. In recent years scientists studying deep sea deposits have also found large deposits of other types of fine opaline material in cores from deep ocean drilling. The opal in these samples has often recrystallised in part, forming rosettes of tiny platelets of crystals of opaline material; these structures have been termed 'lepispheres'. On land, these materials are represented by, often large, deposits of fine grained silica such as diatomite, tripoli or opoka. More details on these materials will be found in later sections.

Plants may also secrete opaline silica in their cells and other structures, and, on the death and decomposition of the plant the silica is released into the soil as minute spicules or other forms. Certain plants, such as the equisitalia ('horse tails') and the myrtacae, are particularly rich in silica; the ash after burning some of these plants has been shown to contain as much as 93% silica.

Evidence of the presence of this opaline material is sometimes found in the form of large pieces of so-called 'haystack glass', formed on the accidental burning of haystacks(pb46). The potash and other inorganic elements in the hay combine and melt in the heat of the burning hay to form lumps of glassy material.

A dramatic example of this may be found in an article by George Baker (R0363), where he describes the results of the burning of two haystacks in western Victoria. He found that some 16 tonnes of hay-silica glass were produced from some 330 tonnes of hay from pasture grass.

When plants such as grasses decay in the soil, the opal in their cells is released as minute spicules and other transparent particles, accumulating in the soil. This can be readily seen with an optical microscope.

The difference in character between the silica-rich grass and hay ash, and the silica-poor ash from, for example, softwoods, is known and used by the traditional art potter. Ash is commonly used for special effects in ceramic glazes; the hay ash (high in silica) is known as 'hard ash', while the low silica ash is termed 'soft ash'.