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Vermiculite as an Industrial Mineral

Vermiculite is the mineralogical name generally applied to a group of hydrated, trioctahedral ferromagnesian aluminum platty or micaceous silicate minerals. In microscopic particles the mineral is differentiated from montmorillonite by a higher Si:Al ratio of approximately 3:1 and a higher layer charges and higher cation exchange capacities. Commercial vermiculite, the vermiculite which is mined, beneficiated and discussed here, is an important industrial mineral which is formed by the alteration of macroscopic particles of biotite and iron-bearing phlogopite. In natural form it has the size and shape of mica, while possessing the ion exchangeable properties of zeolites and some clays.

Within the internal structure of vermiculite lie water molecules, which when rapidly heated to high temperature, transform into steam causing the vermiculite particles to increase in volume. This process of thermal exfoliation produces a lightweight product which finds use in various construction products, agriculture, horticulture, and other industrial applications. In exfoliated form it has the low density and biological inertness of expanded perlite, while maintaining a chemically active surface. Most of the uses of vermiculite are for the thermally exfoliated material. Trade names such as Strong-Lite and Zonolite are sometimes used to denote thermally exfoliated vermiculite and their related products.

In addition, the cation exchange potential of vermiculite and its layer-silicate structure are properties which can be used as the basis for numerous products including intumescent coatings and gaskets, inorganic films, and the treatment of nuclear and other toxic waste. When chemically and physically modified, it possesses some properties common to organoclays and synthetic compounds. In high purity concentrates and manufactured products, these properties can be altered and optimized in applications to produce superior performance.

Basic Definitions

A common shortcoming in a surprisingly large number of technical papers is the lack of definition for the vermiculite being discussed. I sometimes find that even after reading the entire paper, I cannot determine if the work involves thermally exfoliated vermiculite or the raw concentrate produced at the minesite. Clearly, a simple and concise definition of vermiculite is needed. The original description given by Webb in 1824 gives the essential aspect of commercial vermiculite: "If subjected to the flame of a blowpipe, or that of a common lamp, it expands and shoots out into a variety of fanciful forms, resembling most generally small worms having the vermicular motion exact". This definition is elegant in it's simplicity and lack of technobabble. However, it is not adequate for studies dealing with clays, soils and chemically modified materials. The identification and interpretation of vermiculite in these instances requires x-ray diffraction analysis. In order to further contrast the interests of soil scientists and clay mineralogists with industrial applications of the mineral, the term commercial vermiculite is used to indicate the vermiculite of commerce. Commercial vermiculite is normally a beneficiated product composed of particles generally larger than one millimeter in size.

The term exfoliation is used when the layers within the vermiculite structure are separated from each other in a direction roughly perpendicular to the silicate sheets. This term is used in preference to the nondirectional behavior described by expansion (such as with perlite). The physical thickness of the vermiculite particles can be altered by both thermal exfoliation and chemical exfoliation.

A term which should be avoided is hydrobiotite . This term, although widely used, is not applicable to commercial vermiculite. Hydrobiotite suggests that water molecules are incorporated into the biotite structure during the initial growth of the crystal. Although this might be possible, the water molecules in vermiculite have been introduced as a result of the weathering and alteration of biotite. It is very likely that the original papers dealing with hydrobiotite were actually dealing with mixtures of potassium, calcium, and magnesium vermiculites and not interstratified biotite and vermiculite.

Although the confusion caused by these early studies has been resolved, the abuses resulting from the mining and marketing of vermiculite still exist. The common usage of the term vermiculite for the thermally exfoliated product is so entrenched with contractors and the general public that it would be easier to redefine the naturally occurring material. I restrict the term vermiculite to refer to the untreated mineral and modify the term to indicate any modification of the original material. Terms commonly used include thermally exfoliated vermiculite (TEV), chemically exfoliated vermiculite (CEV), and ground vermiculite (CEV).

The production of commercial vermiculite began in 1915 with the unsuccessful marketing of "Tung Ash", the name given to vermiculite mined near Hecla, Colorado. In fact, vermiculite was inadvertently mined in the 1800's as a major accessory mineral in the Jenks Mine, NC, corundum deposit. The vermiculite mine started by the Zonolite Company at Libby, Montana in 1921 was the first successful venture in the vermiculite industry.

The Libby deposit was the oldest and largest producing vermiculite mine. It had been developed and enlarged for almost 70 years until it was closed in 1990. Vermiculite had also been mined for short periods of time in other states including Colorado, Nevada, North Carolina, Wyoming and Texas. Current production in the United States is restricted to mines in South Carolina, Virginia and Montana. Production of vermiculite outside the United States is predominantly from Phalabowra, Republic of South Africa. Other countries currently producing vermiculite include Brazil, China, Egypt, India, Japan, and Russia.

Commercial vermiculite is generally marketed as concentrates of sized particles, with an average grade (i.e. purity) of 90% by weight. Deposits of vermiculite ore may sometimes contain zones of almost pure vermiculite, but vermiculite ore is normally a mixutre of vermiculite and gangue minerals. Common gangue minerals include pyroxenes, magnetite, feldspar, and apatite.

There are various techniques which can be employed to recover vermiculite concentrates from ore. These techniques include both dry and wet techniques such as froth flotation, gravity separations, winnowing, and electrostatic separation. No matter what techniques are used, the resulting concentrates are screened to sized particles to become the vermiculite of commerce. There are two widely used product classifications. The American system is the oldest and is based on the Tyler series of sieves (e.g. 3 mesh, 6 mesh, 10 mesh, etc), while most foreign producers screen their concentrates to Metric specifications (8mm, 4mm, 2mm, etc).

The crystal structure and chemistry of vermiculite is well known, especially to those working with commercial vermiculite. A brief review of the structure and crystal chemistry of vermiculite seems appropriate to clarify the nomenclature used in this area.

Commercial vermiculite (i.e. the material which is mined and beneficiated) is formed by the near surface weathering of rocks containing large crystals of micas known as iron-bearing phlogopite and biotite . The difference between iron-bearing phlogopite and biotite is simply the amount of iron substituting for magnesium in the phlogopite structure.

The basic structure of mica is shown below. The prominent feature of the structure are sheets of silica and alumina tetrahedra (i.e. a pyramid of with three oxygen atoms forming the base, one oxygen atom forming the apex, and a silicon or aluminum atom hidden in the center of the group) linked together in an hexagonal array, much like honeycomb. These sheets, called tetrahedral layers my mineralogists, are shown in edge-on views in the drawing. If one were to view them towards their flat base, one would see only oxygen atoms linked in a thick-walled honeycomb outline with six sided holes that are not quite large enough to contain a sodium ion. The other side of these sheets are made up of the fourth, or apical, oxygen of the tetrahedra. The silicon and aluminum atoms are tightly held in the center of these oxygen groupings and are not generally shown in structure drawings because of their small size.

The resulting sandwich of tetrahedral and octahedral layers is the basic building block of the micas, chlorites, and vermiculites. This sandwich is composed of two tetrahedral layers bound together by one octahedral layer and is often referenced in the literature by a term such as a 2:1 phyllosilicate. (The term phyllosilicate means sheet or layer silicate, and is used to describe minerals such as mica, vermiculite, talc, kaolinite, bentonite, etc.).

What is important to remember in both the genesis and product development of vermiculite is that the tetrahedral layer is the basic building block of the structure, the octahedral layer the connecting unit of the structure, and the interlayer is the the filler unit of the structure. When biotite is altered to vermiculite the interlayer changes from potassium ions sitting on the surface of the silicate sheets, to hydrated units of calcium or magnesium ions surrounded by closely held water molecules. This structure is shown below:

Vermiculite as an industrial mineral shares a number of useful properties with other commodities. In natural form it has the size and shape of mica, while possessing the ion exchangeable properties of zeolites and some clays. In exfoliated form it has the low density and biological inertness of expanded perlite, while maintaining a chemically active surface. When chemically and physically modified, it possesses some properties common to organoclays and synthetic compounds. In high purity concentrates and manufactured products, these properties can be altered and optimized in applications to produce superior performance.

A number of useful products have been developed to take advantage of these properties, and the commercial vermiculite industry has grown because of this. Still, many interesting properties of vermiculite have yet to be fully explored. The area of developing technologies offers the new area of growth in the vermiculite industry, and the promise of profitable new markets for vermiculite.