The 20th century has seen the birth of three Ages, each with profound social implications. These have been called the Nuclear Age, the Electronic Age and the Chemical Age. The latter is the oldest (beginning ca. 1930), and although its impact has been less dramatic than the other two, its consequences have more thoroughly and deeply permeated our day-to-day lives. Our local grocery, hardware, garden and drug stores carry an impressive array of commonly used chemical "tools", such as detergents, adhesives, lubricants, fabrics, pesticides, pharmaceutical drugs, vitamins and a multitude of fabricated plastic items. Industrial applications of chemical tools include explosives, heat-transfer gases and liquids, specialized coatings, fire retardants and high-performance plastic components.
Despite our widespread use of chemical tools, indeed some might say because of our reliance on them, many people fear exposure to these materials, and have deep concerns regarding the use, storage and disposal of chemicals. Paradoxically, we find our desires for more abundant consumer goods, energy and personal mobility in conflict with maintenance of a healthful environment. To be sure, environmental degradation, with accompanying threats to health and disruption of ecosystems, is not a new phenomenon. From the earliest recorded history, human disturbance of the environment by deforestation, air pollution from cooking and heating fires, and careless sewage and waste disposal has been noted. Today, as global populations grow and per capita energy use and material consumption increases, pollution problems are exacerbated, and previously unnoticed secondary effects manifest themselves.
Every day we take risks and avoid others. About 250 people in the U.S. are electrocuted every year in accidents involving home wiring or appliances. This represents a risk of death of about 8*10-7 per year (250 divided by the U.S. population) or 6*10-6 per lifetime (75 yr.). Nevertheless, most of us choose to live in electrically wired homes, and make extensive use of electrical appliances. Likewise, many people would be unwilling to live within 20 miles of a nuclear power plant, yet accept (even request) a 4,000 times greater radiation dose from medical x-rays or 6,500 times greater cosmic radiation at altitudes of a mile or more.
The concept of risk and the notion of uncertainty are closely related. The lifetime risk of dying from cancer is roughly 22%, and is somewhat greater for those who smoke. However, even if an individual is a heavy smoker, we cannot say with certainty he(she) will die of lung cancer. On the other hand, if that individual is dying as the result of a serious automobile accident, the risk of dying from cancer drops to nearly zero.
Using Chemicals
As with any other kind of tool, chemicals must be handled correctly, with proper care and precaution. Although chemicals vary in the hazards they present, it is generally wise to treat all chemicals as though they are potentially dangerous. Among the recognized hazardous properties of chemicals are: explosiveness, flammability, corrosiveness, irritation, sensitivity, toxicity and radioactivity. One of the most useful sources of information about chemical hazards is the material safety data sheet (MSDS). Information about these data sheets is available at MSDSonline. It is an interesting excercise to examine the MSDS for common chemicals such as acetic acid (vinegar) and naphthalene (mothballs).
Of all the hazardous properties noted above, toxicity seems to constitute the greatest concern in the minds of the public. Contrary to popular belief, the fact that a substance is toxic does not mean it will always kill people or animals exposed to it. Virtually all substances are lethal if taken in sufficient amount. As noted by the Swiss Physician Paracelsus, It is the dose that makes the poison! Thus, 1.5 grams of arsenic trioxide will kill a 180 pound man; 2 milligrams will not. Small amounts of vitamin D (ca. 10 micrograms per day) are necessary for good health, but in larger amounts it is more toxic than arsenic compounds.
Humans vary considerably in their sensitivity to natural and synthetic chemicals. Strawberry, peanut and latex allergies are relatively common, and reports of asthma-like symptoms on exposure to synthetic plasticizers exist. A more complex and less well-defined syndrome, known as multiple chemical sensitivity, is the subject of medical controversy, although it is very real to those who suffer its effects. One thing is certain. If you wish to avoid exposure to chemicals, the planet earth is a poor place to live.
Thursday, December 10, 2009
CLAYS
Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products (including weathering of feldspar) and low temperature hydrothermal alteration products. Clay minerals are very common in fine grained sedimentary rocks such as shale, mudstone and siltstone and in fine grained metamorphic slate and phyllite.
Clays are ultra fine grained (normally considered to be less than 2 micrometres in size on standard particle size classifications) and so require special analytical techniques. Standards include x-ray diffraction, electron diffraction methods, various spectroscopic methods such as Mossbauer spectroscopy, infrared spectroscopy, and EDS or energy dispersive spectroscopy. These methods should always augment standard polarized light microscopy, a technique which is sometimes overlooked but often where fundamental occurrences or petrologic relationships are established.
Clays are commonly referred to as 1:1 or 2:1. Clays are fundamentally built of tetrahedral sheets and octahedral sheets, as described in the Structure section below. A 1:1 clay would consist of one tetrahedral sheet and one octahedral sheet, and examples would be kaolinite and serpentine. A 2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, and examples are illite, smectite, attapulgite, and chlorite (although chlorite has an external octahedral sheet often referred to as "brucite").
Structure
Like all phyllosilicates, clay minerals are characterised by two-dimensional sheets of corner sharing SiO4 and AlO4 tetrahedra. These tetrahedral sheets have the chemical composition (Al,Si)3O4, and each tetrahedron shares 3 of its vertex oxygen atoms with other tetrahedra forming a hexagonal array in two-dimensions. The fourth vertex is not shared with another tetrahedron and all of the tetrahedra "point" in the same direction (i.e. all of the unshared vertices are on the same side of the sheet).
In clays the tetrahedral sheets are always bonded to octahedral sheets formed from small cations, such as aluminium or magnesium, coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also form part of one side of the octahedral sheet but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedra. This oxygen atom is bonded to a hydrogen atom forming an OH group in the clay structure. Clays can be categorised depending on the way that tetrahedral and octahedral sheets are packaged into layers. If there is only one tetrahedral and one octahedral group in each layer the clay is known as a 1:1 clay. The alternative, known as a 2:1 clay, has two tetrahedral sheets with the unshared vertex of each sheet pointing towards each other and forming each side of the octahedral sheet.
Bonding between the tetrahedral and octahedral sheets requires that the tetrahedral sheet becomes corrogated or twisted, causing ditrigonal distortion to the hexagonal array, and the octahedral sheet is flattened. This minimizes the overall bond-valence distortions of the crystallite.
Depending on the composition of the tetrahedral and octahedral sheets, the layer will have no charge, or will have a net negative charge. If the layers are charged this charge is balanced by interlayer cations such as Na+ or K+. In each case the interlayer can also contain water. The crystal structure is formed from a stack of layers interspaced with the interlayers.
Clays are ultra fine grained (normally considered to be less than 2 micrometres in size on standard particle size classifications) and so require special analytical techniques. Standards include x-ray diffraction, electron diffraction methods, various spectroscopic methods such as Mossbauer spectroscopy, infrared spectroscopy, and EDS or energy dispersive spectroscopy. These methods should always augment standard polarized light microscopy, a technique which is sometimes overlooked but often where fundamental occurrences or petrologic relationships are established.
Clays are commonly referred to as 1:1 or 2:1. Clays are fundamentally built of tetrahedral sheets and octahedral sheets, as described in the Structure section below. A 1:1 clay would consist of one tetrahedral sheet and one octahedral sheet, and examples would be kaolinite and serpentine. A 2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, and examples are illite, smectite, attapulgite, and chlorite (although chlorite has an external octahedral sheet often referred to as "brucite").
Structure
Like all phyllosilicates, clay minerals are characterised by two-dimensional sheets of corner sharing SiO4 and AlO4 tetrahedra. These tetrahedral sheets have the chemical composition (Al,Si)3O4, and each tetrahedron shares 3 of its vertex oxygen atoms with other tetrahedra forming a hexagonal array in two-dimensions. The fourth vertex is not shared with another tetrahedron and all of the tetrahedra "point" in the same direction (i.e. all of the unshared vertices are on the same side of the sheet).
In clays the tetrahedral sheets are always bonded to octahedral sheets formed from small cations, such as aluminium or magnesium, coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also form part of one side of the octahedral sheet but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedra. This oxygen atom is bonded to a hydrogen atom forming an OH group in the clay structure. Clays can be categorised depending on the way that tetrahedral and octahedral sheets are packaged into layers. If there is only one tetrahedral and one octahedral group in each layer the clay is known as a 1:1 clay. The alternative, known as a 2:1 clay, has two tetrahedral sheets with the unshared vertex of each sheet pointing towards each other and forming each side of the octahedral sheet.
Bonding between the tetrahedral and octahedral sheets requires that the tetrahedral sheet becomes corrogated or twisted, causing ditrigonal distortion to the hexagonal array, and the octahedral sheet is flattened. This minimizes the overall bond-valence distortions of the crystallite.
Depending on the composition of the tetrahedral and octahedral sheets, the layer will have no charge, or will have a net negative charge. If the layers are charged this charge is balanced by interlayer cations such as Na+ or K+. In each case the interlayer can also contain water. The crystal structure is formed from a stack of layers interspaced with the interlayers.
Subscribe to:
Posts (Atom)