Handbook of Fluorescent Gems and Minerals - An Exposition and Catalog of the Fluorescent and Phosphorescent Gems and Minerals, Including the Use of Ultraviolet Light in the Earth Sciences

By Jack DeMent

This vintage book contains a detailed exposition and catalogue of the fluorescent and phosphorescent gems and minerals, and includes detailed information on the uses of ultraviolet light in the earth sciences. This fascinating and comprehensive handbook is highly recommended for those with an interest in fluorescent minerals, and it would make for a worthy addition to collections of allied literature. The chapters of this volume include: “Introduction to Gemmology”, “Radiation Sources and Technique”, “The Florescent Minerals”, “Appendix One”, “Appendix Two”, and “Abridged Bibliography”. Many antiquarian books such as this are increasingly hard to come by and expensive, and it is with this in mind that we are republishing this book now in an affordable, modern, high quality edition. It comes complete with a specially commissioned new introduction on gemmology.

• Language: English
• Publisher: Read Books Ltd.
• Release date: May 31, 2013
• ISBN: 9781473382763

Book preview

BIBLIOGRAPHY.

Part One

Radiation Sources and Technique

The earliest interest in luminescence was the outgrowth of effects of sunlight on minerals and a few synthetic preparations like the Bologna Stone of Cascariola. Later it was found that both natural and artificially created light could be so altered as to make luminescent effects more manifest and thus more easily seen and studied.

A great impetus in interest arose with the development of inexpensive, efficient, and standard light sources. In early years lamps sometimes depended upon acetylene-air flames of low luminosity, or on flames of burning carbon disulfide which had like properties. Later it was found that sparks, arcs, and incandescing metal vapors could be employed as sources of energy for producing fluorescence and phosphorescence.

Prior to a decade ago, ultraviolet lamps were bulky, costly and unsatisfactory for many applications. The development of the modern ultraviolet lamp has unquestionably stimulated studies by a large number of workers. The results of this are the many valuable uses which have been found for luminescence, most of which lie outside the immediate field of mineralogy.

Modern light sources date from the time of the development of the carbon arc lamp and the mercury arc lamp, to a lesser extent the X-ray. Some utilization has been made of incandescent filament lamps, but it was not until the tungsten filament lamp was developed that this source was widely used. Today, increasing favor lies with the mercury vapor lamp; gradually the carbon arc and filament lamps are being forced from the picture.

The carbon arc dates from the time of Sir Humphry Davy who, in 1808, produced an arc three inches long with a 2000-plate voltaic battery. In principle, the modern arc is identical with that of Davy, except for refinements in operation, mechanical construction, energy consumption, and electrode material.

The first observation of the brilliant light emitted from mercury vapor through which passed an electrical discharge is ascribed to Wheatsone in 1835. However, it remained until some years later for this observation to be commercially realized. Between 1852 and 1867 several English patents were granted on enclosed mercury vapor arcs. Later solid metal electrodes and also mercury electrodes were incorporated into the design of this lamp, and finally it was seen that the best light source of all was to be obtained by enclosing at reduced pressure the mercury in a quartz envelope, so that the large amount of invisible ultraviolet light could pass through and be utilized externally.

The study of the discharge of electricity through gases and vapors at reduced pressures not only led to the development of excellent mercury vapor light sources, but also to the discovery of cathode rays or electrons, positive rays, X-rays, and numerous other phenomena, many of which are of interest in luminescence. It was during the course of a systematic research for a possible radiation capable of traversing matter opaque to ordinary light that Roentgen discovered the X-ray in 1895. Here, too, electric discharges passing through gases were used. Roentgen had been studying the ultraviolet light given off from an electric discharge passing through a highly evacuated tube, and had been using crystals of barium platinocyanide spread onto a paper screen to detect the presence of rays too short to affect the eye. These crystals fluoresced and Roentgen subsequently observed a new kind of light which passed through opaque objects, calling this X-rays, a name indicating the then unknown character of the radiation.

The discovery of radioactivity is more or less related to Roentgen’s work in 1895. Immediately after his work it was suspected that the emission of X-rays might in some way be connected with the fluorescence of the glass walls of the X-ray tube. Hence, other observers searched for a penetrating radiation which might be given out by the more common fluorescent and phosphorescent materials. In 1896 Henri Becquerel found that uranium salts, then known to be brightly fluorescent, emitted a penerating radiation. Many invesigations followed that of Becquerel. Such names as Rutherford, Soddy, and the Curies are associated with these early researches. Other elements were ultimately found to give off penetrating rays, and the culmination of this work in the discovery of radium by the Curies is a well known story.

TECHNIQUE

Before luminescence is produced a substance must have what is technically called a radiation history. This means that some form of energy, usually light energy, must be first taken up by the matter. In ordinary work, such as with minerals, the proper radiation history is purposely developed by a special light source like the ultraviolet lamp. Within an exceedingly small part of a second the effects of this light can be seen in certain materials as a glow or fluorescence or as an afterglow or phosphorescence.

Exciting radiation sources are of two kinds: (a) optical, releasing light somewhere in the electromagnetic scale and (b) corpuscular, consisting of streams of moving particles of matter of definite mass and charge. The most popular optical excitant is the ultraviolet or black light source. Likewise, electrons or cathode rays are the more favored corpuscular energy sources.

In the ordinary study of mineral luminescence one wishes to observe with his unaided eye. Thus, one must give some thought to the best kind of light source for creating visible luminescent light. Since the wave length of the luminescent light is generally longer than that of the exciting light it is then obvious that an invisible form of light of less wave length than the visible is best. Such light is ultraviolet radiation.

Most ultraviolet light sources develop visible light along with the invisible, so in order to eliminate the obscuring effect the visible rays would have on the visible luminescence, some means must be taken to exclude the visible light from the lamp, and yet allow the invisible ultraviolet to get through. This is accomplished by means of a device called a filter, which usually is made of high silica glass with purple or blue pigments which absorb the visible, but allow the ultraviolet to pass.

Thus, in most observation and studies of mineral luminescence subjective (unaided eye) methods of inspection, less frequently the objective (instrumental) are employed. The reason for using the unaided eye and words to describe what is seen is fairly evident.

Words consist of a simple and easy, though not overly accurate, means of describing the effect which takes place when a mineral is placed under the ultraviolet lamp. Words are also favored because of the inaccessability of the nontechnical student to expense and specialized instruments. One difficulty in obtaining more accurate results lies also with the lack of standard light units and methods of observation. Such studies would include theoretical and laboratory research on both exciting and emitted light.

Ordinarly specimens of minerals are placed in ultraviolet light in a more or less random fashion, in a darkened room. This suffices for general inspection, since a markedly fluorescent solid will respond irrespective of its position in the beam. It is preferable to examine a freshly broken surface of a mineral. Minerals can also be pulverized and inspected at different particle sizes. This can be accomplished with a mortar and pestle and seives. Acids can be used to etch out constituents of a heterogeneous specimen like the calcite which accompanies willemite.

In all fluorescence examinations the eye should be accustomed to the dark, since the emitted light may be very feeble. All low-order-of-brightness purple and blue fluorescence should be looked upon dubiously, because the filter over the lamp passes these colors and one can easily mistake a residual color for a feeble fluorescence.

For producing phosphorescence the lamp should be used without its filter. The sample is exposed for a minute or so, the observer keeping his eyes tightly shut, the lamp turned off, and the rock inspected in a darkened room. An afterglow may be much weaker than a fluorescence so one should learn the method with known phosphorescent samples.

It is simple to use a small hand spectroscope for observing the spectra of luminescing objects like minerals. These instruments can be obtained for several dollars, having replica grating. The slit on the instrument should be opened wider than usual, up to 0.5 millimeter. The spectroscope, preferably one with a scale, should be mounted at right angles to the ultraviolet beam, with the specimen in between. Spectroscopic examinations are made in a darkened room. Needless to say, spectroscopic studies are far superior to any unaided eye inspection of fluorescence.

In the types of luminescence known as thermoluminescence and triboluminescence some precautions are in order. For triboluminescence the eye should be dark-adapted, and the samples can be gently rubbed together, or against cloth.

In thermoluminescence the sample is placed on a small electric hot plate which has a sheet of metal over the top. A small non-luminous flame does just as well.

These observations should be preceded by a study of known minerals possessing the properties. Sphalerite from Tsumeb, South Africa, is good for triboluminescence, as are uranyl salts. Fluorites from Ohio and England can be studied for thermoluminescence.

VISIBLE LIGHT SOURCES

The best source of intense visible light is the incandescent filament, and to a lesser extent, the carbon arc. Tungsten filaments operating efficiently can be employed as a source of exciting radiation because of the uniform operation of incandescent lamps. This type of light source is especially applicable to photometric measurements. In fluorescent microscopy a small incandescent lamp operating from a dry cell or step-down transformer is effective for some purposes, although the usual glass filter is requisite.

It is possible to employ ordinary flashlight bulbs operating at nearly double their rated current value for micro-sources of exciting radiation, but in this case also a filter must be employed. It should be remembered that the incandescent tungsten filament lamp operated at abnormally high filament temperatures has short life, despite its value as an intense source of continuous visible and long wave length ultraviolet radiation. The commercial photoflood lamp is a good example of filament lamps operating under excessive current.

The General Electric Photoflood number 4, operating at 120 volts, has a rated life of ten hours and is useful for exciting luminescence.

When the exciting energy is to be mostly visible light, consideration need not be taken of the glass envelope on an incandescent filament lamp. However, if this source is to be employed for ultraviolet light the envelope blocks the shorter wave lengths. Certain cases may arise in which a quartz envelope could be employed, but many other sources are better so a choice should be made from a variety of apparatus.

The automobile incandescent lamp emits wave lengths into the middle of the ultraviolet. Even in the case of a lead-glass envelope wave lengths less than 3000 A. U. can be detected when the filament is operated at 3400°K. At this temperature the life is of course quite short, usually but a few hours. With a sodalime envelope radiations can be detected at 2900 A. U., but the amount is negligible for practical purposes.

Strips of burning magnesium ribbon were used some years ago in determining whether a substance exhibited luminescence. The practical drawbacks of this radiation source limited its wide usage. The present source of intense, but brief, visible light which utilizes oxidation of metail foils is the photoflash lamp so familiar in photographic circles. The usual photoflash lamp consists essentially of a very thin sheet of aluminum or alloy of aluminum within a glass envelope which contains an excess of oxygen. The combustion is initiated electrically by wire leads into the tube; only a relatively small potential difference is needed to start the action.

The light from a photoflash lamp is very intense, often greater than 360,000 candlepower, but it only lasts a few hundredths second, and renders this source of little value in most studies on luminescence. It has, however, decided values in studies on phosphorescence where it is necessary to regulate the quantity of light energy absorbed by a system. An apparatus can be easily constructed in which a photoflash lamp is used to add energy to a luminescent system, and immediately after, generally from between 0.03 and 0.06 second, study of the afterglow is made. Such an arrangement provides a novel approach in the study of phosphorescence of quick duration and of feeble