11. Fluorescence of minerals in ultraviolet      

History and introduction

The property of mineral fluorescence was known long before it was observed locally. At Franklin, it was first observed in the mine at the beginning of the twentieth century when the opening and closing of direct-current knife-switches caused electrical arcs, and when electrical flashes from poor connections on electrical contacts emitted sparks. Nearby minerals fluoresced in the spark-emitted radiation, and a new and exciting dimension of Franklin’s mineralogy was born. A substantial number of local minerals fluoresce; they emit visible light when subjected to ultraviolet radiation. The fact that there are more spectacularly fluorescent minerals found here than anywhere else in the world prompted the New Jersey State Legislature on September 13, 1968, in a moment of insufficient humility, to declare the Borough of Franklin the “Fluorescent Mineral Capital of the World.”

Early in this century, the New Jersey Zinc Company developed an in-house, direct-current, iron-spark unit. Later, when alternating current was adopted, an iron-spark instrument was developed by the General Electric Company (Andrews, 1916; Heineman, 1935; Gaines and Bostwick, 1993). These instruments were used to monitor the local ore-handling processes and were used in mineralogical laboratories as well. The ultraviolet fluorescence observations of Palache (1928b, 1935) were largely based on the iron-spark instrument.

Modern observations are made with low-pressure mercury-vapor tubes, which were originally made of fused quartz and were referred to as “cold quartz” tubes. These tubes permitted the development of portable ultraviolet lamps, in use during the last twenty years of the Franklin Mine’s operation and also used extensively at Sterling Hill, by geologists and staff. For extensive discussions of fluorescence and related matters, the reader is referred to the works of Bostwick (1982, 1992) and Robbins (1983).

A perspective is useful: the subjective and general observations of fluorescence given herein, although they have been well-utilized by the local mining industry and are very useful and interesting for the mineral collector, are only of ancillary scientific utility. Comprehensive scientific reviews of mineral luminescence were given by Marfunin (1979) and Waychunas (1988).

Uses in geology, mining, and milling

  Figure 11-1. The slurry on a concentrating table in the Palmer Mill is monitored for willemite and calcite fluorescence using an ultraviolet lamp. Photograph courtesy of the Franklin Mineral Museum.  

Franklin geologists used mineral fluorescence in varied ways. Because the calcite near and within the orebodies fluoresces red, apparent “near-misses,” in which exploratory drill-holes barely missed intersecting the sought-after orebody, were revealed by the fluorescence of the drill-cores. Conversely, the lack of fluorescence in such drill cores suggested the drill-hole had not come very close to the orebody. The nearly consistent local fluorescence and nearly pervasive ubiquitousness of willemite, and to a lesser degree calcite, also aided in mapping geologic relations underground, estimating willemite content and the ore’s economic grade, and tracing the structures of the ore and adjacent marble. Vein- and grain-boundaries, shear zones, the mineral character of veins and selvages, and the textures of deformation structures are all revealed in part by fluorescence, as are otherwise inconspicuous exsolution textures (for example, willemite in tephroite), thin-films, fracture fillings, and a host of other textural features, both megascopic and microscopic. An additional, if often unspoken, benefit lies in the immediacy of this information; it is useful on the spot and requires no additional processing to provide knowledge, unlike complex laboratory procedures. Fluorescence also proved of use to geologists in monitoring stream sediments and in differentiating willemite from tephroite at Sterling Hill, where they can appear visually similar. Local willemite has a protean character, and much material might pass unrecognized as such, if it were not for its fluorescence in ultraviolet radiation.

  Figure 11-2. Monitoring a willemite concentrate using ultraviolet radiation in the Palmer Mill. Photograph from New Jersey Zinc Company publicity releases.  

In the mills, the New Jersey Zinc Company utilized fluorescence to monitor the ore/gangue dividing line on Wilfley tables, to control jig separations, to detect contaminants in concentrates, and to monitor the presence of some lead-bearing minerals at the picking table. Thus was developed the local commercial use of mineral fluorescence in ultraviolet (Fuller, 1933; Fuller and Gamble, 1933).

Not only geologists and mining management enjoyed the direct benefits of fluorescence; it was of benefit and delight to some miners as well (Bostwick, 1979).

The literature of fluorescence

There exists a substantial body of literature on the fluorescence of local minerals, much of it informal and of highly irregular quality. Many specific references are given under the headings of calcite and willemite in subsequent sections. The reader interested in fluorescence is referred to the papers of Palache (1928b), Barrett (1934), Gunnell (1935), Smith and Parsons (1938), Smith (1939), Millson and Millson (1950), McDougall (1952), Mutschler (1954), Jones (1961a, 1961b, 1964, 1981, 1982), Anonymous (1965), and Hochleitner (1984). By far the most helpful, indeed extremely helpful, papers are those by Richard Bostwick (1982, 1992), who provides hands-on guidance and a lot of experience, precautions, and wisdom; bostwickite was named in his honor. For beginners, the work of Warren et al. (1995) is particularly useful.

A number of specialized studies have been made. Among the more interesting are studies on the duration of phosphorescence and on photographic methods (Millson and Millson, 1950, 1964), thermal effects (McDougall, 1952), infrared luminescence (Barnes, 1958), relative brightness (Newsome, 1973, 1984-1985), luminescence spectra (Modreski, 1974), gamma-radiation effects (Rossman, 1974), and the spectral distribution of fluorescent colors (Newsome and Modreski, 1981; Newsome, 1982, 1984-1985).

Influence of fluorescence on collectors and the general public

The introduction of low-cost, low-pressure, mercury-vapor, portable ultraviolet lamps stimulated a whole new emphasis in mineral collecting at Franklin; many collectors and miners could personally own ultraviolet lamps. This branch of local mineral collecting emerged energetically and fostered intense collecting and preservation of fluorescent mineral specimens on a grand scale. The bountiful supply of material fed an increasing demand and ensured the permanent role of fluorescence in the public aspects of Franklin’s mining history and mineralogy.

  Figure 11-3. Robert M. Catlin, the mining superintendent, is here shown examining fluorescent mineral specimens with an ultraviolet lamp. The photograph likely dates from the 1920’s and was obtained from UPI/BETTMANN.  

There were unmeasurable effects as well: miners who had previously collected many visually attractive, uncommon, or scientifically significant specimens were perhaps more likely after the advent of portable, ultraviolet lamps to collect primarily the fluorescent minerals. Many of the non-fluorescent minerals were possibly then undercollected as a result. It is also likely that fluorescence drew some miners to collecting, who otherwise might not have been active collectors. The effects cut both ways, as does a two-edged sword or, locally, a double-headed axe.

Mineral fluorescence was attractive not only to the mineral collectors; it also served as a general stimulant for many who later became interested in both fluorescent minerals and non-fluorescent minerals. Having seen fluorescent specimens, mostly dull in appearance under normal white light, emit intense colors under ultraviolet radiation, many persons, adults and children alike, were first drawn to learn more about minerals in general. This phenomenon was influential in the writer’s first years in mineralogy, and it provided a powerful stimulant.

General discussion

At present, over 80 fluorescent mineral species are known from the Franklin and Sterling Hill area. Of these, about ten are of sufficient brilliance that they are widely regarded as “classic” specimens. Some of the factors which make Franklin specimens the best known in nature are: the intensity, depth, and brilliance of their colors of fluorescence; the sharp contrasts where several (commonly 2 to 4, but up to 7) fluorescent species are present on one specimen; the textural variations for a variety of species; and the large size of some specimens. All these factors add substantially to the cachet of any given specimen.

Some fluorescent minerals are more in vogue than others; such status varies temporally, and the in-demand mineral specimens are assiduously sought after and subject to market forces. Among the minerals very much in demand at this writing are orange-fluorescing wollastonite from early finds at Franklin, blue-fluorescing Franklin margarosanite with platy habit and strong fluorescence, and orange-fluorescing turneaureite from Franklin. Other species in demand include: bright-red-fluorescing manganaxinite from Franklin; green-fluorescing radiating willemite from both deposits; white-fluorescing barite from both deposits; and greenish-yellow-fluorescing Franklin esperite, especially in large masses and/or with interesting textural features and/or numerous associated fluorescent minerals.

Some pleasing fluorescent color combinations include calcite and willemite (orange-red and green fluorescence); hardystonite and clinohedrite (violet and orange); and spectacular “five-color” specimens, such as one containing esperite, hardystonite, willemite, clinohedrite, and calcite (greenish-yellow, violet, green, orange, and orange-red, respectively). The abundance of superb calcite-and-willemite fluorescent specimens has led to the creation of a local idiom, “red-and-green,” to describe them.

The visual characterization of mineral fluorescence is very subjective; reasonable persons can and often do disagree on the observation and interpretation of some of the more subtle fluorescent responses. For these reasons and others, casual observations of mineral fluorescence generally are of limited scientific value. The observations of fluorescence given in this volume were made with widely available, general-purpose lamps; they should be considered biased and subjective. Such information should be used with extreme caution.

Some observations of fluorescence have been made only after much sensitizing of the human eye to darkness; weak or faint responses might not be evident upon casual observation. Additionally, not all specimens of a given species will exhibit the given responses; some may exhibit different responses; and some specimens, or many, may exhibit none. Such fluorescent and phosphorescent responses should not be used alone to identify minerals; they are but an aid to identification, providing in some cases a useful discriminant. Despite such disclaimers, such information is very useful when evaluated carefully. Some of the precautions which should be used by a beginner or by a sophisticated, careful observer are given by Bostwick (1982, 1992) and Robbins (1983).

Scientific research into the specifics of the natural fluorescence of local species is in its infancy and might remain there. Existing minor- and trace-element analyses of many minerals has led to much speculation as to causes of fluorescence, but it is largely unsupported by experimental confirmative studies. The fluorescence of most minerals is not an intrinsic property, but depends on chemical impurities, lattice-defects, and other factors. Some work has been done on the role of Mn and Pb as activators and co-activators of fluorescence at Franklin, but much work remains to be done on natural material. The specific causes of fluorescence in most local species remain ambiguous; see Verbeek in Warren et al. (1995) for a superb discussion of activators.

In general, fluorescence is diminished by the presence of much iron in solid solution, and some fluorescent species (esperite, for example) have an apparently quenched fluorescence in contact with franklinite. Manganese may be hosted by many minerals and in small amounts may contribute to a mineral’s fluorescence (Ward, 1935), especially in the presence of co-activators. In general, however, these are in minor concentrations, in some cases very minor. Most fluorescent minerals do not have large amounts of manganese; hodgkinsonite, some manganaxinite, and some bustamite are exceptions to this general rule-of-thumb.

Lists of fluorescent minerals.

A list of local fluorescent minerals was given by Palache (1928b). Others were provided by Edwards (1972), Kozykowski (1974), Bostwick (1977, 1992), and others.

There exists a small and delightful army of mineral collectors fascinated with mineral fluorescence. They are willing to sit for long periods in total darkness to adapt their eyes for careful observation of faint fluorescence. They utilize strong lamps to stimulate weak-fluorescing minerals, employ filter-goggles to delete unwanted light, and try their very best to see and record everything that is fluorescent. They deserve much praise for increasing knowledge at the margins. The lists of Bostwick (1982, 1992) address all common instances of local fluoresence as well as some uncommon ones; the list of color-terms given by Bostwick (1992) deserves special reading.



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