ABSORPTION STRUCTURES IN THE VISIBLE REFLECTION SPECTRUM OF MINERALS ABSTRACT

Transcription

1 Economic Geology Vol. 60, 1965, pp ABSORPTION STRUCTURES IN THE VISIBLE REFLECTION SPECTRUM OF MINERALS REINHOLD GERHARZ ABSTRACT Narrow and faint absorption lines have been observed in the reflectance spectra of monazite, willemite, scheelite and feldspar. The wavelength of these lines was measured by visual and photographic means, but their cause of origin was not established. Im'ORW^NW sources of information for identifying minerals and rocks and their sub-structure can be provided by examining their light-optical properties, particularly their transmission characteristics (1, 2, 3, 4). Relatively little is known aboutheir reflection properties. The rich spectrum structure of light that is reflected from monazite (Figs. la and b) (5) encouraged the search for similar properties that may help to identify other minerals. With small spectroscopic instruments this endeavor was applied to 50 unassorted rock samples that were obtained from a collection of the U.S. National Museum of the Smithsonian Institution. It was also I I I I i i nm f FIGURE lo 0 I L5.0.5, FIGURE lb Fro. 1. Reflectance spectrum of Ceylonese monazite powder. (Because of the rich spectrum structure, a Beckman type DK-1 spectrophotometer with integrating sphere was used): (a) Range from X 3,500 to 7,000 ix, (b) Near IR Range from X 6,500 to 25,000 ix. 1 nm = 10 ix. 1721

2 1722 REINHOLD GERHARZ Fro. 2. Reflectance spectrum of willemite (Franklin Furnace, N.J.), visible absorption at 3. 4,200 and 4,3003, (see markers). Faint absorption structures were also observable between 3. 8,400 to 8,700 ). intended to survey by photographic detection the spectral reflectance of mineral samples in the blue, the ultraviolet and in the near infrared. The instrument used for photographic examinations was a Hilger medium quartz spectrograph type E 498. The sources of illumination were a hollowcathode Deuterium lamp for the UV and a 100 W Tungsten filament lamp for the visible and the near-infrared. These lights avoided the errors that could otherwise originate from the Fraunhofer lines of sunlight or from the spectral emission of a carbon arc. Preliminary tests had been carried out with a 1,000 W filament lamp and a high-pressure Xe-arc lamp, but excessive heat dissipation, remnant line structures of the Xe-arc spectrum, and the limited far UV transmission of the lamp envelope turned out to be of great disadvantage. Before using the spectrograph, each mineral sample was visibly examined with various types of pocket-spectroscopes. Upon glancing at the reflected spectrum quite superficially, no particularities were observed among the various samples, except for the enhancement of their characteristicolors. After becoming more acquainted with this visual technique, focal length and slit width were carefully readjusted. Then it was found that several minerals Fro. 3. Reflectance spectrum of scheelite (Timmins, Ontario) visible absorption at X 5,700, 5,800 (very weak), 5,830, 5,860 (weak); 5,920 A (weak), doublet structure observable on IR-diagram at X 7,400; more weak lines at 7,550; 8,050; 8,1002. No observable features in the range between 3. 10,000 and 25,0002 ).

3 VISIBLE REFLECTION SPECTRUM OF MINERALS O ;0 9 '1 (-- Fxc. 3.

4 REINHOLD GERH.4RZ , 5 7( FIG. 4. Reflectance spectrum of pink garnets in feldspar (South Park, Col.); visible lines at 5,200, 5,780, 5,820 _, all very faint; weak lines at X 7,300, 7,350, 7,450, 7,600, 7,950 A; other lines at X 8,050, 8,650, 8,700 A; in the near IR: a 5% dip at 14,600 A; a 13% dip at 14,900 A; and a 3% dip at 22,000 _. exhibited persistent absorption features. These were seen very distinctly, after spectroscope slit and focal length were accurately adjusted by observing the green' and the two yellow emission lines from the mercury in a fluorescent lamp. Photographic verification of these reflectance features was then obtained with the Ililger Spectrograph. Examination of the plates from a variety of about 50 mineral samples did not uncover any narrow absorption structures below, = 3,900 A.

5 VISIBLE REFLECTION SPECTRUM OF MINERALS 1725 In the visible spectrum, several cases of band absorption were observed and recorded. Their structures in the green, yellow and orange were clearly visible and their position on the wavelength scale was then estimated with a superposed visible comparison spectrum or with the wavelength reticle in one of the small spectroscopes that were used in these tests. The photographic spectrograms were taken on Kodak type IV-N plates. (This emulsion has sensitivity defects between X 4,600 and 5,000 3,. A somewhat weaker defect is located between X 5,400 and 5,5003,.) The scheelite and feldspar samples exhibited these absorption lines very distinctly. They were later analyzed by emission spectroscopy with a d.c.-arc and they contained a significant percentage of rare earth metals (see below). Sample showing this spectrum absorption in reflected visible light are described as follows and their spectra x are reproduced in Figures 1 through 4: (a) Monazite: (Ce; LaPO4); detaile discussions in (5) Figure 1. (b) Willemite: (Zn.SiO4): Zn-Orthosilicate (color: greenish-yellow, translucent; no analysis made, Figure 2. (c) Scheelite: (CaWO4): Massive milky vein quartz with bands of coarsely crystalline cider-brown scheelite; finely crystalline pyrite and chalcopyrite scattered through scheelite, and as bands, segregating scheelite from quartz matrix. Analysis (in %): Ce: 0.015; Sr: 0.1; W: maj. component; Y: 0.02; Nd: 0.02; few other R.E. present. (The spectrum of pure calcium tungstate is shown as Figure 14 in (6); Figure 3. (d) Feldspar: (K; Na; Ca) A1Si3Os. Coarsely crystalline granitic rock with massive quartz and coarsely crystalline flesh-colored feldspar. Minor amounts of a pale-green fluorite. Analysis of these garnets (minor components only; in %): Fe: 0.15; Ca: 10; Na: 2; Ce: 0.7; La: 0.5; y: 1.8; Nd: 1.5; many other rare earth metals present; Figure 4. The yield of 4 samples among about 50 others gives ample reason for the belief thathe obserwition of reflection spectra with white light might provide simple identification means for many other materials with complex crystalline structure. U.S. A M¾ E.R.D.L., 8703 BU Dv. TTv. RO^D, Br. xhv. s ^, MI , March 19, 1965 REFERENCES 1. Leonard,.B.F., 1960, Reflectivity measurements with a Hallimond visual microphotometer: Ecoa. Gr-o,.., v. 55, p Kayset, H., 1905, Handbueh d. Spectroseopie: v. 3, eh. 5, Leipzig. 3. Wheery, E. T., 1929, Mineral determination by absorption spectra: Amer. Mineral., v. 14, p Vratny, F., and Kokalas, J. J., 1962, The reflectance spectra of metallic oxides in the 300 to 1000 milllmicron region: Applied Spectroscopy, v. 16, p Murata, K. J., et al., 1956, Convenient method for recognizing non-opaque Cerium earth minerals: Science, v. 123, p Nassau, K., and Broyer, A.M., 1962, Calcium tungstate:ezoehralski growth, perfection, and substitution: J. Appl. Physics, v. 33, p x Numbers on wavelength scale t{mes 100 in Angstrfm Units.