Coal Science

9 Significance and Use of Optical Phenomena in Uraniferous Caustobioliths JACQUES JEDWAB

Downloaded by RUTGERS UNIV on May 30, 2018 | https://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0055.ch009

Université

Libre de Bruxelles, Bruxelles 18,

Belgium

Optical anomalies are very widespread in uraniferous carbonaceous materials from different geological ages like bituminous and anthracitic coals, coalified logs, bituminous shales, and asphaltic sandstones. These optical anomalies, expressed by high reflectivity and anisotropic halos, are very stable, once acquired, in the course of history of the sediment. At this time it is not yet known if the intensity of optical anomalies is cumulative and proportional to the radioactive doses received. Nevertheless, one may evaluate through this kind of "natural nuclear plates" the time of introduction of uranium, the epigenetic vers. syngenetic type of its origin, and the plastic and fault-like movements of the organic materials.

^ m o n g the numerous methods used to study uraniferous organic materials, the microscopic a n d autoradiographic approaches and interpretation seem rather neglected. This methodological gap is even more striking when one considers that good instrumental commodities are available. Whatever the reasons, geologists a n d ore microscopists have greatly ignored these fields, the scientific interest of w h i c h w e shall show here. Numerous investigators have already noticed a n d described natural a n d artificial effects related to irradiation i n caustobioliths a n d other carbonaceous materials: Stach (24, 25), M . a n d R. Teichmuller (26, 27), Hoehne, (11), Friedel a n d Breger (8), E r g u n , Donaldson and Breger ( 7 ) , Breger (2) a n d Duchesne, Depireux, a n d V a n der K a a (5) i n coals; Davidson and Bowie (4), Ramdohr (20, 21, 22) Liebenberg ( 1 7 ) , Uytebogaardt (28) i n "Thucholites and Related Materials;" Zoubov (30), Pierce, M y t t o n , and Barnett (18) a n d Pierce a n d Rosholt (19) i n asphalt and bitumen. A n early paper b y G r i p a n d O d m a n (6) displayed halo effects but d i d not explicitly refer to the irradiation as an active agent. 119 Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

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Observations from these workers coupled w i t h ours shows that the dif ferent organic materials, regardless of their origin a n d nature, react quite similarly to irradiation. T h e effects involved, from an optical point of view, are an increase of reflectivity, a n d both opacity a n d anisotropy. These effects, a n d others—e.g., insolubility or microhardness—appear to be related to an increase i n chemical and physical stability. If one considers that these effects are stable towards further geological agents, w e n o w realize that w e have i n h a n d a "geological photographic plate." This optimistic approach should not hide the fact that data a n d information obtained are rather complex a n d that our understanding of the basic phenomena is still inadequate. W e shall thus confine ourselves to describing and interpreting a f e w typical examples. Techniques The observations are performed w i t h a L e i t z Ortholux polarizing micro scope equipped w i t h the O p a k " illuminator, lamps for reflected a n d trans mitted light, immersion objectives, and verniers. Characteristics of the polished thin sections a n d of the nuclear emulsion plates are observed i n transmitted light with the same immersion optics after removing the Berek prism. Polished sections a n d polished thin sections are prepared according to the usual techniques of coal petrography. They are mounted permanently on quad rangular pieces of transparent L u c i t e for location purposes. W e have described elsewhere a graphical method of micro-surveying w h i c h allows an observer to locate any specific autoradiograph on nuclear emulsion plate of a given point i n an opaque section (13). Nuclear emulsion plates are Ilford Κ 5-100 micron. They are processed as recommended b y Bowie ( J ) . Irradiation

Phenomena

U n d e r Reflected L i g h t . Radioactive minerals (zircon, uraninite, coflSnite) enclosed i n organic material are surrounded b y a clear halo of more than 2 0 microns w i d t h (Figure 1 ) . This halo corresponds to a zone of increased aniso tropy, reflectivity, and hardness as first observed b y Stach (24). T h e outline of the halos is determined b y those of the radioactive inclu sions; therefore they may be circular, banded, or irregular a n d asymmetrical. Between the mineral inclusion a n d the clear halo one often finds a dark rim of 2 - 4 microns w i d t h , whose significance is not yet clear (Figure l b , Figure 2 g , Figure 3 f ) . T h e reflectivity within the halo is not constant; one observes a maximum close to the radioactive source w i t h a decrease outwards. T h e tracing of reflectivity curves w i t h a photocell, recorder, and moving stage has shown that the decrease curves are smooth but sometimes stepwise (Figure l a , b ) . T h e w i d t h of the halos is difficult to estimate by a subjective method, and our instru mental measurements are still too f e w to allow, definite interpretation. H o w ever, the w i d t h generally varies between 20 a n d 50 microns. Preliminary observations suggest that the intensity of the halos is proportional to the quan tity of uranium (Figure 4 c ) .

Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.


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Figure 1. Reflectivity curves of halos around uranium inclusions: (a) in Temple Mountain asphalt, (b) in St. Hippolyte Coal. Ordinate axis (R) gives reflectivity in arbitrary linear units. Recordings were made along the white line drawn on the photographs. Notice the stepwise shape of the curves and their asymmetry W h e n observing asphalts and low rank coals, introducing a nicol prism increases the contrast between irradiated zones and background. Rotating the stage usually shows a slight pleochroic effect. Anthracitic coals must be observed w i t h two nicol prisms since their high reflectivity masks secondary effects. Rotating the samples between crossed niçois shows cross- or lemniscatelike figures as w e l l as strongly contrasted bands (Figure 5a, b ) . U n d e r Transmitted L i g h t . T h e former reflecting halos correspond to opaque zones of the same outline which become progressively transparent towards the r i m . T h e color is of the same shades as coal or asphalt.


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One also observes crosses i n polarized light w h i c h are deformed when the stage is rotated. This has been attributed to photoelastic effects of stress applied to a point.


Several Uranium-Caustobioliths

Associations

Apart from these general characteristics, each association presents its own peculiarities determined by the origin of the carbonaceous material, the original structures, its mechanical history, and the time of introduction of the radioactive elements. These peculiarities are most important from a geological and geochemical point of view. T h e few cases described hereafter were chosen accord i n g to their diversity and exemplary value. Asphaltized L o g from Temple M o u n t a i n , U t a h . This type of material has been widely described (e.g., 14). T h e studied samples come from the N o r t h A d i t mine ( C h i n l e formation) (Figure 2 ) . T h e uraniferous minerals (uraninite and cofBnite) form minute isolated inclusions, or are aligned according to the cellular structure of the w o o d (Figure 2a-g). In the same sample one finds compressed relics of woody structures (Figure 2 a ) , together w i t h cellular cavities filled w i t h uranium ( F i g u r e 2 d , g ) . This suggests that uranium was introduced during or before the compression. T h e material shows clearly the difference of optical transparency of irradi ated zones. It seems that the effect is limited to the range of the α-particles, w h i c h w o u l d tend to prove that the radioactivity is unable by itself to induce the entire induration of the asphalt. This does not exclude the effect of the released heat. T h e nuclear emulsion plates show that the bulk of the activity corresponds to discrete inclusions. T h e asphalt by itself shows very l o w activity (Figure 2e,h). Asphaltic Sandstone from Lodeve-Mas A l a r y , Hérault, France. This rock of Permian age contains, together with uraninite, secondary uranium minerals, sulfides, molybdates, etc. (12, 15) (Figure 3 ) . T h e asphalt is distributed as little pellets or stringers. I n the same samples one finds pellets of high reflectivity and nearby, others of d u l l appearance (Figure 3 a ) . T h e last are elastic whereas the former are brittle towards the microhardness test. Figure 2.

Asphaltized

log from Temple Mountain,

Utah (1 nicol)

—*

(a) Compressed cellular structure, without uranium (X 180). (b) The same as a with a few uranium inclusions surrounded by halos (X 180). (c) The same as a with heavy uranium filling. In the center: structureless asphalt (X 90). (d) and (g) Cellular structures, showing the shape of halo (d) (X 180) and black narrow bands (g) (X 562). (e) and (h) Uranium inclusions (X 112) and corresponding autoradiograph (6 hours exposure) (X 112). (f) Contiguous inclusions of uranium; vanadium and nickel minerals "pushed in the matrix. Obliterated woody structure at left (X 180).


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T h e most interesting features are the traces of mechanical movements w h i c h gave rise to manifold or stretched halos. These movements occurred when the material was plastic and still sensitive to irradiation ( F i g u r e 2e, g ) . One may observe halos separated from the uranium minerals w h i c h remain only as fine threads; other elongated halos have one end showing a sharply defined limit, and the other fading out smoothly.


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C o a l from St. H i p p o l y t e , Vosges, France. This coal of Stephanian age contains an epigenetic mineralization of uranium (Figure 4 ) ( 9 ) . Microscopic uraninite fills cracks of different size especially visible i n massive vitrinite ( F i g ure l b , Figure 3 d ) . Symmetrical halos r u n along the cracks following their finest details. O n e observes sometimes thin dark zones between filled crack a n d bright halos, of unknown origin. These banded halos are similar to oxidation bands found i n highly altered coals. However, i n the St. H i p p o l y t e coal,



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Figure 4. Coal from St. Hippolyte, Vosges, France (1 nicol) (a) Banded coal with transversal crack in vitrinite. Symmetrical dark grey halo (X 180). (b) Enlarged part of (a) showing that the crack is actually filled (X 562). (c) Uranium concentration in massive vitrinite showing the decrease of halo intensity with the narrowing of active zone (X 180). (d) Two fine veins of uranium. Notice the different reflectivities of left and right sides (X 562). fractures devoid of uranium filling or filled only w i t h sulfides, are also devoid of halos. Samples of banded coals show i n the vitrinite portion transversal bands of high relief (owing to polishing) w i t h a fine median crack filled w i t h uranium minerals (Figure 4a, b ) . Distribution of the radioactivity corresponds to the filled cracks. T h e main coal shows very l o w radioactivity.

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