Blue John) from Castleton, Derbyshire, England
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MINERALOGICAL MAGAZINE, JUNE I97 9, VOL. 43, PP. 243-50 The Blue coloration in banded fluorite (Blue John) from Castleton, Derbyshire, England A. K. GALWEY, 1 K. A. JONES, 2 R. REED 3 1 Department of Chemistry, 2 Department of Geology, 3 The Electron Microscopy Laboratory, The Queen's University, Belfast BT7 INN, Northern Ireland AND D. DOLLIMORE Department of Chemistry and Applied Chemistry, University of Salford, Salford, M5 4WT, Lancashire, England s u MM A R Y. From the microscopic examination of lightly of chromophore that have been proposed to etched, cleaved (I I I) surfaces of banded blue fluorite explain the blue coloration. These are incorporated (Blue John, from Castleton, Derbyshire) it is concluded hydrocarbon, inorganic impurities, and lattice that the coloured lamellae are not necessarily or exclu- sively associated with lattice-line imperfections. Ele- imperfections or damage produced mechanically mental analyses, by electron dispersive methods, of fresh or by radiation. These workers applied the usual cleavage (I I I) surfaces in the immediate vicinity of small complementary chemical and physical techniques inclusions (c. IO #m diameter) possessing associated and conclude that the chief colouring material is coloured haloes detected no appreciable concentrations colloidal Ca, banded as a consequence of changing of impurities. From the evidence available, it is suggested conditions during mineralization. Braithwaite et al. that the zones of blue colour consist of colloidal calcium (I973) find no correlations between the distribu- resulting from radiation damage caused by the inter- tions of colour and of either inorganic or organic mittent deposition of radioactive material on the surfaces impurities in Blue John and related minerals. They of fluorite during crystal development. The dispersion of colloidal calcium produced is particularly stable as a conclude that physical rather than chemical factors consequence of the close correspondence between lattice are responsible for the coloration and suggest that spacings in calcium fluoride and in calcium metal. colloidal particles of Ca have been formed by the aggregation of Ca atoms preferentially within the more defective regions of the crystal. These atoms THE unusual and aesthetically pleasing banded are believed to have been released from the lattice fluorite, often referred to as 'Blue John', occurring during long-term irradiation by radioactive ele- near Castleton in Derbyshire has been the subject ments, which are known to have been available by of long and continuing investigations of the their continued presence in associated uraniferous reasons for the striking blue coloration. Compari- deposits (collophane). sons have been made with the properties of colour- Calas (I972) concluded that colloidal Ca is not less material from the same location and, or, necessarily the colouring matter in all banded banded or colourless fluorites from other widely fluorites and discusses the possible existence of scattered sources. However, despite many alternative light-absorbing centres containing in- investigations no general agreement concerning the organic impurities, notably yttrium. Holgate identity of the chromophore has yet been reached (I973), from consideration of the dichroism of perhaps partly because much of the experimental coloured fluorites, believes that the most probable evidence has been negative in character. More chromophore in Blue John is a hydrocarbon, possible explanations have been eliminated than though the particular compound or molecular proposals positively proved. grouping is not identified. Thus, from these and MacKenzie and Green (I97I) summarize the other recent reports, we must conclude that the earlier literature concerned with Blue John and light-absorbing centres in coloured fluorite have related fluorites and identify the three general types not been positively and finally characterized and Copyright the Mineralogical Society 244 A. K. GALWEY, ET AL. that the nature of the chromophores present dark blue. In addition to these readily recognizable remains a matter for active discussion. patterns of colour distribution, with which we are The present article reports a study of two aspects particularly concerned here, there were also zones of the structure and composition of Blue John. of less regular tinting, ranging from what appeared Firstly, we have investigated the relationship be- to be lamellae of restricted extent to regions of ill- tween the distribution of lattice-line imperfections defined outline containing no recognizable struc- (dislocations) now present, as revealed by etching of ture. It should be remembered that any explanation cleaved surfaces, and the distribution of the blue of the coloration, based on consideration of the coloration. This was intended to determine whether properties of representative lamellae and the most direct support could be obtained for the theory that concentrated groups of point features with asso- dispersed Ca in the bulk crystal was precipitated or ciated haloes, must also be capable of accounting aggregated in the form of a stable colloid prefer- for the existence of the ill-defined and more diffuse entially within defective zones. Although deforma- regions of tinting. tion and annealing under natural conditions could Lamellae of coloration. Planar lamellae of colora- be expected to cause continuing variations in lattice tion, many being c. o.I mm thickness, obliquely imperfection distribution, it was thought probable traverse the thin plates (o.2-1.o mm) of fluorite that extensive precipitation of colloidal material prepared for examination by parallel cleavages at within dislocations would stabilize them against the (i I I) plane. This appearance is consistent with further movement. It seems inherently more likely the production of the thin coloured lamellae at that progressive dispersal of single Ca atoms would crystal planes parallel to cubic growth faces, during occur more readily than the annihilation of the or following addition of CaF2 in mineralization: dislocation networks within which such atoms were natural deposits of fluorite are often produced in preferentially accommodated in the first place. the form of assemblages of intergrown cubes, for Secondly, we have attempted to establish the which the (ioo) faces predominate. Growth direc- composition of features having the appearance of tions, and their relict structures, therefore, appear inclusions and which characteristically occurred in as planes orientated at 54 ~ 44' to the strongly irregular groups comprised of numerous individual preferred (Ill) fracture direction of the fluorite members, each possessing an associated zone of crystal. blue coloration. We have been unable to identify Assemblages of inclusions. Slivers of fluorite, any foreign constituent and conclude that there are obtained on cleavage, occasionally contained zones no appreciable local concentrations of any of irregular dispositions of large numbers of small included non-volatile impurity. features provisionally identified as inclusions. The In this article we also make brief reference to individual members of these assemblages were too certain other of our measurements. These report no small (diameters c. Io/zm) to permit the resolution new information but are entirely consistent with of detail by optical microscopy. In some places each and serve to confirm earlier observations by others. opaque zone was surrounded by an indistinct blue The nature of the colour centres resulting in the halo. blue banding and the manner of its production Line dislocations in Blue John under natural conditions are discussed. The preferential dissolution of crystal surfaces initiated in the vicinity of imperfections has been Distribution of coloration in Blue John widely used as a method of locating points of The present study was exclusively concerned intersection of line dislocations with surfaces. Pre- with samples of natural banded fluorite (Blue John) liminary work with the present mineral showed collected from a surface exposure directly above that triangular etch pits were developed after the entrance to Treak Cliff Cavern, near Castleton, c. 2 min dissolution in IO ~o aqueous hydrochloric Derbyshire (Nat. Grid Ref. SK 135 83I). From acid at c. 35 ~ K. An unidentified deposit produced relatively perfect single crystals (c. Io-3o mm linear in this treatment was removed from etched sur- dimensions) selected specimens were cleaved, by a faces by washing with hot distilled water before sharp blow with a knife edge, into thin plates microscopic examination or analysis. suitable for microscopic examination. Coloration About twenty different thin cleaved (i i i) plates in banded fluorite occurred in two characteristic of Blue John, each specimen having been selected to distributions: in the form of planar lamellae and in include at least one strongly coloured lamella, were the immediate vicinity of individual members of individually etched under an appropriate range of features having the appearance of irregular groups conditions (of severities greater than and also less of inclusions. There were significant variations of than the optimum) and the treated surfaces hue, even within a single crystal fragment; examined in detail in a scanning electron micro- perceived colours varied from pink-magenta to scope. This technique has the disadvantage that the BLUE COLORATION IN BANDED FLUORITE 245 precise position of the coloured zone does not located in a previous optical examination.