Diccionario de la EDT

a) Results

A detailed investigation of the variation of the

reflectivity of graptolites cut at various angles to the plane of the

bedding was peiformed on a block of black mudstone collected from an

exposure of the clingani Zone at Hartfell in an attempt to determine

the position of any isotropic sections and determine the

crystallographic indicatrix to which the graptolites of the Moffat

Reflectivity measurements were made in air and in oil at

various wavelengths over a specimen of Climacograptus scalaris (liisinger), which exhibited a good polish with negligible relief effects, firstly, in the plane of the bedding and then after repeated cutting and repolishing, at angles of 57°, 74° and 90° to the plane

of the bedding (Fig.120). Maximum and minimum reflectivities in oil and in air at each of the four positions are given in Table 5~1.

inspection of the reflectivity data indicates that there is a linear relationship with wavelength (Fig.121). This relationship has been quantified by means of linear regression analysis which

indicates that the reflectance values for each of the four sections are linearly related to wavelength at the 99.9 per cent confidence level. Random fluctuations in the results have been eliminated by calculating estimated reflectivity values from the regression parameters which are given in Table 5-2. Estimated bireflectance values were then found from the difference between estimated maximum and estimated minimum reflectivities (Table 5-3).

In no instance do the bireflectance values indicate an isotropic section. The nearest they approach a zero bireflectance is in the section parallel to the bedding measured at 410 nm where even then there is a noticeable difference between maximum and minimum

values of 0.3. Bi reflectance is at a maximum of 1,6 in the 57° section measured in air at 710 run. Values then decrease in the 74° section

and finally drop to an intermediate value of 0.5 in oil and 0.75 in air at 410 nm in the section cut perpendicular to the bedding plane.

The variation in birefLectance with the angle of section

is obviously a function of the trends in maximum and minimum -i

reflectance values (Fig.122). For a fixed wavelength the maximum J*

s $

117

reflectivity decreases slowly while the minimum drops more markedly between the parallel and the 57°sectiono At angles of between 57°

and 74° both minimum and maximum reflectivities increase at similar

rates while between 74° and the perpendicular section both minimum

and maximum values decrease in the sections measured in oil but there

is a slight tendency for minimum reflectivities measured in air to

increase.,

b) Discussion

The bireflecting property of coals and cokes is often

likened to the birefringence of many rock-forming mineral crystals

and the concept of a birefringence indicatrix has been translated to

a bireflectance indicatrix, Such an indicatrix represents by mean©

of three orthogonal axes the relative reflectivities of the organic

material measured in differing directions usually with respect to the

bedding plane. The lengths of the axes are proportional to the

reflectivities in these directions.

Normal coals usually have a uniaxial negativebireflecting

indicatrix with the axes in the horizontal (bedding) plane equal in

length and longer than the single vertical axis (Fig.123), A positive

uniaxial figure is the reverse of this configuration ie, the single

vertical axis is longer than the horizontal axes, The biaxial

configuration is rather more complex with all three axes of unequal

length (Fig,123).

In the uniaxial configura1 ion there is only one possible

circular section ie. a section containing the two equal axes which

section lies in the horizontal plane. With a biaxial indicatrix

there are two possible circular sections, both of which are equally

inclined to the horizontal. In common with terminology related to

birefringence indicatrices, the term optic axis is retained to

indicate that axis which lies in a direction perpendicular to the

circular section. A uniaxial indicatrix has thus one and a biaxial

i nd i ca trix two opt i.c axes«

The results presented for the variation of graptolite

reflectance values with the angle of section are inconclusive. The

variation in both maximum and minimum values from one part of the

sample to another renders the possibility of a uniaxial configuration

unlikely. Until the positions of the isotropic sections are located,

it must however remain a tentative suggestion that the graptolites

of the Moffat Shales are behaving as a biaxial material.

The anistropy displayed by the graptolite fragment in the

plane of the bedding may be due to a number of causes which may have

acted singly or jointly to produce the observed effects. Firstly,

as graptolites tend to be long and slender it may be that they are

displaying a form of zoological anistropy in the same way as some of

the form anistropy of vitrinite is mimetic after that of the parent-

plant material (SaLeh 1968).

Another cause might be that the growth of structural

elements in the graptolites was governed by the pressure field acting

upon them. In coals, the lamellae appear to grow so that the plane of

the lamellae is orientated perpendicular to the direction of maximum

stress (Cook et al., 1972a). Ln a triaxial. stress regime it is

119

circular discs, with the long- axes of the disc parallel with the

minimum stress direction. Such a structure should have biaxial

optical properties and with a small degree of elliplicity it would

be optically negative. Pressure may well have influenced the

anisotropy of the graptolite fragment because it and the enclosing

sediment have without doubt suffered the full effects of the

Caledonian orogenic movements.

Thermal alteration might also give a biaxial character to

the graptolite fragment. This type of effect has been described for

a British Devonian meta-anthracite coal (Cook et al., 1972b). It is

of relevance to observe that both this meta-anthracite and the

graptolite fragment show a pronounced increase in reflectance values

from the blue to the red in the visible spectrum whereas coals of

lesser rank show decreasing values fi-om the blue to the red (Cook

et al., 1972b).