6. CAPÍTULO I CARACTERIZACIÓN DE LOS CENTROS DE DESARROLLO
6.1. CENTROS DE DESARROLLO EMPRESARIAL DEL MUNICIPIO DE
a) Component minerals
The sediments are composed mainly of quartz with subsidiary
amounts of feldspar (plagioclase), mica (sericite), pyrite, chlorite and
occasionally haematite. Strong plagioclase reflections corresponding to
d - spacings of 6.37
X [ooi] ,
4.02 X (20l| , 3.85 $ [ill] , 3.66 R[l30 : 13l] and 3.18 X [002 : 040| have been recorded. These spacings
match closely those quoted for low temperature albite (Brown 1961,p.480).
Out of a total of 130 samples analysed, only 6 gave indications
of the presence of a carbonate mineral phase. These are samples ID-3,
ID-4, ID-5, ID-6, 2E-6 and 2F-10 all of which were collected from Dobb’s
Linn, Samples ID-3 to ID-6 are grey mudstones from the D.anceps Zone
while 2E-6 is a claystone from the M.convolutus Zone and 2F-10 is a
greywacke from the M.sedgwickii Zone (Fig.16).
It was possible by using the quartz reflection at 2.46
X [lioj
as an internal standard to assess accurately the d - spacing for theprincipal reflection from the carbonate mineral in each of the six samples.
The calculated cl - spacings approximate to 3.04 X (corresponding to the
calcite |l04) at 29.43°) except only in sample 2F-10 in which a reflection
at 29.56° corresponds to the dolomite [l04] peak at 2.95 X.
The observed reflections were all at slightly higher spacings
than those published for the pure minerals, indicating the presence of
iron within the carbonate. This was confirmed by staining thin sections
of the samples with a 3:2 ratio mixture of alazarin red S and potassium
ferricyanide in 1.5 per cent hydrochloric acid. When stained, sample 2F-10
assumed a patchy appearance. Some patches were mauve in colour whereas
- 17 -
ferroan dolomite in the sample. The other five samples assumed a mauve
coloration when stained, indicating the presence of ferroan calcite.
b) Layered minerals
Preliminary X-ray examination revealed the presence of a
7 ft layered mineral in most samples. The exact nature of this material
was not immediately obvious, and therefore, a series of experiments was
undertaken upon a sample displaying strong reflections in the region of
12.5° in order to identify each reflection occurring between 2 and 14 X.
The sample chosen was DCYP-3 which was collected from the M.cyphus Zone
at Dobb’s Linn.
After eliminating from the diffractogram those reflections
attributable to quartz, there remained a series of unidentified peaks with d - spacings of between 2 and 14 X. These are listed in Table 2-1.
Three of the peaks correspond to spacings of 4.01 X, 3.65 ft and 3.18 ft
representing albite [20T] , [l3l : 130] and [002] reflections
respectively. Peaks at 3.12 X and 2.70 X correspond to pyrite jlll]
and [200] reflections.
The remaining peaks are illustrated in Fig.17. The
relatively strong peaks which occur at 3.52 ft and 7.01 ft and the weaker
peaks at 4.70 ft and 13.86 ft are assigned to chlorite [004], [002], [003J
and [OOl] reflections respectively. The dominance of the [004~j and
[002] reflections diagnosis' the ferroan as opposed to the magnesium /
* /
chlorite (Brindley, 1961 p.262).
The presence of expanding lattice chlorites and
montmorillonite minerals may be ruled out,because after treatment with
glycerol, the peaks exhibited no apparent change in d-spacing (Fig.17).
It is to be noted that the peaks corresponding to spacings of 3.52 ft and 7.01 ft, coincide with kaolinite reflections. Kaolinite,
however, has a diagnostic though weak reflection corresponding to 2.38 X.
If kaolinite is present only in small amounts the diagnostic reflection
could be submerged by the background and its absence is not to be taken
that chlorite alone is present (cf. Weir 1974).
Treatment in dilute acid dissolves chlorite whilst leaving
kaolinite substantially unaffected to continue production of its strong reflections corresponding to spacings of 3.5 X and 7.0 X (Brindley 1961
p.264). Sample DCYP-3 was digested for 15 hours in 10 per cent
hydrochloric acid at 80°C, washed until neutral to litmus paper and then
dried for 4 hours at 100°. A diffractogram was then run at the more
sensitive instrumental setting quoted above.
Acid treatment completely removed the reflections corresponding
to 13.86
X and
4.70X
and considerably weakened those corresponding to 4.95X,
7.01X
and 3.52X
(Fig. 17). No change in intensity was observed for the other peaks. The former two are assigned to chlorite [00l] and[003] while the latter two are assigned to chlorite [002] and {004J. The
calculated d - spacings of 9.86
X,
4.95X,
4.46X
and 3.75X
are assigned to sericite [002],[004], [lio]
and [023^ reflections (A.S.T.M. Powder Data File 2-0056). The assignment of each peak in the range from 2X
to 14X
is given in Table 2-1.Thermal treatment is also a valuable auxiliary technique for
identifying chlorites positively. Chlorites tend when heated to dehydrate
in three stages (Caill&re and Benin 1957). The first stage corresponds
to loss of water from the brucite layers with only minor resultant changes
in unit cell parameters. There is however, considerable rearrangement in
structure. This is manifested by changes in X-ray reflection intensities.
The second stage corresponds to loss of water from the talc layer. The
19
olivine (MgFe) SiO^. The temperature of the dehydration reactions
and the extent to which they are clearly separated depends on the
composition, on the crystallinity and on the particle size of the
chlorites as well as on the thermal conditions (Brindley 1961).
In general,chlorites which are low in iron and predominantly magnesian in character, dehydrate between 600° and 800°C, whereas the
reaction of chlorites which are rich in iron take place at lower
temperatures (Brindley and Youell 1953). Chlorites when heated to the
dehydration temperature exhibit increased intensity of the [00l]
reflections and decreased intensity of the [002], [003] and [004j
reflections. Iron rich chlorites show slightly greater shrinkage
parallel to ’c* than magnesian chlorites (Martin 1955).
As with chlorite, the dehydration of muscovite also depends
on the crystallinity of the material. Fine grained muscovite is reported to dehydrate at 750°C whereas coarse grained material
dehydrates in the region of 900°C (Bradley and Grim 1961). The
results of step-heating and continuous heating of fine grained
muscovites indicate that water is released over three temperature
intervals. These correspond to the release of superficial adsorbed
water, the release of water in cracks and inclusions and the release
of the water of constitution, (Zimmerman 1970). The first interval occurs between 100°C and 400°C, the second between 400°C and 700°C
and the third in the region of 700°C. Associated with the release
of water there is progressive dilation in the basal spacing. This
commences at about 300°C (Reif 1966).
Portions of sample DCYP-3 were heated for 60 minutes at temperatures of 450°, 500°, 600°, 800° and 820°C and diffractograms
of the roasted powder were run at the more sensitive settings given
This illustrates that with increasing temperature, up to a maximum of 600°C, there is a progressive increase in the intensity of the chlorite
[OOl] peak relative to the other chlorite reflections. The change in
intensity can readily be picked out by comparing the ratio of the
intensity of the chlorite [OOl] reflection to the intensity of the
chlorite [002] reflection. This ratio increases from 0.51 for the
untreated sample to 0.80 for the sample heated at 450°C and then to 0.86
at 500°C before attaining a maximum value of 1.79 at 600°C.
The peaks which are present at 2.56 X and 3.52 X in the
diffractograms of the sample heated at 800°C are attributable to sericite
[202] and [I14] reflections. It is noteworthy that the chlorite basal
reflections re-appear in the sample which was heated at 820°C. An
explanation is not immediately forthcoming but the re-appearance of the
peaks may be related to disequilibrium within the resultant mineral
assemblage.
The reflection intensities of sericite exhibit similar changes.
At temperatures of 450°C and above, the sericite [lio] reflection is the
most intense. Increasing temperature decreases the intensity of the sericite [004] and [002j reflections. The peak attributable to a mixture
of the sericite {§02j and the chlorite [l32 : 20'lJ reflections is not
evident in the sample heated to 820°C. Associated with the variation in
peak intensities the sericite basal spacings increase with increasing
temperature. This may be illustrated by examining the sericite [002j
reflection which occurs at 9.86 X in the untreated sample and at 9.90 X
in the sample heated at 600°C.
The above experiments were repeated on samples collected from
the P.linearis Zone at Hartfell, the D.clingani Zone at Dobb’s Linn, and
the A.acuminatus Zone at Dobb’s Linn. In each case, results similar to
21
those outlined have been obtained. It is therefore concluded that both
sericite and an iron-rich chlorite, possibly clinochlore.contribute along
with quartz, albite and pyrite to the mineralogy of the Moffat Shales.
c) Interpretation
The observed mineral assemblage of quartz-albite-sericite-
chlorite is Characteristic of the low-temperature zone of the greenschist
facies (Winkler 1967). This facies, occurring at the boundary between
diagenesis and metamorphism, is related to temperatures of 300° C and is
almost independent of pressure. Such a temperature can be attained in
burial metamorphism at normal geothermal gradients (30°C/km) at depths
of 10 km. It is likely however that since the Moffat Shales have been
subjected to large scale orogenesis there has been additional thermal
energy produced so that the critical transitional temperature which
delineates diagenesis from metamorphism must surely have occurred at
much shallower depths.