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| hexagonal
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| citrine |
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| amethyst |
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| 2
tonne quartz crystal |
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| quartz
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SCIENTIFIC
DISCOVERIES |
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|
The
Chemical Composition of Opal |
In this section we will
look at the following areas:
|
Introduction |
| Most
collectors, lapidarists and others
interested in minerals and gemstones
are aware of the fact that opal is
a poorly crystalline form of silica
containing some water. Silica itself
is an oxide of silicon (silicon dioxide)
with the chemical formula SiO2.
At least one other oxide of silicon
exists, silicon monoxide (SiO), but
can only be formed in the laboratory;
it does not occur in nature.
Silicon dioxide exists in many forms.
The commonest is the mineral quartz,
which occurs in a variety of physical
forms. It contains no water, and is
represented by the chemical formula
SiO2. It is best known
in the form of hexagonal
crystals generally colourless,
but may be almost black,
brown, yellow, purple,
pink, or more rarely, greenish or
bluish in colour.
The coloured varieties are well known
gemstones: cairngorm or morion (brown),
the traditional stone of the Scots;
citrine (yellow); amethyst (purple);
rose quartz (pink). The colourless
material is termed rock crystal.
The crystals may be of great size;
a crystal exhibited in the Heimat
Museum in Idar-Oberstein in Germany
is nearly two metres high and weighs
two
tonnes.
However, Bauer and Bouska (R1599)
in their book on precious stones state:
|
| |
One
such crystal had a circumference of
7.5 metres and weighed some 45 metric
tons. In September 1958 a crystal
was found in Kazakhstan in the USSR
that was as high as a two-storeyed
house. Its estimated weight is 70
metric tons.
Flawless quartz is an important industrial
material in the electronics field,
where it is used as an oscillator,
although nowadays most of the quartz
for this purpose is synthetic. In
fact quartz crystals of any colour
can be grown, and the corresponding
natural gemstones can thus be easily
synthesised.
Quartz is also found abundantly as
a constituent of many rocks. It is
a necessary constituent
of granite, and is the main constituent
of sedimentary rocks such as sandstone,
and metamorphic rocks such as quartzites.
It occurs in appreciable amounts in
most sedimentary rocks except the
carbonates (limestones and dolomites,
for example), and in most metamorphic
rocks, again except for the carbonates.
The weathering of these rocks (quartz
being the most resistant to weathering),
frequently forms accumulations of
grains to form the sands of our beaches
and creeks. Fine grained quartz often
accumulates with clays.
Very finely crystalline aggregates
of minute quartz crystallites are
collectively called chalcedony (pj81).
It is a hard, tough material with
which, in the form of flint, man established
his first manufacturing industry.
It can be chipped and flaked to form
useful tools and weapons,
often with very keen edges.
Flints are abundant in the chalk deposits
of southern and eastern England, as
well as in Europe, and the remains
of large prehistoric industries of
this type are to be found in pits
in the chalk deposits. In the middle
ages, and even up to more recent times
in these areas, flints were used as
a building
stone. They were of a convenient
size to handle, so did not need physical
quarrying or shaping, and are extremely
resistant to weathering. |
| |
Many
other forms of chalcedony occur; chert
is an abundant type which commonly
forms grey bands in bedded limestones.
There are various forms of coloured
chalcedonies, which, because of their
beauty and durability, have been used
as ornamental stones for thousands
of years.
These include such stones as onyx,
agate,
chrysoprase,,
prase, bloodstone, carnelian and others.
These materials were commonly used
for cylinders and other types of seals
by people of ancient civilisations.
They were often intricately carved
and engraved, and were very durable.
Some forms of chalcedony are so fine
grained that they are sometimes difficult
to distinguish from common opal and
in some cases may have been formed
by the transformation of the less
stable opal to the more stable chalcedony.
Chalcedony also differs from other
types of quartz in that it usually
contains a small amount, about 1%,
of water, probably held in minute
pores between the tiny quartz crystallites.
The colours of the various types are
caused by 'impurity' elements absorbed
during formation of the material.
Other types of silica are rare or
non-existent in nature. The best known
are cristobalite and tridymite. These
are occasionally encountered in small
cavities in volcanic rocks; they are
formed at high temperatures.
Theoretically, on heating, quartz
transforms to tridymite at 873°C,
and tridymite transforms to cristobalite
at 1470°C. However, the changes
occur only slowly, and are complex,
as each of these minerals themselves
have two
or more forms (polymorphs).
Quartz has two forms, tridymite three,
and cristobalite two. These latter
forms convert readily into one another
within their groups. This is an important
phenomenon industrially, as all of
these forms of silica are frequently
encountered, and are often essential
components of refractories, slags,
and other ceramics. |
| |
| Even
rarer are those forms of silica which
are formed only at very high pressures.
Such forms are coesite, stishovite
and keatite. They are formed in the
laboratory under very high pressures,
and the first two have been found
in the environments of craters formed
by the impact of large meteorites
with the earth. Enormous pressures
are generated on impact, and quartz
in the impacted rocks has in some
cases been converted to one or another
of these forms. One of the best known
localities for coesite, for example,
is the great Meteorite Crater in Arizona.
Keatite has not been found in nature.
The melting point of pure silica is
1723°C. It melts to a very viscous
liquid, and when cooled does not tend
to crystallise, but forms a glass.
Occasionally natural silica glass
is found. Such high temperatures are
rare in nature, but can occur in,
for example, a lightning strike. Silica
glass has sometimes been found in
sandy areas where lightning has struck;
it has been called lechatelierite.
Silica glass is readily made industrially,
and is a valuable material for a variety
of purposes in industry and in the
laboratory.
'Haystack
glass', formed by the combustion
of large quantities of hay or similar
materials, is an impure form of silica
glass. Its melting has been assisted
by the absorption of impurities such
as potassium, sodium, calcium and
aluminium in small amounts.
Other forms of silica containing water
are known in nature. The rare form
silhydrite has the chemical formula
3SiO2.H2O. It
is known mainly from a spring deposit
in California, and has been formed
by the natural leaching of sodium
from the silica-rich hydrated mineral
magardiite. The other form of hydrated
silica common in nature is opal. |
|
The Water
in Opal |
|
Water
is regarded as an essential
constituent of opal and opaline
silica. Its content is variable,
ranging from about 3% in true
hyalites to up to 20% and more
in some earthy types such as
diatomite. The chemical formula
of opal is therefore usually
represented as SiO2.nH2O.
A large number of determinations
of water in opal have been made
over the last 150 years or more,
and it is of interest to plot
the percentage of water found
(at, say 0.5% intervals) against
the number of analyses for each
0.5% range. An
illustration has been compiled
from some 170 analyses.
In some cases the analyses are
weighted towards a specific
occurrence, but the general
trend is clear. Two main maxima
occur, one around 3-3.5% water,
and the other in the range 4.5-9.5%
of water. Many of the opals
associated with the lower maximum
are hyalites, the glass-clear
mineral found mostly in association
with volcanic rocks.
The main maximum covers most
other opals, including common
and gem varieties. The samples
showing around 1% water are
most likely to be chalcedonies
wrongly reported as opals, although
it is possible that some volcanic
opals have been dehydrated.
Those with very high water contents
are most commonly earthy types
such as diatomites. |
| Dehydration
of Opal |
|
| Most
of the water in opals
is not tightly held. In
fact careful weighing
between summer and winter
can sometimes detect a
weight change, the higher
temperatures of summer
driving off a small amount
of water. Further heating
results in a greater loss
of water.
The loss of water by heating
is most clearly shown
by differential thermal
analysis (DTA), a technique
in which the powdered
sample is heated at a
uniform rate, and the
temperature of the sample
monitored against a neutral
material. In general,
loss of water from the
sample causes a change
in its temperature; it
becomes slightly cooler
than the furnace temperature.
This is recorded on a
graph; if the change in
temperature is sudden,
a peak appears on the
graph. If the water loss
is slow and gradual, no
peak is seen. Parallel
with the DTA determination
it is usual to monitor
the weight change of the
sample. This technique
is called thermogravimetric
analysis (TGA) and records
continuously any weight
change, usually a loss
in weight.
In the case of opals,
variable results are obtained.
A
graph shows a selection
of DTA curves for different
specimens of opal. They
range from a typical sedimentary
gem opal, to common opals.
Gem and related opals
(potch) seldom show a
peak (termed an endotherm)
associated with water
loss, indicating that
the water is slowly driven
off as the temperature
rises (Curve A).
Common opals show a good
deal of variability in
the shape of their DTA
curves. Curves B and C
are the type usually obtained
from common opal (opal-CT
in the main). However
the white and translucent
opals illustrated
here, which are identical
in most respects, give
different DTA curves.
The white material shows
a marked endotherm, while
the translucent gives
a result similar to that
obtained for gem opal.
In this case the difference
was simply due to a system
of microcracks in the
white opal. A scanning
electron micrograph clearly
shows the cracks
in this opal. Other samples
showed minor variations
in the shape of the endotherm,
and one sample even showed
two endotherms.
It can be seen from these
curves that water is easily
driven off from opal.
Most of it is gone by
200°C, although small
amounts cling on until
much higher temperatures.
In pure synthetic silica
powder, traces of water
are retained up to 1100°C. |
|
How
is the Water Held? |
|
|
The
different types
of differential
thermal analysis
(DTA) curves obtained
from opals and opaline
silicas were the
catalyst which resulted
in a considerable
amount of work being
carried out in recent
years to determine
the nature of the
bonding of the water
in opaline silica.
Most of the earlier
work, some of which
has been outlined
previously, dealt
mainly with the
actual amount of
water in the opal,
and its effect on
physical properties.
The DTA work of
Jones et al (R0371)
suggested that there
were different ways
in which water was
held in opals from
different sources.
They noted three
main types of DTA
curves which
they classified
as being derived
from, in general,
three types of opal:
| 1.
Most
glassy opals,
including precious
opal. These
produced a DTA
trace showing
few or no inflections,
and certainly
no peak corresponding
with a loss
of water. |
| 2.
Most
opaque opals.
These usually
gave a definite
rounded endothermic
peak between
100°C and
200°C corresponding
with a loss
of water. |
| 3.
A small
group of translucent
red or brown
opals, which
gave a sharp
water-loss peak
at a little
above 100°C.
|
This
classification was
somewhat simplistic,
as later work was
to show, although
it was observed
at the time that
this limited evidence
indicated that most
of the water in
opal was not chemically
bonded.
Detailed work had
been published on
the bonding of water
by synthetic amorphous
hydrous silica,
and this was relevant
to the nature of
the water in opal.
Later work on opal
took note of this,
with investigations
on the nature of
the water in opal
using infrared spectroscopy,
nuclear magnetic
resonance, and a
more detailed examination
by thermal techniques
(see bibliography).
All of the techniques
indicated that most
of the water was
physically absorbed
in the internal
pores of the opal.
The rate of release
of this water, either
by dehydration at
constant temperatures,
or by continuous
heating was variable,
a phenomenon explained
by the physical
structure of the
particular opal.
A good example of
this is the Lake
Eyre wood opal,
described in the
previous section.
The clear variety
lost water slowly,
the white rapidly.
Scanning electron
micrographs showed
that the latter,
milky variety, contained
a network of minute
cracks which
allowed water to
escape rapidly,
while, with the
clear variety, it
must necessarily
diffuse mainly through
the silica gel structure.
Nevertheless, most
of the water, in
both cases, was
lost by 200°C
under isothermal
conditions. This
result was confirmed
by other methods,
indicating that
most of the water
was physically held.
|
|
|
|
|
Other
opals gave similar
results, but, in
some cases, less
water was evolved
at the lower temperatures;
this was particularly
the case with X-ray
amorphous opals,
those with a physical
structure of sub-microscopic
silica spheres,
either uniform in
size and close packed
(precious opal)
or irregular in
size (potch).
It was clear that
the spaces between
the spheres could
hold free water,
but, after this
was evolved, there
was found to be
a considerable proportion
of the water still
in the structure.
This was attributed
to surface bonding
of hydroxyl groups.
The silica spheres
themselves were
composed of very
small (10-20 nm)
particles of
silica which suggested
a very high internal
surface area.
A calculation indicated
that the internal
surface area of
the opal based on
these small particles
was 200-100m2/gram.
A complete coverage
of these surfaces
by hydroxyl groups
requires a water
content of 2.4-1.2%
water.
Many of these phenomena
may be explained
by the widely accepted
view that all silica
surfaces tend to
be covered by a
layer of (OH)
groups, either
as single hydroxyl
groups, or as pairs
of hydroxyls.
The twin groups
may lose water by
condensation of
neighbouring hydroxyls
at as low a temperature
as 200°C under
vacuum, but the
reaction is reversible.
At 400°C, however,
changes take place
in the nature of
the silica surface,
and the twin groups
cannot be regenerated.
With increasing
temperature, further
condensation of
hydroxyls to form
water from the silanol
groups occurs, with
consequent gradual
loss of water.
A more detailed
analysis of the
water in several
types of opaline
silica was carried
out by Langer and
Flörke (R1551).
Using polished plates
for infrared spectroscopy,
they were able to
differentiate more
clearly between
molecular water
and -SiOH groups
in the opals. They
were able to evaluate
quantitatively the
contents of 'water'
of different types
in the samples.
They gave the
estimates for
the three different
types of opal they
examined (four samples
of opal-AN, five
of opal-AG and three
of opal-CT).
The figures for
opal-AG and opal-CT
agree fairly well
with the figures
suggested by Jones
and the author,
who did not examine
opal-AN in detail.
|
|
|
|
|
Langer
and Flörke
also noted two types
of molecular water:
They
found the highest
fraction of type
A water in opal-AN,
and the lowest in
opal-CT. They also
found two types
of -SiOH groups,
A and B, the latter
with stronger hydrogen
bonding.
In the case of precious
and related potch
opals (opal-AG),
with their silica
sphere microstructure,
the type A water
is held within the
silica network 'cages'
inside the larger
spheres. The type
B water is held,
largely by hydrogen
bonding, in the
pores caused by
the close packing
of the larger spheres.
|
|
|
|
Constituents
other than Silica and Water |
|
| An
indication of the amounts of
the foreign constituents may
be obtained by evaporation of
the opal sample with hydrofluoric
and sulphuric acids, followed
by ignition at about 1000°C.
The residue will consist largely
of sulphates of alkali and alkaline
earth elements, and oxides of
the heavier elements.
Occasionally, opals are found
which are remarkably low in
any components other than silica
and water. A hyalite from Victoria
yielded a residue of 0.14 weight
percent of the original sample,
and an opal-CT from South Australia
left a residue of only 0.08%.
Most opals contain less than
three percent by weight of foreign
elements calculated as oxides.
Of 75 opals analysed in this
manner, 70% had a residue of
less than 3%. Six opals had
a residue of more than 10%;
the maximum was 23.4%.
A triangular plot of silica/water/residue
ratios is instructive; the
concentration of compositions
in the 5-10% water and 0-5%
residue area is clearly shown.
|
|
Major
Constituents of the Residue |
|
The
major constituents of
the residue are usually
iron, aluminium, calcium,
magnesium, sodium and
potassium, although in
widely varying proportions.
A survey of published
and unpublished chemical
analyses of opals of all
kinds, as well as analyses
of the residues from the
hydrofluoric-sulphuric
acid treatment, also shows
that there are certain
patterns in the chemistry
of the 'impurity' elements.
A triangular
plot shows the distribution
of residue compositions
on a three component diagram
with iron oxide at one
corner, calcium and magnesium
oxides at another, and
aluminium oxide at the
third corner.
It can be seen that there
is one concentration of
analyses (group A) towards
the iron oxide corner;
that is, if the iron content
is high, there are only
small amounts of the other
elements. Along the base
of the diagram, there
are scattered a group
which tend to be high
in alumina, often with
some alkaline earths,
but always low in iron.
Those samples labelled
'S' on the figure are
siliceous sinters and
geyserites; these clearly
make up a characteristic
group (group C). There
is another group which
plots towards the (Ca,Mg)O
corner of the diagram
(group B). These tend
to be rich in magnesium;
they are labelled 'M'.
The latter are, in fact,
compositions of the residues
of an opaline material
which was called 'menilite'
in earlier days.
Finally, in the central
part of the diagram there
are numerous samples which
form a scattered field.
These largely comprise
precious opals (opal-AG)
and ordinary common opals,
mainly opal-CT, which
can have a wide range
of ratios of the common
elements of the residues.
|
|
|
|
Such
concentrations of certain
elements suggest that
the pattern is related
to the mode or environment
of formation of the particular
groups of opals. A selection
of analyses of some iron-rich
opals and the corresponding
residues is given in a
table. All of these opals
were of the opal-CT variety.
The optical
microstructure usually
showed an inhomogeneous
mixture of iron oxide
material in the opal.
The
iron content was present
in various forms in these
samples:
| 19023:
This opal, of a dark
brown colour, occurs
in pods in a basalt.
The XRD pattern was
of opal-CT; no form
of iron was detected,
so it was evidently
present as an amorphous
admixture. |
| 19236:
This sample was yellow
brown in colour, with
a shiny conchoidal
fracture. It also
occurred in a basaltic
environment, but was
collected from decomposed
surface material.
XRD indicated opal-CT,
but no crystalline
iron oxides. |
| 21588:
This was obtained
from veins in a volcanic
tuff; it was brown
in colour. The XRD
pattern showed highly
disordered opal-CT;
the iron was present
as goethite, and there
was a little quartz. |
| 19002:
Another opal-CT with
no crystalline iron
oxide. Its associations
are not known. |
| 19032:
Also opal-CT with
goethite and a little
quartz; associations
are not known. |
A red opal (residue content
14.6%) from Cooma, New
South Wales was highly
disordered opal-CT; at
least some of the iron
was present as hematite.
Most of these opals appear
to have been formed at
a late stage in basaltic
rocks, which have presumably
been the source of iron.
One would also expect
other ions to be present
in this type of environment.
However, iron hydroxide
can precipitate at a much
lower pH than most other
common elements; it can
begin to precipitate at
pH3. Therefore, if silica
precipitates in a slightly
acid environment, and
iron is present, the two
materials can co-precipitate,
with only minor contamination
by other common ions.
|
|
|
|
| Bayliss
and Males (R1504)
analysed precious
and potch opals from
three of the major opal
fields of Australia. The
major constituents of
the residues, which were
low in iron, were alumina
and lime. These analyses
have been recalculated
and presented in a similar
form to those of the iron-rich-opals
for more direct comparison.
The total residue content
is considerably less than
in the iron-rich opals.
There were no very significant
differences between the
compositions of potch
and precious opals, and
this is not to be expected
in the present knowledge
of the relationships between
the formation of these
two types.
In general, where a high
aluminium content is found
in opals of whatever type,
there is usually a low
iron content, and vice
versa. There are, of course,
some exceptions to this;
a precious opal from Nevada,
for example, is recorded
as containing 3.22% Al2O3
and 1.85% Fe2O3.
Precipitation at a somewhat
higher pH will engender
the co-precipitation of
these two elements.
The major 'impurity' elements
in the opals from the
Australian opal fields
are dominated by the major
elements in the weathering
environment, in this case,
aluminium and calcium.
The Cretaceous beds associated
with opal deposition are
largely pale sandy clays,
and the movement of aluminium
in the environment is
shown by the presence
of alunite, which is abundant
in some areas. Calcium
is also plentiful, especially
in the opal horizons,
in the form of gypsum.
Another group of opaline
silicas which are low
in iron but high in aluminium
are the geyserites and
siliceous sinters. These
are formed around volcanic
vents and hot springs
where they have been brought
to the surface and deposited
by steam and hot water.
The compositions
of the residues from this
group form a narrow zone
across the low iron area
illustrated in a triangular
plot.
In the case of common
opals (opal-CT mainly)
the residue compositions
vary widely, and are clearly
to be related to the environment
in which the opal formed.
This is also illustrated
and shows the distribution
of the main constituents
of the residues of a wide
variety of opals and opaline
silicas, including precious
opals.
In general, therefore,
it can be stated that
the main 'impurity' elements
which are found in opals
and opaline silicas are
iron, aluminium, calcium
and magnesium, and smaller
amounts of sodium and
potassium. The relative
amounts are determined
by the chemical environment
in which the opal is formed;
in some cases, especially
with those with a high
iron content, pH probably
plays an important part. |
|
Minor
Constituents of the Residue |
|
| In
addition to the major
constituents of the residue
from the evaporation of
opaline silicas with hydrofluoric
and sulphuric acids, a
wide range of minor constituents
have been found. Apart
from alkalies, which are
usually present to some
extent, there occur a
variety of elements in
trace amounts. Opaline
silicas, have, by the
nature of their physical
structure, a large internal
surface area exposing
a large silica/hydroxyl
surface to the chemical
environment.
This surface is capable
of adsorbing significant
amounts of the trace elements
occurring in the environment
of formation of any particular
opal. In other cases,
where we note a high phosphorus
content, for example,
it is likely that another
mineral has co-precipitated
with the opal. This could
be the case, for example,
with sample
17048, where high
rare earth contents are
associated with a high
phosphorous content. In
the case of C26, it is
probable that barite has
been coprecipitated with
the opal.
As an example of the range
of elements, the diagram
shows the results of semi-quantitative
spectrographic analysis
of four opals from Australia
and New Zealand, together
with the average content
of these elements in the
earth's crust for comparison.
These analyses, as well
as the germanium analyses
referred to below, were
carried out on behalf
of J.B. Jones and the
author by the Australian
Mineral Development Laboratories,
South Australia.
The high content of gallium
in the siliceous sinter
probably reflects the
association with aluminium,
which appears to be a
common element in this
type of opal. Similarly,
the high lithium and rubidium
contents reflect the volcanic
(hydrothermal) origin
of the opal.
The heavy metal content
of some opals is surprisingly
high; these metals are
probably absorbed on the
silica surface. They must
reflect an abnormally
high content of these
metals in the local environment,
suggesting the possibility
that opaline silicas may
be a useful geochemical
exploration tool.
There are many references
in the literature, right
back to the 19th century,
of opals containing surprisingly
high contents of unexpected
elements or compounds.
For example, Royer (R1552)
described an opal rich
in arsenic from St. Nectaire
(France). Ahlfeld (R0389)
examined opal from an
antimony deposit in Argentina,
and found up to 17.9%
antimony, probably present
as the oxide. Gerasimovski
(R1023)
found, on blocks of sodalite
syenite from the Kola
Peninsula (Russia), incrustations
of opal containing nearly
8% sodium oxide, as well
as fluorine.
Payne and Mau found (R1346),
in an altered basalt from
Kilauea (Hawaii), an opal
containing 10.9% titanium
oxide. High contents of
sulphur, usually as sulphate,
have been encountered
in opal from hot springs.
Rammelsberg (R1602)
found 7.8% sulphur (19.5%
sulphur trioxide, SO3)
in a geyserite from Pozzuoli
(Italy), while Payne and
Mau reported 2.7% SO3
in the Kilauea opal.
Some of the older references
also reported carbonate
in opal. Kokta (R1582)
quotes Schaffgotsch as
finding 9.1% calcium carbonate
in a 'Schwimkiesel' (geyserite)
from St Ouen (France),
and Damour as finding
3.2% carbon dioxide (=
6.2% calcium carbonate)
in an opal from Buttes-Chaumont
(France).
The low germanium figures
are also of interest.
Of 48 opals and opaline
silicas, both opal-A and
opal-CT, analysed for
this constituent by emission
spectrography, 42 yielded
less than one part per
million (ppm) germanium;
the maximum content observed
was 2 ppm. For comparison,
a chalcedonic (mineralogically
quartz) petrified wood
contained 50 ppm. It is
clear that the conditions
under which opal forms
separates germanium from
silicon. |
|
Organic
Constituents |
|
| From
time to time, organic
constituents have been
reported in opaline materials.
One area of interest in
this field is in the nature
of the famous black opals
from the Lightning Ridge
field in northern New
South Wales. This variety
of opal, which, when it
was first displayed on
the world markets was
considered to have been
treated in some manner,
has a dark background
against which the diffracted
colours look their best.
The origin of the black
colour of the background
has long remained a mystery,
but recent work suggests
that it is caused by the
presence of small amounts
of dark organic material
or finely dispersed carbon.
An analysis of black potch
from Lightning Ridge,
using the technique for
determination of carbon
in steel, showed the presence
of 0.16% carbon. If very
finely dispersed, this
is probably enough to
cause a dark body colour.
It may be mentioned that
elements such as iron,
sulphur and manganese
were not detected in the
sample.
Organic matter in opaline
materials has been reported
from other sources. A
black opal was reported
from a pegmatite at Volyn,
(former) USSR (Gigashvili,
R0255).
It appears to have been
an opal-CT, and firing
under vacuum at 600°C
caused blackening of the
sample, with condensation
of an oily liquid on the
cold parts of the sealed
tube. The organic material
had a carbon/hydrogen
ratio of 84.7/15.3, which
was close to the ratio
for natural petroleum.
Theories
of origin were:
| 1.
Migration of bituminous
materials from country
rock and concentration
with the primary minerals
at a late stage of
mineralisation. |
| 2.
Formation of organic
materials from aqueous
colloids and co-deposition
with colloidal silica.
|
| 3.
By reaction between
carbon monoxide and
hydrogen in the presence
of bivalent iron (from
pyrite) as a catalyst.
|
A
third reported occurrence
was in an opal claystone
from Kulkurna Station,
west of Wentworth, south
western New South Wales.
The earthy material was
white in colour, but DTA
recorded a sharp exotherm
at 228°C. Chloroform
extracted footnote
organic compounds which
made up some 0.1% of the
opal claystone. Much of
this organic matter comprised
a series of alkanes and
branched and cyclic hydrocarbons.
The conclusions were:
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