Eur. J. Mineral.
2002, 14, 837–848
How grey limestones become white marbles
JACQUELINE SCHMID1 and IVO FLAMMER2
1 Geologisches
Institut, Universität Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland
e-mail: [email protected]
Present address: UNESCO Cairo Office, Abdel Rahman Fahmy Street, Garden City, Cairo, Egypt
2 Institute of Applied Physics, Universität Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
Corresponding author, e-mail: [email protected]
Present address: ALCATEL, Optical Fiber Division, 53 rue Jean Broutin, F-78703 Conflans, France
Abstract: The conditions required for generating white marbles are investigated in the contact aureole of the Adamello Pluton in
northern Italy, where grey limestones pass into white marbles with increasing metamorphic grade. The grey-level or whiteness of the
carbonate rocks is quantitatively measured using an integrating sphere, and values are linked to the mineralogical components and
textural features. Nearly pure calcite rocks (> 99 wt% CaCO3) undergo a transition from dark and light-grey very-low-grade
metamorphic limestones to white and dull white marbles close to the intrusive contact. The grey colour of the limestones is caused
by as little as 0.05 wt% of finely dispersed organic carbon. Approaching the pluton, the organic carbon content decreases with
increasing metamorphic grade, producing a pure white to dull white marble with less than 0.02 wt% of organic carbon. Grain size and
grain-boundary width have a secondary control on the appearance of high-grade white marbles by controlling light absorption, and
thus brightness as well. The photon path is longer leading to higher absorption in coarse-grained marbles, thus explaining their dull
appearance. Otherwise, changes in refractive index caused by wide grain boundaries are believed to enhance the light reflection in
statically recrystallised marbles compared with very-low-grade metamorphic limestones which appear darker. Secondary phases
such as silica or ore minerals, or significant fluid flow, do not induce colour changes.
Key-words: grey limestone, white marble, organic carbon content, optical properties.
1. Introduction
Marbles s.s. are metamorphosed carbonate rocks, occurring in a number of different colour varieties. Several authors have investigated the colour of marbles. Lepsius
(1890) described some ancient quarries in the Mediterranean, where he observed grey, bluish and brownish varieties in addition to the common white marbles. This author
attributed the different colours to finely dispersed mineral
inclusions. Camisasca (1941) and Papageorgakis (1961)
found that grey marbles owe their colour to the presence of
Fe-rich silicates or ore minerals, thus confirming the observations of Lepsius (1890). Papageorgakis (1961) stated,
that the mineral content, and thus the colour of the grey
marbles, is mainly due to the initial mineral composition of
the limestone protolith.
White marbles have been widely used as building and
sculptural stones in ancient and modern times. Nevertheless, to our knowledge, the conditions required for producing such a marble have not been investigated. Nevertheless,
Deines & Gold (1969) noted that the metamorphism of grey
limestones into recrystallised marbles may be accompanied
by a general “bleaching” of the carbonate rock. Todd (1990)
suggested that fluids caused bleaching of thin silicate-rich
limestone layers in a contact-metamorphic setting.
This paper addresses the genesis of white marbles by
studying the change in colour associated with the transition
from very-low-grade grey limestone to a high-grade metamorphic, white marble in the contact aureole of the Adamello Batholith, in Northern Italy.
2. Geologic framework and sampling
The Adamello Pluton is the largest Tertiary intrusion in the
Alps, and is located north of Brescia in northern Italy (Fig.
1). During Eocene to Oligocene times, the calc-alkaline
suite of the Adamello Batholith intruded and contact-metamorphosed the Southern Alpine crystalline basement and its
Permo-Mesozoic cover. The Pluton consists of interfingered stocks of granite, granodiorite and tonalite, with numerous small bodies of diorite, gabbro and gabbro-diorite
occurring along the contact with the sedimentary rocks
(Brack, 1984; Ulmer et al., 1983, and references therein).
We examined the Middle Triassic “Calcare di Dosso dei
Morti” formation located in the Valle di Daone area at the
0935-1221/02/0014-0837 $ 5.40
DOI: 10.1127/0935-1221/2002/0014-0837
ˇ 2002 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart
838
J. Schmid, I. Flammer
Fig. 1. Geological map of the study area (modified from Brack, 1984), showing sample locations and metamorphic temperatures. The position of the Forsterite-in (Fo-in)-isograd is given according to Gerdes et al. (1999). For clarity, sedimentary formations other than the Calcare
di Dosso dei Morti and Verrucano Lombardo are omitted.
south-eastern margin of the Adamello Massif. This massive
carbonate formation can be followed from its non-metamorphic facies at Dosso dei Morti towards a strongly recrystallised marble close to the contact with tonalitic intrusive rocks
(Fig. 1).
The Calcare di Dosso dei Morti consists of a very pure,
fine-grained, massive, light- to dark-grey calcareous limestone. Minor amounts of dolomite can be observed as well
as rare detrital white mica, quartz and feldspar. Ore minerals
are finely dispersed in the matrix. The Calcare di Dosso dei
Morti formation represents massive shallow marine platform carbonates. Biomicrites to biopelsparites are mainly
built up by stromatoporoids. Additional bioclastic components in these limestones include dasycladacean algae, echinoderms, bivalves, foraminifera and gastropods. Large cavities are filled with rhombohedral and sparry calcite. The
contact metamorphic equivalent is a coarse-grained, clear to
dull white calcite marble containing less than 1 % by weight
of dry residue (tremolite, phlogopite, quartz, feldspar, very
rare dolomite and a few ore minerals, Schmid, 1997).
How grey limestones become white marbles
2.1 Metamorphic geology
The Calcare di Dosso dei Morti in the Valle di Daone area
was metamorphosed at ~ 1.6 kbar, due to the stratigraphic
overburden at the time of intrusion (inferred from Callegari,
1962; Matile & Widmer, 1993; Riklin, 1983a). In the underlying Verrucano Lombardo, pre-intrusive temperatures are
estimated as e 200 °C (Riklin, 1983b). Contact metamorphic temperatures in the area can be characterised as follows: peak metamorphic temperature of 600 °C attained in
the sediments at the intrusive contact (Matile & Widmer,
1993; Riklin, 1983a; Vogler, 1985). South of La Uzza, Bucher-Nurminen (1982) mapped a forsterite-tremolite-spinel-chlorite isograd in dolomitic and calcitic marbles stratigraphically overlying the Calcare di Dosso dei Morti (Fig.
1). This author estimated the metamorphic temperature at
the isograd to be near 530 °C. Talc present at the Passo del
Frate could indicate a maximum temperature of about
400 °C (Bucher-Nurminen, 1982). However, this author
discussed a number of uncertainties related to this temperature estimate. Additional metamorphic temperatures
throughout the studied calcitic formation were determined
by calcite-dolomite geothermometry (Schmid, 1997), and
are indicated in Fig. 1. The temperature profile shows a distinct step at a distance of about 1000 m from the plutonic
rocks (Fig. 6), indicating a strong heat source underlying the
sedimentary sequence, possibly caused by an irregular
shape of the granodioritic or tonalitic intrusion, or by smaller gabbroic bodies occurring at the margin of the tonalite
near the summit of La Uzza (Brack, 1984; Ulmer et al.,
1983). This abrupt step may be confirmed by the temperature estimates at the Fo-in isograd (530 °C), and the (uncertain) temperature value of 400 °C for the Passo del Frate
(Bucher-Nurminen, 1982, and Fig.1).
839
yses and observations under the reflected light microscope.
3.1 Microprobe analyses
Compositions of the carbonates were obtained by wavelength-dispersive spectrometry (WDS) using the CAMECA
SX-50 electron microprobe at the Institute of Mineralogy
and Petrography, University of Bern. Analytical conditions
for major and trace elements were: spot size 5 x 7 µm, accelerating voltage 15 kV, beam current 13 nA for Ca, 15 nA for
Mg, Fe, Mn and Sr, counting time 20 s for Ca, 45 s for Mg,
and 200 s for Fe, Mn and Sr. Other elements could not be detected by WDS. In view of the long counting times described, quantitative analysis of trace elements required a
procedure to reduce the accumulation of surface contamination. The “anticontamination device” installed on the SX-50
consists of vacuum cold traps and an oxygen jet.
3.2 Stable isotope analyses
Measurements of oxygen and carbon stable isotope ratios
were carried out at the Institute of Geology, University of
Bern. Approximately 10 mg of powdered sample was reacted in H3PO4 at 90 °C in an on-line automated preparation
system. The resulting CO2 was analysed on a VG Prism II
ratio mass spectrometer. Reproducibility is better than
0.1 ‰ for · 18O and approximately 0.05 ‰ for · 13C (Burns
& Matter, 1993). · 18O values are reported relative to
SMOW, · 13C relative to the PDB standard, whereas the internal standards are calibrated against the VPDB standard.
3.3 Quantifying the grey-level of the samples
2.2 Samples
Rocks were supplied from 35 localities in the Calcare di
Dosso dei Morti formation, starting with very-low-grade
metamorphic limestones at a distance of about 4.5 km from
the pluton; successive samples were taken approaching the
contact between the sediments and the intrusive rocks (Fig.
1). The distance of each sample to the intrusive contact is referred to as the “apparent distance”, which is defined as the
shortest distance between the sample site and the pluton.
The subsurface position of the intrusive contact was hypothetically extrapolated assuming a constant dip.
3. Analytical methods
Thin sections of each sample were used for transmitted
light optical microscopy, cathodoluminescence microscopy and grain size measurements. In samples containing
dolomite (detected by cathodoluminescence microscopy),
X-ray analysis was used to determine its modal abundance
(in wt%) by comparing the dolomite peak with a previously established curve of known calcite-dolomite mixtures.
Polished thin sections were prepared for microprobe anal-
To establish a correlation between the grey-level of each
sample and its mineralogical components, a method had to
be found to quantify the grey-level or “brightness”. In a first
approach, thin sections of marbles and limestones were irradiated with light. The power of the transmitted light was
measured and given as an indication of the grey-level of a
given sample. We used a laser beam expanded to 8 mm in order to smooth out spatial heterogeneity, and every thin section was measured at eight different positions. However,
this method involves a number of uncertainties: The thickness of the thin-section, glass and glue support, as well as
the grain size of the analysed sample, all have a major influence on the transmitted power, and we were not able to distinguish the effect of each variable. This approach was
therefore abandoned, and we tested a light reflection method using an integrating sphere (Grum & Becherer, 1979).
This method provided an appropriate quantification of the
visual appearance of the samples.
Grey-level measurements were performed using a CARY
219 spectrometer equipped with an integrating sphere, at the
Institute of Applied Physics, University of Bern. Rock
cubes were illuminated perpendicularly to the surface with a
spot size of approximately 2 x 6 mm. To avoid surface ef-
840
J. Schmid, I. Flammer
Fig. 2. Illustration of the proposed model: a homogeneous medium
with randomly distributed voids acting as scatterers (white ellipsoids) and randomly distributed colour centres (black dots). Also depicted is a light path scattered several times before being re-emitted
at the rock surface.
fects, all rock cubes were prepared in the same way, using a
diamond saw, without further processing. Since the intensity of the reflected light depends on the viewing angle, and in
order to measure the total reflected radiation, light is collected from all viewing angles using the integrating sphere.
The measured value is called the “total reflectance”,
which is defined to be the amount of reflected light power of
the sample compared to the reflected light power measured
on a MgCO3 standard, and is given in per cent. We measured
the total reflectance for 28 samples from the Calcare di Dosso dei Morti. Sample size was at least 20 x 20 x 10 mm, and
the thickness was chosen large enough to prevent any transmission of light through the sample. For different wavelengths in the visible region, the variation in total reflectance was less than 5 %. Thus, experiments were conducted
using a constant wavelength of 700 nm ± 2 nm. The reproducibility of the measurements was better than 1 %. Up to
five measurements were conducted on each sample in order
to obtain a representative total reflectance value.
3.4 Theoretical considerations on the reflectance
The total reflectance measured by the integrating sphere is
used to characterise the grey-level or brightness of the rock
sample. Reflected light is composed of two parts: One part
of the light is reflected directly due to the refractive index
difference between the air and the solid sample. This contribution depends also on the roughness of the sample surface
(Ogilvy, 1991). Identical surface preparation was used for
all samples to minimise such surface effects. The second
part of the illuminating light penetrates into the sample,
which is highly heterogeneous in refractive index due to the
void spaces within the sample. In the case of the analysed
marbles, void spaces are located at the grain boundaries. Because the voids act as scattering centres, the light is repeatedly scattered when travelling through the rock. The birefringence of CaCO3 enhances scattering, but its effect is
small compared to void scattering since the refractive index
contrast is much smaller. Since no light is transmitted
through the sample, photons are either absorbed when they
hit a colour centre (such as mafic accessory minerals or organic phases in this case) or re-emitted at the sample surface
(Fig. 2). The sum of directly reflected light and re-emitted
light represents the total reflectance. The mechanism of
multiple scattering has been discussed in depth by various
authors (Chandrasekar, 1950; Chernov, 1960; Ishimaru,
1978; Tatarski et al., 1993). However we only utilise some
of their qualitative results in the present discussion.
The light scattering power of a rock is determined by the
size and density of the scatterers. An increase in the size of
the scatterer increases the amount of scattered light (Van der
Hulst, 1957). In our case, the scatterer size is represented by
grain-boundary width. The scatterer density, i.e., the number of scatterers per unit volume, is determined by grain
size. The larger the grains, the less abundant are the grain
boundaries and thus the scatterers. Without light absorption,
all rocks would have equal brightness independent of their
scattering power. However, in rocks with low scattering
power, light penetrates much deeper into the rock and the
light travels over long paths before being re-emitted at the
surface. In fact, the mechanism of light absorption by colour
centres makes the rocks appear darker. More abundant colour centres will reduce the brightness or increase the greylevel. Furthermore, the interaction between light scattering
and light absorption determines the grey-level or brightness.
For an equal number of colour centres, rocks with low scattering power appear darker because the light travels further
into the rock and there is a higher photon absorption probability (Mandelis et al., 1990; Leutz & Ricka, 1996).
Hence, the grey-level or brightness of a sample depends
on three factors: the abundance of colour centres, the void
size and the grain size.
3.5 Insoluble residue extraction
Seven bulk samples were processed by Carbon Consultants
International, Inc. Carbondale. The rocks (200 to 900 g)
were cleaned with H2O prior to digestion to remove surface
contamination and were subsequently digested in 20 % HCl.
Hydrochloric acid was added periodically to maintain pH <
2. Total reaction time varied from 20 to 26 days, the HCl
processing being ended when the carbonate in each sample
was totally digested. Upon completion of acid digestion,
samples were neutralized and dried at 40 ± 4 °C. To remove
residual mineral matter, 48 % HF was added. Upon completion of HF digestion after about two hours, samples were
neutralized and dried at 40 ± 4 °C.
Two additional samples (33CdM43; 34CdM45) were
processed at the Geological Institute, University of Neuchâtel, in order to analyse the accessory mineral content of the
limestones. Samples were crushed into about 3-mm-sized
rock chips, which were immersed in deionised water and
How grey limestones become white marbles
841
Fig. 3. Trace element composition of the calcite
plotted against apparent distance from the contact. Fe, Mn and Sr were detected by microprobe
analysis. Circles represent mean values per sample. Error bars indicate standard deviation. Filled
circles: samples used for calcite-dolomite geothermometry. Sample numbers are given on the
Sr-plot. Numbers on Mn-plot: individual measurements per sample. Fe and Mn contents are
close to the detection limit of 0.016 wt%.
subsequently decarbonised by adding 10 % HCl. The insoluble residue was washed with deionised water until a neutral
pH was obtained. No HF treatment was performed on these
samples. Because the procedure is not optimised for the extraction of organic carbon, we cannot rule out the loss of
some organic matter.
3.6 Quantitative determination of organic matter
Bulk-rock analyses
Analyses of organic carbon content in bulk rocks were performed at the Institute for Mineralogy and Petrography, University of Fribourg, Switzerland. Carbon content was determined in 34 powdered samples, using a LECO RC-412 furnace. The method is based on the principle that various forms
of elemental carbon and carbon compounds react with oxygen
over specific temperature ranges, giving rise to carbon dioxide (Cizek et al., 1990, and references therein). 400-500 mg of
powdered rock were reacted in an oxygen flow by stepwise
heating from 100 to 550 °C in 440 s, and held at the maximum
temperature for 120 s. Evolved carbon dioxide was continuously detected by an infrared spectrometer. Peak areas are
proportional to the amount of carbon (Cizek et al., 1990). Repeated analyses of several samples show a relative error of 4
% at organic carbon contents of 0.04-0.05 %. At contents of
0.01-0.02 % the relative error is 9 %.
Amorphous carbon typically reacts to form CO2 in the
temperature range 420-490 °C, whereas graphite combustion requires temperatures higher than 780 °C (Cizek et al.,
1990). Carbon from the calcium carbonate (CaCO3 = Calcite) reacts at temperatures above 600 °C (Picouet, pers.
comm.). To avoid interference of organic carbon with the
calcium carbonate, which is the main constituent of limestones, analyses were performed at temperatures below
550 °C. This implies that totally recrystallised graphite is
not combusted.
Insoluble residue analyses
The carbon content in insoluble residues of 9 samples was
analysed using the STRÖHLEIN CS-mat 5500 instrument
at the Geological Institute, University of Bern. 1-25 mg of
insoluble residue were reacted in an oxygen flow at temperatures of approximately 1350-1550 °C. The evolved CO2
gas was detected by NDIR-spectrometry. Under these conditions, there is a complete combustion of all organic carbon, including graphite. The two samples that had not been
digested in HF (33CdM43; 34CdM45) were combusted in a
nitrogen atmosphere after the combustion in oxygen. Nitrogen is inert to organic carbon, so only mineral carbon is
measured by this procedure. Organic carbon was calculated
by difference. From measured values, we calculate the organic carbon content of the bulk rock.
842
J. Schmid, I. Flammer
Table 1. Total reflectance and organic carbon measurements. * indicate measurements performed on samples that have been processed
at the University of Neuchâtel. In the case of repeated analyses, an
average value is given.
Sample
Fig. 4. Values of · 18O and · 13C for calcite plotted against apparent
distance to the plutonic intrusion.
Bulk-rock analyses were carried out by the same procedure. However, because of the very high carbon content in
calcite (~12 wt%) compared to the very low amount of organic carbon in the analysed samples ( e 0.07 %), organic
carbon cannot be detected by this method.
3.7 Grain-size determination
Complete grain-boundary networks of 135 to 283 grains
were traced onto transparent paper from direct projection of
the thin sections, using a standard petrographic microscope.
This method allows rotation of the specimen about the microscope axis and switching from crossed to parallel polarisers, thus enabling the identification of a maximum number
of grainboundaries. Working on single enlargements (without rotating the specimen) would result in overestimating
the grain sizes, as not all of the grain boundaries can be identified (Burkhard, 1990; Pfiffner, 1982). Because the present
study deals with relative changes in grain size, we did not
take account of the systematic errors produced by the method (e.g. Covey-Crump & Rutter, 1989; Tullis & Yund,
1982). From grain-boundary network tracings, we calculated the mean area using the program NIH IMAGE, Version
1.43 (Rasband, 1992). The mean grain diameter L (= grain
size) was determined assuming circular grain shapes of
equal area.
4. Results
4.1 Calcite geochemistry
The trace element composition of the carbonate minerals
was measured for very-low-grade metamorphic, transitional and highly recrystallised samples of the Calcare di Dosso
dei Morti formation. Besides Ca and Mg, the trace elements
Fe, Mn and Sr were detected by WDS analyses (Fig. 3). Fe
and Mn are present at very low contents (Fe < 0.45 wt%; Mn
< 0.04 wt%). Sr contents for most samples vary between
0.015 and 0.025 wt%. Three samples show Sr contents that
are anomalous for shallow-marine platform carbonates:
33 CdM 43
32 CdM 42
27 CdM 37
34 CdM 45
28 CdM 38
35 CdM 47
36 CdM 47
40 CdM 54
38 CdM 50
38 CdM 52
42 CdM 55
42 CdM 56
65 CdM 96
67 CdM 99
67 CdM 100
66 CdM 98
55 CdM 83
54 CdM 82
52 CdM 79
51 CdM 78
50 CdM 77
49 CdM 76
48 CdM 75
56 CdM 85
57 CdM 86
47 CdM 74
46 CdM 73
59 CdM 88
60 CdM 89
62 CdM 91
63 CdM 92
69 CdM 102
69 CdM 103
70 CdM 104
71 CdM 105
Apparent
distance
from Pluton
[m]
10
250
345
480
515
865
880
910
920
920
970
970
990
1050
1050
1600
1580
1748
1965
2065
2120
2235
2358
2380
2390
2515
2560
2650
2840
2930
2920
4060
4060
4285
4511
Total re- % C
%C
flectance ( CS-mat ) ( LECO )
[% ]
47
56
50
45
74
53
43
43
24.5
70
27
24.5
75
76
45
23
21
30
39
32
36
39
24
46
46
28
30
40
*0.015
*0.005
0.052
0.040
0.022
0.026
0.07
0.015
-
0.016
0.015
0.016
0.016
0.017
0.013
0.011
0.016
0.040
0.020
0.053
0.044
0.012
0.014
0.029
0.057
0.064
0.050
0.018
0.034
0.029
0.029
0.050
0.023
0.056
0.019
0.034
0.023
0.028
0.032
0.018
sample 38CdM51 with 2.8 ± 0.21 % (1 c ), 38CdM52 with
1.3 ± 1.2 %, and 33CdM43 with 0.76 ± 0.02 %. No distinct
trend can be observed for these elements.
Measurements of oxygen and carbon stable isotope ratios
in the Calcare di Dosso dei Morti yield mean values of · 18O
= 24.3 ± 1.1 ‰ (1 c ) and · 13CPDB = 1.8 ± 0.6 ‰ (1 c ) (n = 52)
throughout the aureole (Fig. 4).
4.2 Mineral content of the limestone
The Calcare di Dosso dei Morti consists of a very pure limestone. X-ray analyses reveal the presence of minor amounts
of dolomite (0-4 wt%) in samples at apparent distances
greater than 920 m from the intrusive contact. Samples closer to the pluton contain no dolomite at all. The very-low-
How grey limestones become white marbles
843
Fig. 5. Total reflectance measured in Calcare di Dosso dei Morti samples. Numbers on the figure indicate measured values. a-d: recrystallised
rocks, unpolished. e-h: non-metamorphic and low-metamorphic limestones, unpolished (top) and polished equivalent (bottom). a) 65CdM96,
b) 28CdM38, c) 88CdM118, d) 27CdM37, e) 46CdM73, f) 71CdM105, g) 52CdM79, h) 54CdM82. See Fig. 1 for sample position.
grade metamorphic rocks as well as the highly recrystallised
equivalents contain less than 1 wt% of residue. In the grey
limestones, accessory minerals determined by optical microscopy, cathodoluminescence microscopy and X-ray
analysis comprise phengites, quartz and feldspar, whereas
marble contains minerals such as tremolite, phlogopite,
quartz and feldspar. Reflecting optical microscopy shows
the presence of a few pyrite crystals, sometimes altered to
hematite.
4.3 Total reflectance measurements
Total reflectance was measured in 28 Calcare di Dosso dei
Morti samples, including the entire transition from very-lowgrade metamorphic limestones to highly recrystallised marbles. Measurements are given in Table 1. Fig. 5 shows the
spectrum of observed dark grey to pure white rocks, together
with the measured total reflectance. Comparing polished and
unpolished samples shows the effect of surface preparation:
grey limestone becomes almost black upon polishing (Fig.
5f-h). The figure shows that the total reflection measurements
provide a very sensitive quantitative indication of qualitative
variations perceived by the human eye.
White to dull white marbles occur in the vicinity of the
pluton, displaying total reflectance of 43 to 75 %. Values
close to 43 % are measured in coarse-grained marbles,
which appear dull white in contrast to the finer grained pure
white marbles. Further away from the intrusive contact, values vary from 20 to 46 %, representing dark grey to faint
grey or dull white limestones (Table 1, Fig. 6a).
Fig. 6. a) Total reflectance plotted against apparent distance from the
intrusive contact. b) Carbon content versus apparent distance from
the intrusive contact. 2 represent measurements performed using
the LECO-analyser. Circles group repeated analyses on the same
sample. c) Grain size plotted against apparent distance from the contact. Grain size is given as an indicator of the progressive recrystallisation at increasing metamorphic temperature. d) Peak metamorphic
temperature plotted against apparent distance from the intrusive contact. The dashed line indicates a tentative temperature profile. The
dashed bar marks the region of abrupt decrease of grain size, which
correlates with decreasing temperature, decreasing total reflectance,
and increasing organic carbon content.
4.4 Organic carbon content
No organic matter or graphite was detected by reflecting optical microscopy, except in sample 33CdM43, where clusters of graphite occur in small dark zones within a strongly
recrystallised white marble.
Organic carbon contents from the bulk rocks were measured using the LECO RC-412 instrument. Values were extremely low at 0.011-0.064 wt% (Table 1). Also, these measurements do not allow the detection of highly ordered
844
J. Schmid, I. Flammer
graphite. To check the reliability of the analyses, we conducted a series of analyses using the Ströhlein CS-Mat instrument, in which the insoluble residue of samples was
combusted at temperatures high enough to detect all types of
carbon. Measurements obtained by both methods correspond fairly well, and no systematic deviation occurs, notably even in the high-grade marbles at elevated metamorphic
temperatures (Table 1). Most data were obtained by the LECO instrument, and these measurements are used in the following discussion.
Fig. 6b shows the variation of carbon content with increasing distance from the intrusive contact. In the vicinity
of the pluton, up to an apparent distance of 1050 m, the organic carbon content is e 0.02 % for all samples except
38CdM50, 42CdM55 and 42CdM56, which display higher
values (Table 1). Further away from the pluton, contents
range from 0.018 to 0.064 %, and are randomly distributed
throughout this part of the contact aureole.
In the Calcare di Dosso dei Morti, the trace element composition is uniform throughout the entire aureole, although
some samples show slightly increased trace element contents (Fig. 3). However, these samples do not display altered
colours compared with other marbles. We therefore conclude that the trace element composition of the studied carbonates has no influence on their colour. This is in accordance with the findings of Frondel et al. (1942). These authors investigated the distribution of trace elements in calcite crystals, but failed to establish a relation between the
distribution of Fe, Cu, Mn, Al, Sr and Mg, and the colour of
the crystals.
In three of the analysed samples, Sr contents are considerably higher than expected in middle Triassic platform carbonates (Kranz, 1978). These samples were collected at the
contact with mafic dykes (38CdM51, 38CdM52), and at the
immediate contact with the pluton (33CdM43). The high Sr
content probably indicates a very minor influx of fluid into
the rock. These samples are not used for further interpretations in the present study.
4.5 Grain size
Fig. 6c shows the relative coarsening of calcite grains, given
as an indicator of progressive recrystallisation at increasing
metamorphic temperature. The abrupt increase of grain size
at about 1000 m from the intrusive contact closely reflects
the increase of peak metamorphic temperature approaching
the contact (Fig. 6d). This step in the grain-size profile also
corresponds to a threshold in the organic carbon content and
the total reflectance (Fig. 6a, b).
5. Discussion
5.1 Fluid flow
Stable isotope measurements of oxygen and carbon in samples from the most distant right up to the intrusive contact,
yield uniform values (Fig. 4). The preservation of pre-metamorphic isotopic ratios throughout the contact aureole rules
out the pervasive regional passage of fluids during the contact metamorphic event. Gerdes et al. (1999) also argue
against any significant amount of fluid infiltration through
the bulk of carbonate country rock in lithologies overlying
the present studied formation. The pervasive infiltration of
magmatic fluids would have generated variations in oxygen
isotopic values towards the fluid source (e.g. Valley, 1986;
Baker & Matthews, 1995). We therefore conclude that the
bleaching of the studied rocks towards the intrusive contact is
not caused by fluids derived from magmatic bodies.
5.2 Trace element composition
Although most calcite is colourless or white, natural varieties in different faint colours are also known. Substitution of
Ca2+ by other divalent cations can have an effect on the colour of the calcite aggregate. For example, the entry of appreciable Mn into the calcite structure usually introduces a pale
pink to rose-red colour (Deer et al., 1992).
5.3 Mineral content of the limestone
Deer et al. (1992) state that dark carbonate rocks may owe
their colour to fine dusty mineral inclusions such as chlorite.
However, only colourless or white accessory silicate minerals are present in the Calcare di Dosso dei Morti (phengites,
quartz and feldspar). None of these minerals can account for
the grey colour of the rocks. Apart from these minerals, rare
pyrite crystals, sometimes altered to hematite, occur in the
analysed rocks. The presence of finely dispersed pyrite in a
calcareous matrix could cause the grey colour of limestones
(Bläsi, pers. comm.), whereas hematite dust may cause a
pink colour (Papageorgakis, 1961). However, pyrite and hematite crystals in the studied samples are relatively coarse
grained ( & 2 µm) and cannot act as pigments. Hence, accessory silicate or ore minerals have no influence on the colour
of the studied carbonate rocks.
5.4 Organic carbon content
The threshold observed in Figure 6a and b for total reflectance and organic carbon content, at equal distance from the
pluton, suggests a correlation between the two variables. In
Figure 7a, total reflectance is plotted against the organic carbon content. Two trends can be distinguished: at total reflectance values < 46 %, a clear correlation exists between organic carbon content and total reflection; at total reflectance
values > 43 %, samples with similar organic carbon content
cluster around two different brightness levels (Fig. 7b).
Most total reflection values < 45 % represent limestones
at more than 1000 m from the intrusive contact (filled circles
in Fig. 7a), which display various carbon contents between
0.018 wt% (dull white limestones) and 0.064 wt% (dark
grey limestones). These samples are not or only slightly recrystallised, and have experienced metamorphic temperatures below about 390 °C (Schmid, 1997). Under these conditions, grain size is relatively constant (6-9 µm), and we as-
How grey limestones become white marbles
845
atile CO2 or CO (Holloway, 1987), thus explaining the absence of carbon in the analysed marbles.
The organic carbon compounds are too fine grained to be
detected by reflecting optical microscopy. In the form of
sub-microscopic finely dispersed dust within the calcite aggregates, the carbon compounds form colour centres, which
absorb light within the marble sample. The organic carbon
thus acts as a dark pigment.
5.5 Fabric of the rock
Grain size
Fig. 7. a) Organic carbon content plotted against total reflectance. b)
Grain size versus total reflectance. Filled symbols: non-metamorphic and low-metamorphic limestones, open symbols: recrystallised
marbles, encircled area: non-metamorphic rocks with low initial
contents of organic carbon. A clear correlation exists between total
reflectance and organic carbon content. Pure white samples correspond to fine-grained marbles, whereas coarse-grained marbles appear dull white.
sume the inter-granular void size and density are also constant. An organic carbon variation of as little as 0.05 % or
less can lead to the change from dark grey to white.
Recrystallised samples 42CdM55 & 56 and 38CdM50
(open symbols) represent grey marbles with relatively high
organic carbon content, lying within the trend described
above. With the exception of these three samples, all of the
recrystallised marbles at distances of less than 1000 m are
white and display organic carbon contents below 0.02 %.
Metamorphic temperatures experienced by these rocks
range from 430 to 600 °C (Schmid, 1997).
We therefore conclude that organic carbon content variation of as little as 0.05 % can produce the change from dark
grey to white. Thus, if the initial organic carbon content is
low enough, an initially grey limestone can recrystallise to a
white marble, provided temperatures are above approximately 430 °C. At increasing temperatures, organic matter
undergoes various physico-chemical modifications. At first,
there is release of water and C in form of carbon dioxide. As
temperature further increases, liquid hydrocarbons and finally gaseous hydrocarbons are released (Rouzaud & Oberlin, 1990), while elemental C may crystallise as graphite.
Although graphite clusters were detected in one sample only
(33CdM43), we cannot completely rule out the recrystallisation of organic C to graphite, favoured by shear stresses
along the grain boundaries (Oh, 1987; Jehlicka & Rouzaud,
1995). The clustering of graphite flakes within small domaines of the marble prevents the graphite acting as a pigment.
Alternatively, in the presence of fluid, C may oxidize to vol-
The second trend in Figure 7a consists of white marbles with
total reflection values greater than 43 %, which cluster in
two groups: one group with dull white, the second group
with pure white marbles. Fig. 7b shows grain size versus total reflection. Grain sizes of the recrystallised samples (open
circles) range from 20 to 870 µm. Dull white marbles are
composed of coarse grains (465-870 µm), whereas the grain
size of pure white marbles is notably smaller and varies
from 20 to 255 µm.
According to the discussion in the theoretical section,
photon paths are longer in coarse-grained samples. More
light is thus absorbed and the samples look dull white,
whereas the fine-grained marbles are bright white due to
less light absorption caused by increased scattering at grain
boundaries.
Inter-granular voids
One group of samples that is belonging to the first trend does
not confirm this behaviour: the non-recrystallised limestones with organic carbon contents of ca. 0.02 % and total
reflection of ca. 40 to 46 % (filled symbols encircled in
Fig. 7a and b). These very-fine-grained limestones with
very low organic carbon are expected to be pure white, according to the principle that smaller grain size causes more
intensive scattering. Nevertheless, these limestones are dull
white.
Closer observations of the fabric of the rocks show that
very-low-grade metamorphic limestones have grain boundaries about 0.1 µm wide, while the recrystallised marbles
have grain boundaries wider than 1 µm. The widening of
grain boundaries in statically overprinted carbonates is not
uncommon (e.g. Herwegh & Pfiffner, 1999), being attributed to the absence of coalescence between calcite grains that
typically display smooth grain boundaries. Breaking up of
the sintered calcite aggregate during the cooling episode is
favoured by the large thermal anisotropy of calcite (e.g. Olgaard & Fitz Gerald, 1993). Further widening of the grain
boundaries in statically overprinted marble during the preparation for REM observation and polished sections cannot
be avoided, but was kept minimal by embedding the samples in epoxy. Examples representing the limestones and
marbles, respectively, are shown in Figure 8. We believe
that this difference in grain boundary width by one order of
magnitude leads to the darker appearance of the limestones.
Scattering power decreases with void size (grain boundary
846
J. Schmid, I. Flammer
Fig. 8. Secondary electron (SE) images typical of non-recrystallised limestone (71CdM105) and recrystallised marble (38CdM52) respectively. a) and c) show the fabric of the rocks, b) and c): detailed views of the grain boundaries. The grain-boundary width increases remarkably
in the statically recrystallised marble. For electron microscopy, rock fragments were mounted without polishing, thus minimising the additional widening of grain boundaries.
width), so recrystallised marbles with wider grain boundaries look brighter than limestones with similar organic carbon content.
We should stress that three samples, namely 42CdM55 &
56 and 38CdM50, all of them recrystallised marbles (grain
size of about 70 µm), nevertheless show organic carbon contents exceeding 0.04 % (Table 1). This is interpreted as the result of a higher initial carbon content than in the other samples, so not all of the organic carbon is eliminated at the metamorphic temperature reached in these samples (< 530 °C). It
is noteworthy that the grey-levels of these marbles correspond
to the grey-levels of very-low-grade metamorphic rocks with
similar carbon contents (Fig. 7). We therefore conclude that
the amount of organic carbon present in a sample is the main
factor determining the grey-level in limestones and marbles,
whereas microstructural features determine gradations within
the white calcareous rocks.
6. Conclusions
The light to dark grey Calcare di Dosso dei Morti formation
owes its “colour” to the presence of minor amounts of organic
carbon. No organic carbon is detected by reflecting optical
microscopy, thus indicating that organic carbon is finely dispersed within the calcite aggregate. Present in the form of fine
dust, as little as 0.03-0.07 % of organic carbon can act as a
grey pigment, and the grey-level decreases linearly with the
amount of organic carbon in the sample. Oxidaton or recrystallisation to graphite clusters leads to removal of organic
matter with increasing metamorphic temperature, thus explaining the fading of colour from grey to white.
Microstructural features such as grain size and grainboundary width have a secondary control on the brightness
of the rocks: In very fine-grained limestones, light scattering is limited due to the narrow grain-boundary width.
How grey limestones become white marbles
These non-recrystallised rocks look dull white compared
with the bright white, fine-grained recrystallised marbles,
which in turn owe their brightness to intense light scattering
at wider grain-boundaries. Coarse-grained marbles invariably look darker than fine-grained marbles, which is due to
greater light absorption as a result of longer photon paths
and lower abundance of grain boundaries.
Acknowledgements: M. Maggetti kindly permitted the use
of the LECO analyser at the Institute for Mineralogy and Petrography, University of Fribourg. Many thanks are due to P.
Picouet for technical assistance with the measurements. T.
Adatte performed insoluble residue extraction at the University of Neuchâtel and helped with the interpretation of Xray analyses. K. Hindenberg and B. Hofmann provided
valuable help with the reflected light microscopy, and M.
Herwegh kindly produced the REM images at the University of Bern. We greatly appreciate the valuable comments of
K. Ramseyer and the anonymous reviewers. Research was
supported by the Swiss National Science Foundation (grants
20-34’091 and 20-43’351.95). The same organisation (grant
21-62’579.89) provides financial support for the microprobe laboratory at the University of Bern.
References
Baker, J. & Matthews, A. (1995): The stable isotopic evolution of a
metamorphic complex, Naxos, Greece. Contrib. Mineral. Petrol., 120, 391-405.
Brack, P. (1984): Geologie der Intrusiva und Rahmengesteine des
Suedwest-Adamello (Nord-Italien). Dissertation Nr. 7’612, ETH
Zürich.
Bucher-Nurminen, K. (1982): On the mechanism of contact aureole
formation in dolomitic country rock by the Adamello intrusion
(northern Italy). Am. Mineral., 67, 1101-1117.
Burkhard, M. (1990): Ductile deformation mechanisms in micritic
limestones naturally deformed at low temperatures (150-350 °C).
Geol. Soc. Spec. Publ., 54, 241-257.
Burns, S.J. & Matter, A. (1993): Carbon isotopic record of the latest
Proterozoic from Oman. Ecl. geol. Helv., 86 (2), 595-607.
Callegari, E. (1962): La Cima Uzza (Adamello Sud-Orientale) Parte
I. Studio petrografico e petrogenetico delle formazione metamorfiche di contatto. Memorie degli Istituti di Geologia e Mineralogia del’Università di Padova, 23, 116.
Camisasca, O. (1941): Il marmo di Candoglia ed i suoi minerali. Atti
della Società italiana di science Naturale, LXXX, Milano.
Chandrasekar, S. (1950): Radiative transfer. Fowler, Oxford.
Chernov, L. A. (1960): Wave Propagation in a Random Media. Mc
Graw-Hill, NY.
Cizek, Z., Borek, P., Fiala, J., Bodgain, B. (1990): New analytical
method for determination and speciation of forms of carbon. Microchimica Acta, 111, 163-170.
Covey-Crump, S.J. & Rutter, E.H. (1989): Thermally-induced grain
growth of calcite marbles on Naxos Island, Greece. Contrib. Mineral. Petrol., 101, 69-86.
Deer, W.A., Howie, R.A., Zussman, J. (1992): Rock-forming minerals: Non-Silicates, 5. London: Longmans, 371 p.
Deines, P. & Gold, D.P. (1969): The change in carbon and oxygen
isotopic composition during contact metamorphism of Trenton
limestone by the Mount Royal pluton. Geochim. Cosmochim. Acta, 33, 421-424.
847
Frondel, C., Newhouse, W.H., Jarrel, R.F. (1942): Spatial distribution of minor elements in single-crystals. Am. Mineral., 27, 726730.
Gerdes, M.L., Baumgartner L.P., Valley, J.W. (1999): Stable isotopic
evidence for limited fluid flow through dolomitic marble in the
Adamello contact aureole, Cima Uzza, Italy. J. Petrol., 40, 853872.
Grum, F. & Becherer, R.J. (1979): Optical radiation measurements,
Vol. 1. Academic Press, N.Y.
Herwegh, M. & Pfiffner, O.A. (1999): Die Gesteine der Piora-Zone
(Gotthard-Basistunnel). In Löw, S. & Wyss, R. (Eds.): Vorerkundung und Prognose der Basistunnels am Gotthard und am Lötschberg: Berichte des Symposiums/Zürich/15-17. Februar, 1999,
77-88.
Holloway, J.R. (1987): Igneous fluids. In Carmichael, I.S.E. & Eugster, H.P. (Eds.): Thermodynamic modeling of geological materials: Minerals, fluids and melts. Rev. Mineral., 17, 211-232.
Ishimaru, A. (1978): Wave propagation and scattering in random
media. Academic Press, N.Y.
Jehlicka, J. & Rouzaud, J.N. (1995): Structural and microtextural
characteristics of natural “cokes”. In Pajares, J.A. & Tascón,
J.M.D. (Eds): Coal Science, Elsevier Science, 223-226.
Kranz, J.R. (1978): Strontium, ein Fazies-Diagenese-Indikator im
Oberen Wettersteinkalk (Mittel-Trias) der Ostalpen. Geol. Rundschau, 65, 593-615.
Lepsius, G.R. (1890): Griechische Marmorstudien. Königl. Akad.
Wiss., Philosoph.-hist. Klasse, Abh., 17 (Berlin).
Leutz, W. & Ricka, J. (1996): On light propagation through glass
bead packings. Optics Comm., 126, 260-268.
Mandelis, A., Boroumand, F., vanden Bergh, H. (1990): Quantitative diffuse reflectance spectroscopy of large powders: the Melamed model revisited. Applied Optics, 29, 2853-2860.
Matile, L. & Widmer, T. (1993): Contact metamorphism of siliceous
dolomites, marls and pelites in the SE contact aureole of the Bruffione intrusion (SE Adamello, N Italy). Schweiz. Mineral. Petrogr. Mitt., 73, 53-67.
Ogilvy, J.A. (1991): Theory of wave scattering from random rough
surface. Adam Higler, Bristol.
Oh, J.H. (1987): Etude structurale de la graphitation naturelle
(exemples de bassins Sud-Coréens). Dissertation, Université
d’Orléans, France.
Olgaard, D.L. & Fitz Gerald, J.D. (1993): Evolution of pore microstructures during healing of grain boundaries in synthetic calcite
rocks. Contrib. Mineral. Petrol., 115, 138-154.
Papageorgakis, J. (1961): Marmore und Kalksilikatfelse der Zone
Ivrea-Verbano zwischen Ascona und Candoglia. Schweiz. Mineral. Petrogr. Mitt., 41, 157-254.
Pfiffner, O.A. (1982): Deformation mechanism and flow regimes in
limestones from the Helvetic zone of the Swiss Alps. J. Struct.
Geol., 4 (4), 429-442.
Rasband, W. (1992): NIH IMAGE, Version 1.43. National Institute
of Health, Research Services Branch (Public domain software).
Riklin, K. (1983a): Contact metamorphism of the Permian “red
sandstones” in the Adamello area. Mem. Soc. Geol. Ital., 26, 159169.
– (1983b): Kontaktmetamorphose permischer Sandsteine im Adamello-Massiv (Nord-Italien). Dissertation, Nr. 7415, ETH Zürich.
Rouzaud, J.-N. & Oberlin, A. (1990): The characterization of coals
and cokes by transition electron microscopy. In Charcosset, H. &
Nickel-Pepin-Donat, B. (Eds.): Advanced methodologies in coal
characterization. Coal Science and Technology, 15. Amsterdam:
Elsevier, 311-315.
848
J. Schmid, I. Flammer
Schmid, J. (1997): The Genesis of White Marble: Geological Causes
and Archaeological Applications. Inauguraldissertation der Philosophisch-naturwissenschaftlichen Fakultät Bern.
Tatarski, V.I., Ishimaru, A., Zavorotny, V.U. (Eds.) (1993): Wave
Propagation in Random Media (Scintillation). SPIE, Washington.
Todd, C. (1990): Bleaching of limestones in the Notch Peak contactmetamorphic aureole, Utah. Geology, 18, 83-86.
Tullis, J. & Yund, R.A. (1982): Grain growth kinetics of quartz and
calcite aggregates. J. Geol., 90, 301-116.
Ulmer, P., Callegari, E., Sonderegger, U.C. (1983): Genesis of the
mafic and ultramafic rocks and their genetical relations to the tonalitic-trondhjemitic granitoids of the southern part of the Adamello batholith (Northern Italy). Mem. Soc. Geol. Ital., 26, 171222.
Valley, J.W. (1986): Stable isotope geochemistry of metamorphic
rocks. in Valley, J.W., Taylor, H.P. & O’Neil, J.R. (Eds.): Stable
isotopes in high temperature geological processes. Rev. Mineral.,
16, 445-489.
Van der Hulst, H.C. (1957): Light scattering by small particles. John
Wiley & Sons, NY.
Vogler, R. (1985): Geologie und Petrographie am Südost-Rand des
Adamello XII: Val di Daone. Diplomarbeit an der philosophischen Fakultät der Universität Zürich.
Received 31 August 1999
Modified version received 20 January 2001
Accepted 28 January 2002