Geology Reference
In-Depth Information
Box 7.14.
Descriptive criteria of marbles in thin sec-
tions. Visual differentiation of marble samples takes
into account criteria related to the petrogenesis of
marbles, such as shape, size distribution, and spatial
distribution.
Texture
Crystal size:
Average size, maximum size, and size
ranges. At least 100 crystals should be determined
(use line-intercept method or digital image analysis,
Lumbreras and Serrat 1996). Maximum diameter and
diameter ranges are used in distinguishing marbles
of different provenance. The increase in the mean di-
ameter may be caused by thermal control (Schmid
1997; Taguchi and Aoki 1998). Equi- or non-equi-
dimenional fabric (see Fig. 7.19)? Uni- or bimodal
size distribution? Bimodal distribution may indicate
growth of crystals at the expense of adjacent crys-
tals.
Crystal boundaries:
Straight or gently curved, or ir-
regular and serrate? Boundaries reflect grain growth
history.
Crystal shape and fabric
can be described by using
the terms proposed by Friedman (1965) or those in-
troduced by Sibley and Gregg (see Fig. 7.22).
Fig. 7.24.
Carrara marble is one of the best known marbles,
appreciated by structural geologists interested in deforma-
tion tests, loved by artists like Michelangelo, and already used
in the antique for sculptures and as building stones. The bright-
ness and the translucity of the marble is due to the strikingly
homogeneous composition and texture. Calcite crystals are
very regular in shape, and consistent in size, and exhibit regu-
lar boundaries. The marble developed from a Jurassic lime-
stone, which was affected during the Tertiary by folding, meta-
morphism, and heating processes. Carrara, Alpi Apuani, Italy.
Scale is 1 mm.
Microstructural fabrics
Some of the criteria that describe conditions of crys-
tal growth are twin and glide surfaces, annealing and
segregation of a crystal into subcrystals, and alterna-
tion of undulose extinction.
Calcite twinning and growth of twins (Pl. 38 /4, 7, 8)
is related to the formation of dislocations and to the
glide of dislocations within slip planes. Twinning
types are used to estimate paleostress history, defor-
mation temperature, and relative timing of tectonic
events (Wenk et al. 1983; Simonsen and Friedman
1992; Railsback 1993). Twinning types are differen-
tiated according to the abundance, thickness and ori-
entation of twin lamellae (thin, thick, straight, or su-
tured twins; Burghard 1993, with many references).
Thin twin lamellae record small strains in marbles
deformed under little cover. Curved thick twin lamel-
lae and patchy twin lamellae indicate large deforma-
tion. The appearance of twin lamellae changes as a
function of deformation temperature.
Crystal fabric analyses allow marble types (Schmid
et al. 1999) and deformation styles to be distinguished.
except that metamorphism occurs at higher tempera-
tures and pressures than diagenetic processes. The ulti-
mate products of increasing strain and temperature are
carbonate marbles (Pl. 38/4-8).
The fabric of marbles seen in thin sections (Fig. 7.24)
is predominantly the result of the deformation and re-
crystallization history of the rock. Lithotypes of marbles
are caused by tectono-metamorphic deformation of
sedimentary lithologies and ductile processes leading
to tectonic fabrics (e.g. schistosity and fractures). Dy-
namic recrystallization of the protolith (the precursor
rock) changes the size, shape, boundaries, and the crys-
tallographic orientation of calcite crystals. Increasing
recrystallization leads to complete obliteration of sedi-
mentary textures and the formation of pavement mosa-
ics of completely interlocking crystals. The main ef-
fects of metamorphic alterations of limestones are an
increase in the matrix crystal size, followed by a pro-
gressive loss of the original texture, and, ultimately,
total obliteration of fossils and other grains. Increas-
ingly more thin-section studies are being done of marble
exhibiting relicts of the texture, composition and even
the microfacies of the precursor rock.
Accessory minerals
Common accessory minerals in calcite and dolomite
marbles are quartz, clay minerals, mica, pyrite, and
graphite.
Fossils
Fossils are very rare in marble thin sections except
for scarce crinoidal fragments or ghosts of shells (Pl.
38/6). Insoluble residues of some marbles may yield
microfossils, particularly pyritized, originally sili-
ceous radiolarians and sponge spicules (Kiessling
1992).
Methods
The methods used for investigating marbles depends
on the questions that need to be answered and requires
a multi-method approach applying different micro-
