Geoscience Reference
In-Depth Information
97
producer to change the colour in order to increase sales.
Figure 97 shows a microscopical view of sandstone that
was deliberately heat-treated to change its colour from
light brown to a more marketable deep red. Other
important heat-induced changes in stone include
cracking or shattering of quartz resulting from the
α
- to
β
- quartz phase transition at 573°C and the calcination
of limestone and marble at 800-1000°C. The main heat-
induced changes that affect different stone types are
summarized in Table 11 .
97 Sandstone that has been heat-treated to redden its
appearance. Iron-bearing minerals have been
oxidized (black). Quartz grains appear white and pore
spaces are shown yellow; PPT, ×150.
Table 11 Changes caused by heating of various types of natural stone that may be observed either visually
or microscopically (Ingham, 2007)
Heating
Stone type
temperature
°C
Limestone
Sandstone
Marble
Granite
250
Pink or reddish-
Red discolouration
Heating marble
At less than 573°C, if
brown discolouration starts at 250-300°C
through a range of
heating rate is <1°C
300
starts at 250-300°C
but may not become
temperatures causes
per minute the thermal
but may not become visible until 400°C
nonreversible
expansion is fully
visible until 400°C
expansion known as
reversible. If heating
thermal hysteresis
rate is >5°C per minute
400
Discolouration
the expansion is not
becomes more
totally reversible
reddish at 400°C
600
Calcination of
Heating above 573°C Above 600°C
Develops cracks or
calcium carbonate
causes internal
complete disruption
shatters at 573°C due
commences at
rupturing of quartz
due to differential
to quartz expansion
600°C
grains with
expansion, becomes
associated weakening
friable and reduces to
and friability.
powder
Clay minerals in the
cement disintegrate
(kaolinite up to 600°C,
chlorite above 600°C)
800
Calcium carbonate
Red discolouration
Differential thermal
calcines to a grey-
may persist until
expansions at higher
white powder at
1000°C. Any
temperatures (900°C)
800-1000°C with
calcium carbonate
gives rise to tensile and
associated loss of
cement calcines to
compressive stresses
strength
powder at 800-
causing permanent
1000°C causing
strain in the stone
disintegration
1000+
Melting starts
Melting starts
Melting starts
Melting starts
 
 
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