Geoscience Reference
In-Depth Information
2
(
(
Figure 2.16
Use of a grain-size chart to determine the average grain size. (a) In this case the average grain
size is 500
µ
m. The grain size varies between 375 and 750
µ
m. (b) Close up view of (a).
whereas calcite will not. Feldspars have distinct cleavage and
tend to go white and powdery as they weather.
Colour variation can be ascertained using Munsell colour
chips compiled onto charts. This is particularly useful in
successions where one of the main variations is colour (for
instance in mudstones) and/or colour has been demonstrated to
be an important indicator of the composition. A full Munsell
colour chart is bulky for fi eld use but short versions with the
typical colours of rocks can be obtained from stockists of
geological equipment and various Geological Society
publishers.
A streak plate (ground glass or ceramic plate across which the
rock is scratched) can be useful for getting an even colour from
a rock or mineral that is then easier to compare with the colour
chart.
The Munsell colour system is based on all variation in
colour being defi nable by a point in a sphere. To defi ne
where the colour lies in the sphere there are three aspects to
consider.
•
Value (or Lightness)
: This is the paleness or darkness of the
colour, i.e. for the colour grey where it lies between black
(1) and white (9). Value defi nes the vertical axis through
the sphere (Figure 2.17, p. 30).
