Geoscience Reference
In-Depth Information
C LEAVAGE
Slate is found in mountainous areas where tectonic
forces have compressed the rocks by folding. In response,
minerals grow perpendicular to the direction of
maximum stress producing slaty cleavage ( 102 ).
Cleavage forms independently of sedimentary bedding
(along which some sedimentary rocks can be split) and is
often at high angle to it. The type and intensity of
cleavage controls every aspect of slate production, from
ease of extraction to the characteristics of the finished
product. At a microscopical level, cleavage planes can be
As slate is very fine-grained, optical resolution of
minerals is difficult and consequently, definitive
determination of mineralogy usually requires
combination of several different microscopical and
analytical methods. The approximate quantity of each
mineral present may be estimated visually through the
microscope or by point counting to give a volume
percentage. A more accurate approach involves a
combination of optical microscopy, SEM, elemental
analysis by EPM or XRF, and mineralogical analysis by
XRD.
99
100
99 Close view of purple coloured slate in thin section
showing the haematite (red/black) that imparts its
colour. 'Penrhyn' slate (Wales); PPT, ×600.
100 'Westmorland Green' slate (Cumbria, England)
formed from deposits of volcanic ash and containing
chlorite (green), which provides its green colour; XPT,
×150.
101
102
101 Close view of a green reduction spot in 'Penrhyn'
slate (Wales), showing a concentration of chlorite
(grey) that gives its colour; XPT, ×300.
102 Slate with a continuous, well developed cleavage
(running diagonally in the field of view). 'Cwt-Y-
Bugail' slate (Wales); XPT, ×300.
 
 
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