Geology Reference
In-Depth Information
MOH'S RELATIVE
HARDNESS
COMMON
OBJECTS
MINERAL
Talc
1
Gypsum
2
Finger nail (2.5)
Calcite
3
Copper coin (3.5)
Flourite
4
Apatite
5
Steel nail (5.5)
Glass plate (6)
Orthoclase
6
Quartz
7
Streak plate (7.5)
Topaz
8
Corundum
9
Diamond
10
Figure 3.9
Moh's harness scale.
individual abundances rarely exceed 10% by volume of a rock. Whereas apatite,
sphene (titanite), garnet and leucite sometimes form easily identified euhedral
crystals, the three opaque iron-titanium oxide minerals, which often fill the
interstices between major silicate minerals, have similar appearances in hand
specimens. The list of secondary (that is, post-magmatic) minerals includes
(i) those found as amygdale minerals in lavas (for example, calcite, zeo-
lites); (ii) those found in hydrothermal veins cutting igneous rock outcrops
(for example, calcite, chlorite, but also quartz) and (iii) those which replace
primary silicates in rocks that have been subject to pervasive hydrothermal or
metasomatic alteration and weathering reactions at submagmatic temperatures
(for example, clay minerals and/or epidote replacing feldspars and mica; chlorite
replacing mafic minerals).
3.5 Naming and Classification
When describing a medium- or coarse-grained field specimen you should esti-
mate, as a percentage, the volume of the rock occupied by each identified mineral
and, for those you are unable to identify, make a brief note of their properties for
later reference. Observations of colour, texture and mineral content can now be
