Geology Reference
In-Depth Information
Table 3.6
Checklist for describing igneous rock hand specimens in the field
(to be used in conjunction with Table 2.1 for outcrop description).
1
Examine weathered surface of rock outcrop, noting texture and colour:
sometimes weathered surfaces provide a clue to the relative hardness of
minerals and to their composition (e.g. the red-brown weathering residues
of orthopyroxenes and olivines)
2
From the outcrop, collect and number representative sample(s) with fresh
surfaces. When collecting for thin section or analytical work, trim off as
much weathered material as possible in the field to save effort later and to
lighten the load for carrying. Clean any waste chips away, especially from
farmland, and do not leave the outcrop looking a mess
3
Record colour of the fresh surface and, where possible, estimate the colour
index (% ferromagnesian minerals)
4
Record the colour of the weathered surface and any features that were not
visible in the fresh surface
5
Examine the grains under a handlens:
(a) If the rock is aphyric, note any other textural features and record felsic
or mafic composition (see Table 3.2)
(b) Record coarse, medium or fine grain size of the rock (see Table 3.3)
and note textural relationship between minerals (e.g. Figures 3.5 and 3.6)
(c) If the rock is porphyritic, record grain size and textural relationship of
phenocrysts to groundmass
6
Record the degree of homogeneity, the presence of layering, lamination,
flow banding, vesicles and other special textural characteristics, such as
the presence of inclusions
7
Estimate the number and proportions of the different minerals present and,
for each, record where possible: colour, cleavage, lustre, habit, hardness.
Use Table 3.4 for identification purposes
8
Use mineral associations to predict the possible presence of other minerals
you can not immediately identify and in classifying the rock. Combine
your observations to give the specimen a field name
9
Combine your observations to give the specimen a field name, using
Figures 3.11 - 3.14 for medium- and coarse-grained rocks and Table 3.5 for
porphyritic fine-grained rocks. If you are unable to be specific, at least use
general/generic names, such as 'mafic lava'
