Geology Reference
In-Depth Information
5. Volcanics 2-Pyroclastic Rocks
4. Volcanics 1-Lava Flows
3. Igneous Textures & Classification
2. Field Skills & Outcrop Structures
1. Introduction and Occurrence
Table 3.4
Mineral properties in igneous rock hand specimens.
Mineral
Typical chemical
formula
Colour
Cleavage
Lustre
Habit
Hardness
Felsic minerals
Quartz
SiO
2
Colourless to pale
grey when surrounded
by dark minerals;
transparent
None; irregular or
curved fracture
surfaces
Glassy, shiny
Rare trigonal pyramids but
usually irregular, anhedral
7
Two sets at 90
◦
poorly visible
Alkali
feldspar
(K,Na)AlSi
3
O
8
White or pink,
sometimes orange or
yellow
Usually dull,
sometimes
silky or
vitreous
Tabular crystals; shiny
cleavage surfaces may show
simple twins. Elongate
rectangular 'laths', lamellae,
or irregular masses of
6
