Geology Reference
In-Depth Information
Figure 4.3 Photo of porphyritic lava, Faroes. Large white/light tabular feldspar
crystals in a fine grained basalt matrix. Note that tabular shapes show many long
intersects in 2D cross section.
Table 4.1 Composition of major phenocryst phases in porphyritic lavas.
Olivine basalt/
Basalt
Basaltic
Andesite
Dacite
Rhyolite
picrite
andesite
Plagioclase
**
**
***
***
***
**
Olivine
***
**
**
*
-
-
Pyroxene
*
**
**
**
*
-
Hornblende
-
*
*
**
**
*
Biotite
-
-
-
*
**
**
Alkali feldspar
-
-
-
*
**
***
Quartz
-
-
-
-
**
***
Fe-Ti oxide
*
*
*
*
-
-
**Frequently present.
***Often present.
*Rarely present
- absent (or very rare)
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