Practical Igneous Petrology (for Dummies). Minerals in Hand Specimen A.Have the right tools B.Make...
-
Upload
keon-kennington -
Category
Documents
-
view
213 -
download
0
Transcript of Practical Igneous Petrology (for Dummies). Minerals in Hand Specimen A.Have the right tools B.Make...
Minerals in Hand Specimen
A. Have the right tools
B. Make the most of the properties
C. Know what to expect where!
D. Know the key properties of the key minerals.
Make the Most of the Properties.
1. Colour
2. Hardness
3. Transparency (transparent, translucent, opaque)
4. Lustre (adamantine, dull, metallic, pearly, vitreous etc)
Make the Most of the Properties.
5. Cleavage and fracture (number of cleavages, concoidal fracture, irregular fracture).
Make the Most of the Properties.
6. Crystal system (cubic, tetragonal, orthorhombic, monoclinic, trigonal, hexagonal)
Beryl
Barytes
Make the Most of the Properties.
6. Crystal habit (fibrous, prismatic, tabular)
fibrous
Acicular
Prismatic
Tabular
Know Where to Expect What.
<1/3 dark minerals(e.g. Granite, rhyolite)
Most common light coloured minerals (in order of frequency):QuartzAlkali feldsparPlagioclase feldsparMuscoviteMost common dark coloured minerals (in order of frequency):BiotiteAmphibole (hornblende most common but also reidbeckite in alkali granites). PyroxenesTourmaline (abundant in some granites and pegmatites).Most common opaque minerals:PyriteChalcopyriteAzuriteGalenaMalachiteMagnetiteHematite
Leucocratic
Know Where to Expect What.
~50% dark minerals(e.g. Diorite, andesite)
Most common light coloured minerals (in order of frequency):Feldspars (either alkali or plag can dominate).QuartzFeldspathoids (such as nepheline and leucite).
Most common dark coloured minerals (in order of frequency):HornblendePyroxenes (augite, hypersthene, pigeonite, diopside)Biotite and/or phlogopiteOlivine (rare)
Mesocratic
Know Where to Expect What.
>1/3 dark minerals(e.g. basalt, gabbro)
Most common light coloured minerals (in order of frequency):Plagioclase feldsparQuartz (rare)Alkali feldspar (rare)Feldspathoids (rare)
Most common dark coloured minerals (in order of frequency):Pyroxene (augite, hypersthene, pigeonite, diopside)OlivinePhlogopite (a mica)GarnetAmphiboles (rare)Biotite (rare)
Melanocratic(mafic and ultramafic)
Know the Properties of Important Minerals.
Quartz (SiO2)
Characteristic propertiesColour is as variable but clear quartz is by far the most common color followed by white or cloudy (milky quartz). Purple (Amethyst), pink (Rose Quartz), gray or brown to black (Smoky Quartz) are also common. Cryptocrystalline varieties (agate, chalcedony etc) can be multicolored. Luster is glassy to vitreous as crystals, while cryptocrystalline forms are usually waxy to dull but can be vitreous.Crystal System: trigonal.Cleavage and Fracture: No cleavage, concoidal fracture.Crystal Habit: variable but often anhedral. Hexagonal prisms or three sided prismatic crystals are common in veins and pegmatites.Hardness: 7 (less in agate and chalcedony).Occurrence: any leucocratic rock is likely to contain quartz, melanocratic rocks are unlikely to contain quatz.Best Indicators are first the fact that it is very common (always assume transparent clear crystals may be quartz), crystal habit, hardness, striations on the surfaces of crystals, good conchoidal fracture and lack of good cleavage.
Minerals in Thin-Section
• Relief• Colour (in PPL)• Pleochroism (in PPL)• Birefringence (colours in XPL)• Crystal Shape• Twinning (e.g. Plag)• Extinction Angle
Textures of Igneous Rocks
Coarse-grained – crystals are > 5mmMedium-grained – crystals are 1-5 mm
Fine-grained – crystals are <1 mm (not including phenocrysts)