Felsic minerals and classification Flashcards
What are the most common continental felsic rocks?
granite
rhyolite
What is the bulk geo-chemistry of felsic like compared to mafic?
Much more silica rich (70% for F, 50% for M)
Rich in alkalis
Poor Mg, Fe and Ca oxides
What process is responsible for the difference in bulk Geo-chemistry of felsic and mafic?
Fractional crystallisation
What are alkalis that felsic can be rich in?
Na2O- sodium oxide
K2O- potassium oxide
How can the composition of felsic magma also be referred to as?
Evolved
What does it mean to say felsic magma is evolved?
magma experienced modification from primitive mafic mainly via fractional crystallisation
What is fractional crystallisation?
early formed crystals removed- thus the minerals that form them are used creating a dilution affect of liquid melt
What are the first minerals to form in a cooling basalt?
olivine
pyroxene
Ca-rich plagioclase
What will the the same and what will be variable in the evolved rocks?
Not all same silica content
Rich be rich in felsic minerals
What elements typically make up felsic minerals?
Al, Si, K, Na
What are some examples of minerals rich in felsic minerals ?
Feldspars (alkali feldspars)
Quartz or feldspathoids
What is the general reason rocks will become evolved?
as the magma crystallises the minerals forming have a chemistry that doesn’t match liquid melt
What are the first minerals to form from cooling basalt usually composed of?
rich in- Mg Fe Ca
Poor in silica
What are compatible elements?
elements that go into a specific mineral
What are incompatible elements?
those which dont go into phases
What will happen to the liquid melt during crystallisation involving incompatible?
liquid melt will become enriched in incompatible elements
What can fractional crystallisation produce graphically?
a continuous spectrum of real magma compositions from mafic starting to felsic end
What are the common minerals in felsic igneous?
Quartz
Feldspars
Micas- muscovite and biotite
Hornblende
Magnetite (opaque oxide phases)
What is the equation for quartz?
SiO2
Where is quartz commonly found?
widespread in upper continental crust
What is quartz common in?
felsic igneous rocks
metamorphic rock
clastic sediments
What will the framework of quartz be like?
SiO2 tetrahedra
How many polymorphs of silica are there?
6
What is the stability of the silica polymorphs like?
all stable at different pressure and temperature
What is the stability of quartz like?
stable at low temp and pressure (upper continental crust)
highly resistant to weathering
Very hard
What will quartz appear like in igneous hand specimen?
grey glassy looking mineral
no cleavage instead glassy conchoidal fractures
What is quartz like in thin section?
colourless
low relief
low birefringence
no twinning
no cleavage
Clean even when around altered minerals
Undulose extinction
What is undulose extinction and why does it occur in quartz?
occurs when structure is strained
it is extinction sweeping across grain as platform is rotated
What elements will you typically find in feldspars?
(Ca, Na, K)
Al
(Al, Si)
Si2O8
Where are feldspars dominant?
the crust- continental and oceanic
What is the framework of feldspars like?
alumino-silicates
Si and Al in tetrahedral framework
What 2 series exist for feldspar in solid solution?
Plagioclase feldspars
Alkali feldspars
What ion exchange occurs in plagioclase feldspars?
Na-Si
Ca-Al
What do you transfer to and from with plagioclase feldspar?
Albite (Na) to Anorthite (Ca)
What endmembers is there no natural transfer between? (feldspars)
Potassium and Calcium (orthoclase and Anorthite)
What can alkali feldspars transition inbetween?
Orthoclase
Albite
What ion exchange takes place with alkali feldspars?
Na and K (sodium and potassium)
What are the common elements found in plagioclase feldspar?
(Ca, Na) Al (Al, Si) Si2O8
What can the more complex exchange in solid solution of plagioclase feldspar lead to?
reaction to changing magmatic conditions can be slow and lead to compositional zoning
What is zoning in crystallisation?
bands of light and dark crystal contained inside crystal border almost radiating from a central nucleus
How will plagioclase feldspar appear in hand specimen?
white blocky grains
What will plagioclase feldspar be like in thin section?
low relief
colourless
low birefringence
Unclear cleavage - can show 2 (90*- cubic)
Polysynthetic twinning (multiple or lamellar)
Altered (dusty)
What is K feldspar?
K-rich alkali feldspar
What will K feldspar in hand specimen?
pinkish colour can also be white
What will K feldspar be like in thin section?
Similar to plagioclase
but instead simple twinning
What will twinning be like in a low T-form microcline?
might be cross-hatched
What can the texture of coarsely-crystalline alkali feldspars be called?
perthite- “flame”, “patchy” and “lamellar”
What are the 2 common mica minerals in igneous rocks?
Biotite
Muscovite
What are the important bits to remember from biotite formula?
Hydrous- OH- anion
Range of cations- Al, K, Fe, Mg
What is biotite more commonly referred to as?
Dark mica
What is the general thing to take away from muscovite formula?
Al rich (different from biotite as absence of Fe and Mg)
What is muscovite more commonly referred to as?
white mica
What type of silicates are micas?
sheet silicates
What is the structure of micas like?
tetrahedra link in sheet form (perfect cleavage between sheets)
Thin platy grains break with basal cleavage
Grain shape hexagonal in other plane
How can micas be recognised in hand specimen?
strong cleavage
reflective surface- biotite: black, muscovite: silvery
What will biotite be like in thin section for ppl?
Pleochroic brown
Moderate relief
One very good cleavage
Alteration prone
What can happen to the cleavage in thin section for biotite?
kinked due to mineral weakness causing irregular crystal ends
What will alteration look like in biotite thin section?
patchy
What is biotite like in xpl thin section?
high birefringence (third order)
Extinction grains have speckled texture
What is muscovite like in thin section ppl?
Colourless
One very good cleavage
What can the kinking of the cleavage in muscovite lead to?
irregular crystal and development of woody texture
What is muscovite like in thin section xpl?
high birefringence (upto 3rd order)
What does crystal size relate to in igneous rocks?
cooling rate
What is the factor that distinguishes basalts from gabbro due to chemical and mineralogical sameness?
texture- crystal size
What will a slow cooling rate do to crystallisation?
coarser crystals
Where will extrusive rocks form?
on the surface
What does the surface cooling environment mean for extrusive rocks?
heat lost rapidly
liquid magma will crystallise quickly
finer grained
Where do intrusive rocks cool and solidify?
in the crust
What are some examples of shallow intrusions?
Sills
Dykes
What can shallow intrusions also be called?
hypabyssal rocks
What is cooling rate like with hypabyssal rocks?
small size lose heat relatively quickly
What is the typical grain size of hypabyssal rocks?
medium to fine grained
What 2 broad types of intrusive body?
hypabyssal
plutonic
What are plutonic intrusive bodies?
larger
greater depth (few km)
What is cooing rate and crystal size like in plutonic bodies?
generally slower forming coarser crystals
What will the hand specimen of plutonic rocks be like?
granular texture - formed due to large interlocking crystals
How will crystal formation and nucleation be affected by rapid cooling?
many crystals nucleate but dont have time to grow into larger crystals
What is nucleation for crystallisation?
initial process of crystal formation from solution
