Title: Magma
1Magma
- Differentiate magma based on its chemical
composition ? felsic vs. mafic
2Melt Composition freezing T
- Liquid magma freezes into crystals ? the
composition of what freezes first is governed by
the melts composition - Analogous to the composition of seawater ice ?
icebergs are composed of pure water pure water
freezes first, leaving the concentrated brine
behind - In magmas ? More silica lower T more Ca,
Mghigher T - Silica polymerization also affected by T and how
much Si there is!
3- Discontinous series Structures change, harder
to re-equilibrate - Continuous Series ? plag re-equilibrates quicker
and if not is a continuum in composition rather
than a change in mineral as T decreases
rock
Mg2
Fe2
cooling
Mg2
4(No Transcript)
5Silicate structures
nesosilicates
phyllosilicates
sorosilicates
inosilicates
cyclosilictaes
tectosilicates
6Mineral Structures
- Silicates are classified on the basis of Si-O
polymerism
SiO44- Isolated tetrahedra
Nesosilicates Examples olivine
garnet Si2O76- Paired tetrahedra
Sorosilicates Examples lawsonite nSiO32-
n 3, 4, 6 Ring silicates
Cyclosilicates Examples benitoite
BaTiSi3O9 axinite
Ca3Al2BO3Si4O12OH beryl Be3Al2Si6O18
7Mineral Structures
- Chain Silicates single and double
SiO32- single chains
Inosilicates Si4O114- Double
tetrahedra pryoxenes pyroxenoids
amphiboles
8Mineral Structures
- Sheet Silicates aka Phyllosilicates
Si2O52- Sheets of tetrahedra
Phyllosilicates micas
talc clay minerals serpentine
9Mineral Structures
- Framework silicates aka Tectosilicates
low-quartz
SiO2 3-D frameworks of tetrahedra fully
polymerized Tectosilicates quartz
feldspars feldspathoids zeolites
10Characterizing minerals
- WITHIN classes (like the silicate classes)
Minerals put into groups based on similar crystal
structures differing typically in chemical
substitution - Groups usually named after principle mineral
- Feldspar group, mica group, feldspathoid group
- Sites designated M1, M2, etc. designate spots
where cations go into structure - different site designations have different
characteristics (see different charge, have
different sizes, etc.) and accommodate different
ions based on this
11Tectosilicates
Substitute Al3 for Si4 allows Na or K to be
added Albite-Orthoclase
Substitute two Al3 for Si4 allows Ca2 to be
added Albite-Anorthite
Albite NaAlSi3O8
12Equilibrium
- Need a description of a minerals equilibrium
with its surroundings - For igneous minerals, this equilibrium is with
the melt (magma) it forms from or is a
representation of the Temperature and Pressure of
formation
13NASA News 03-15-06
- Scientists say the minerals found in Stardust
aerogels include magnesium olivine (forsterite) - "In the coldest part of the solar system, we have
found samples that have formed at extremely high
temperatures. So, the hottest samples in the
coldest place."
14Melt-crystal equilibrium 1
- When crystal comes out of melt, some ions go in
easier ? more Ca rich crystals form 1st - Precipitated crystals react with cooling liquid,
eventually will re-equilibrate back, totallly
cooled magma xstals show same composition - Magma at composition X (30 Ca, 70 Na) cools ?
first xstal bytownite
X
liquidus
solidus
15Melt-crystal equilibrium 1
- Magma at composition X (30 Ca, 70 Na) cools ?
first crystal bytownite (73 Ca, 27 Na) - This shifts the composition of the remaining melt
such that it is more Na-rich (Y) - What would be the next crystal to precipitate?
- Finally, the last bit would crystallize from Z
X
Y
Z
16Melt-crystal equilibrium 1b
- Precipitated crystals react with cooling liquid,
eventually will re-equilibrate back, totally
cooled magma xstals show same composition - UNLESS it cools so quickly the xstal becomes
zoned or the early precipitates are segregated
and removed from contact with the bulk of the
melt
17Why arent all feldspars zoned?
- Kinetics, segregation
- IF there is sufficient time, the crystals will
re-equilibrate with the magma they are in and
reflect the total Na-Ca content of the magma - IF not, then different minerals of different
composition will be present in zoned plagioclase
or segregated from each other physically
18Melt-crystal equilibrium 2 - miscibility
- 2 component mixing and separation ? chicken soup
analogy, cools and separates - Fat and liquid can crystallize separately if
cooled slowly - Miscibility Gap no single phase is stable
- SOUP of X composition cooled in fridge Y vs
freezer Z
100
SOUP
X
Temperature (ºC)
50
Y
0
fats
ice
Miscibility Gap
Z
-20
10
70
30
90
50
Water
Fat
fat in soup
19Melt-crystal equilibrium 2 - miscibility
- 2 component mixing and separation ? chicken soup
analogy, cools and separates - Fat and liquid can crystallize separately if
cooled slowly - Miscibility Gap no single mineral is stable in
a composition range for x temperature