Magma

1
Magma

• Differentiate magma based on its chemical
  composition ? felsic vs. mafic

2
Melt 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)
5
Silicate structures
nesosilicates
phyllosilicates
sorosilicates
inosilicates
cyclosilictaes
tectosilicates
6
Mineral 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
7
Mineral Structures

• Chain Silicates single and double

SiO32- single chains
Inosilicates Si4O114- Double
tetrahedra pryoxenes pyroxenoids
amphiboles
8
Mineral Structures

• Sheet Silicates aka Phyllosilicates

Si2O52- Sheets of tetrahedra
Phyllosilicates micas
talc clay minerals serpentine
9
Mineral Structures

• Framework silicates aka Tectosilicates

low-quartz
SiO2 3-D frameworks of tetrahedra fully
polymerized Tectosilicates quartz
feldspars feldspathoids zeolites
10
Characterizing 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

11
Tectosilicates

• Feldspars

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
12
Equilibrium

• 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

13
NASA 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."

14
Melt-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
15
Melt-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
16
Melt-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

17
Why 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

18
Melt-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
19
Melt-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