CLAY MINERALOGY AS A
GUIDE TO ALTERATION
ENVIRONMENTS ON MARS
David Bish and David Vaniman
Indiana University
Los Alamos National Laboratory
What can clay minerals tell us?
• That liquid water was present!
• That alteration took place at (relatively)
low temperatures
• Water compositions-open vs closed
hydrologic systems
• Whether any post-formation alteration has
taken place (diagenesis, metamorphism)
Cheto bentonite
hematite,
gibbsite
kaolinite
mixedlayer I/S
o
g
mica,
chlorite
n
e
t
In
y
sit
ea
W
f
rin
e
th
Tundra
Prairie
Laterite
Mineralogy vs. intensity of weathering
(modified from Velde, 1985).
Solutions are supersaturated with the respective
phase to the left of or above the solid line.
Stability diagram for minerals in the Al2O3-SiO2-H2O system
at 25ºC and 1 atm. The “p” in axis labels refers to –log[].
From Kittrick (1969)
Smectites vs. Zeolites
• Smectites or zeolites can form from
volcanic ash, depending on conditions
– smectite in near- or below-neutral pH
conditions
– zeolites under alkaline conditions
– cannot assume that basaltic ash will always
alter to phyllosilicates
– both smectites and zeolites would indicate a
more persistent and evolved hydrogeologic
system
Alteration of Volcanic Glass to
Clinoptilolite
St. Cloud Mining, Buckhorn, NM
Smectites vs. Kaolin Minerals
• Kaolin minerals form on Earth most commonly in tropical
climates, usually under more-acidic conditions and with
high water:rock ratios (well drained).
• Can form hydrothermally, accompanied by amorphous
silica and TiO2 minerals such as anatase.
• On Mars, a Ti-Si association has been considered to
support acid-vapor alteration (Yen et al. 2007)—not
unique.
• Detection of 10Å hydrated halloysite, a more hydrated
kaolin mineral, on Mars would imply that the mineral had
never experienced dehydration after formation, as
hydrated halloysite irreversibly dehydrates to a 7.2Å
phase under low-RH conditions.
• Kaolinite stable
at low pH, low K,
and a(SiO2) > qtz
• Mica stable at
low a(SiO2), med
pH, and medhigh K
Stability relations of phases in the K2O-Al2O3-SiO2-H2O system at
25ºC and 1 atm. Solid circles represent of analyses of waters from
arkosic sediments.
From Garrels and Christ (1965)
Short-Term Clay Mineral Stability
and Sample Return
• ANY change in RH (or T) will change
smectites (and other hydrous minerals)
– Reversibility of rxn is important
– sealed samples that are heated will change
due to change in RH!
– as T goes up, RH goes down, etc.
• Large changes in RH or T will have little or
no effect on kaolin or chlorite minerals.
RH Effects
16
NG-1 Nontronite
(Na-Exchanged)
d(001) reflection, Å
15
14
i
s
a
e
r
Dec
13
RH
%
ng
12
asi
e
r
Inc
11
RH
%
ng
10
3
4
5
6
7
8
9
10
2-Theta (CuKa)
9
0
10
20
30
40
50
60
70
Relative Humidity (%)
80
90
100
11
12
Long-Term Clay Mineral Stability
from Mars
• Poorly ordered clay minerals (e.g., smectites and
illite/smectites) do not occur in old rocks on Earth--it is often
assumed that they gradually transform to more stable
phases such as illite, micas, and chlorites.
• The discovery of smectites in Noachian terrains has
important implications for the long-term stability of clay
minerals and suggests an alternative hypothesis, namely
that tectonic activity on Earth eventually results in the
progressive alteration of low-temperature minerals to
higher-temperature assemblages.
• Smectites on Mars in rocks >3 Gya would rewrite our
understanding of clay mineral stability and suggest that, in
the absence of (plate) tectonic activity, “metastable” clay
minerals may be “stable” for times on the order of the age
of our planet.
250
T (ºC)
200
illite ± chlorite
150
100
smectite and
mixed-layer illite/smectite
50
0
0
50
100
150
200
Time (My)
Time-temperature limits on clay minerals (modified from Velde, 1992).
This figure implies that mixed-layer illite/smectites are not stable over
long times even at low temperatures.
250
Relationship between T and extent of smectite-to-illite reaction
(Hower and Altaner, 1983)
What is Lost on Return?
• Hydration states of minerals (not just
smectites)
• So-called H2O of hydration can be
reversibly driven off—isotope info lost
• Potential for reaction between minerals
– hydrous sulfates can react together
– Ca-smectite + epsomite⇒Mg-smectite +
gypsum!
What is Gained by Return?
• Ability to perform analyses not easily done
remotely
– Clay mineral analyses are best done in the
laboratory
– Discrimination between different smectites,
smectite and illite/smectite, stacking order
– Isotope analyses (e.g., Ar-Ar)
Discrimination of Clay Minerals
3200
3100
3000
16.9Å
2900
2800
2700
2600
2500
2400
2300
2200
2100
2000
)1900
s1800
p
C
(1700
ni1600
L1500
Treatments used in terrestrial
laboratories will not be accessible
on Mars (e.g., saturation with
ethylene glycol vapor).
1400
1300
9.6Å
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
3
10
20
30
40
50
60
2-Theta - Scale
XRD patterns for air-dry and glycolated smectite (SWy-1)
Summary
• The occurrence of clay minerals ⇒ aqueous
alteration has occurred.
• Specific clay minerals can put limits on the
conditions of mineral formation, e.g.,
–kaolin ⇒ high water:rock, low pH
–zeolites ⇒ low water:rock, closed system, high pH
–smectites ⇒ open system, med pH, I/S ⇒T
• ID of “old” smectites on Mars can rewrite our
understanding of clay stability.
• Clay mineralogy can clarify alteration mechanisms-the entire mineral assemblage can greatly constrain
processes responsible for today’s martian
mineralogy.
• Unlike other hydrous minerals, there is no
significant loss on sample return.
Cheto bentonite