Pseudobrookite, is found in volcanic rocks, and the mineral armalcolite ((Fe,Mg)Ti2O5) found in the Apollo 11 and subsequent lunar samples seems to be unique to the moon. In plutonic rocks on the earth, ilmenite and rutile were found with what appears to be an equilibrium liquidus texture, while on the moon armalcolite often appears to be the primary liquidus phase among Ti-oxides. This suggests that total pressure may be a factor in the formation of these minerals, and a knowledge of the phase relations in this system yields information about the pressure under which a given magma crystallized

Friel, J. J.

English

NASA Technical Reports Server

FINAL TECHNICAL REPORT
(NASA-CR-140490) PHASE EQUILIBRIA STUDY N714-33881
OF PSEUDOBEROOKITE TYPE MINERALS Final
Technical Report (Pennsylvania Univ.)
17 p HC $4.00 CSCL 08G Unclas
__G3/13 50356
Phase Equilibria Study of Pseudobrookite Type Minerals
John J. Friel
Univ. of Penna.
Dept. of Geology
NSG 7031
9/27/74
https://ntrs.nasa.gov/search.jsp?R=19740025768 2020-03-23T04:53:46+00:00Z
INTRODUCTION
Minerals of the pseudobrookite-ferropseudobrookite solid
solution series (Fe2TiO 5-FeTi205 ) do not seem to form in
deep-seated plutonic rocks on the earth, but rather ilmenite
and rutile are found. Pseudobrookite, however, is found in
volcanic rocks, and the mineral armalcolite ((Fe,Mg)Ti205 )
found in the Apollo 11 and subsequent lunar samples seems to
be unique to the moon. In plutonic rocks on the earth ilmenite
and rutile have been found with what appears to be an equilibrium
liquidus texture, while on the moon armalcolite often appears to
be the primary liquidus phase among Ti-oxides. This suggests
that total pressure may be a factor in the formation of these
minerals, and a knowledge of the phase relations in this system
should yield information about the pressure under which a given
magma crystallized.
Haggerty (1973) divided lunar armalcolites into three types
based on chemistry. These are: Zr-free armalcolite characteristic
of high titanium mare basalts, Cr-Zr-Ca armalcolite, and Zr-
armalcolite, both of which are characteristic of the non-mare high-
alumina basalts. This division, which was also observed by Steele
(1974), indicates that zirconium may have an important role in the
formation of armalcolite. It is therefore necessary to know the
high pressure stability of armalcolite as a function of ZrO2
content.
1
Hartzman and Lindsley (1973) found that armalcolite in
equilibrium with metallic iron reacts below 1000 C to oxidize
the iron and reduce some of the Ti4+ to Ti3+ according to
the following reaction:
Feo + 4FeTi2 05 = Ti3 05 + 5FeTiO3
(in arm.)
It would therefore be interesting to know if there is any
difference in the pressure stability between this reduced
armalcolite and one in which all of the Ti is quadrivalent.
The objective of this study was to determine the pressure-
temperature curve for the stability of armalcolite with and
without ZrO 2 and also of armalcolite in equilibrium with
metallic iron. In order to predict the behavior of armalcolite
at high pressure, the difference in molar volumes was calculated
for the probable armalcolite decomposition reaction. These are
listed below:
2(Fe.5 g.5)Ti 20 = 2TiO2 + MgTiO + FeTiO
112.6 37.6 30.9 31.7 V = -12.4cm 3
Since the ilmenite-rutile assemblage on the right has a smaller
molar volume than the pseudobrookite type phase, the reaction
should proceed to the right at high pressure.
2
SAMPLE PREPARATION
Mixtures of MgO, TiO 2 , Fe 2 03 , and Feo were prepared to
make armalcolite according to the following reaction:
1/6Fe + 1/6Fe20 3 + MgO + 2TiO 2 = (Fe 5Mg.5)T1205
In the case of Zr-armalcolite, 4% and 10% by weight ZrO2 was
substituted for TiO 2 , and a fourth composition was prepared
with excess metallic iron. These mixtures were then ground
in a mixer-mill for 30 minutes, after which time the mixtures
appeared homogeneous at 1OX magnification, and no white streaks
of MgO or TiO 2 were observed.
Three of these mixtures (Zr-free, 4%, and 10% ZrO 2 ) were
placed in silica glass capsules sealed at one endand these
capsules were placed in a larger SiO 2 glass capsule containing
metallic iron at the bottom. The outer glass capsule was then
evacuated and sealed. The Fe-rich material was prepared in
the same manner but in a separate capsule. These silica tubes
were suspended in a furnace at 1300 0 C for ten days and then
quenched. Each armalcolite composition was then reground for
30 minutes in the mechanical shaker, evacuated, and resealed,
and placed in the furnace as before for seven more days.
When this was quenched, the armalcolite appeared to be single
phase when examined by X-ray diffraction, but optical
examination revealed some birefringent TiO 2 (estimated to be
about 1%) probably due to the presence of some Fe 3 +. The ZrO 2
3
armalcolites also showed some unreacted ZrO2 , and the Fe-rich
material produced a two phase starting material (armalcolite
and ilmenite).
Semi-quantitative data from the energy dispersive detector
on the scanning electron microscope indicate that the two
starting compositions containing ZrO2 contained approximately
equal amounts of ZrO2 , on the order of 1 wt.%. This is
consistent with the amount found in lunar armalcolites by
Steele (1974) and may represent saturated armalcolite under
these conditions. The remainder of the ZrO2 weighed in was
apparently lost to the silica glass capsules. Zircon (ZrSi04 )
was observed by X-ray diffraction of the glass after the
runs; however, no zircon or other silicate was observed in
the starting material.
EXPERIMENTAL TECHNIQUE
The pressure runs were made in a single-stage, piston-
cylinder apparatus set up by Drs. Meyer and Weidner at Goddard
Space Flight Center. The armalcolite samples were sealed in
silver-palladium capsules which were then placed in crushable
alumina between pyrophyllite plugs all within a graphite furnace
and talc outer-sleeve. The Ag60Pd40 alloy was chosen for its
ability to withstand the temperatures used without absorbing too
4
much iron from the sample. The runs were not buffered because
it was felt that the small amount of air trapped in the capsule
would not significantly change the composition of the sample.
All runs were in the piston-out configuration, and the
calibration of the press was done by Dr. Gene Ulmer against
the melting point of gold. The apparatus was calibrated to
±5% pressure and ±80C temperature, and when the temperature
calibration was checked during this work, it was found to
be ±60C.
Three capsules each containing a sample of different
composition were run at the same time in the press. These
capsules were in an equilateral triangle configuration with
their sides touching each other and all void space filled with
crushable alumina as the pressure transmitting medium. This
presumably allowed three compositions to be run under identical
conditions, but to check this assumption three capsules containing
the same composition material were run together at 1000 0C and
5 kb. All produced a three phase assemblage in which the
amounts of each phase were constant among the three charges as
estimated from X-ray diffraction data.
The maximum drift during a run was 1000 and 0.2 Kb, and
all runs were held for one hour at the temperature and pressure
of interest. Reversed runs were held for one hour at a
5
pressure above the nearest phase boundary (still piston-out)
and then dropped to the point of interest and held for
one hour.
RESULTS
The results of this work are plotted in figures 1 and 2
in the form of a P-T plot. Figure 1 shows the 1-,2-, and
3-phase stability regions of synthetic armalcolite with a
0.5 Fe/Fe+Mg ratio. The dashed curve is for the Zr-free
material while the solid curve is for the material containing
about 1% ZrO2 . The armalcolites produced by weighing in
different amounts of ZrO2 produced identical results in terms
of the phases present, but this was expected after discovering
that equal amounts of ZrO2 were taken into the armalcolite.
The phases present were determined by X-ray diffraction,
and a crude estimation of the amount of each phase was made from
relative intensities in the X-ray pattern. While this is not
very precise, it was used as a guide in placing the phase
boundaries in figures 1 and 2. All two-phase regions were
approached from lower pressure; i.e., arm.- ilm. + rut.
All one-phase regions were approached from higher pressure; i.e,
rut. + ilm.-* arm., and the three-phase regions at 10000C were
approached in both directions; i.e., arm.-) rut. + ilm. + arm.'
6
and rut. + ilm. - arm. + rut. + ilm. It was assumed that if
equilibrium could be demonstrated by reversed runs at 1000 0C,
then equilibrium should also be attained at higher temperature.
It is apparent from figure 1 that ZrO 2 has the effect of
destabilizing armalcolite at higher pressure; however, this
effect is not great, and armalcolite containing approximately
1% ZrO2 breaks down at about 1-2 kb lower pressure than
Zr-free armalcolite. Figure 2 shows the stability of
armalcolite prepared in equilibrium with metallic iron. No
significant differences between this and the Zr-free
armalcolite of figure 1 are apparent. This may be due to
the limits of precision of the X-ray analysis used in this
study, and possibly microprobe analysis of the resulting phases
would uncover differences.
INTERPRETATION
Armalcolite has been interpreted as a primary liquidus
phase in lunar rocks, and Akimoto et.al. (1970) found that
this liquidus is about 12000 C at 1 atm. sloping toabout 1500 0C
at 40 kb. for one armalcolite bearing rock from Apollo 11.
Figure 3 shows the P-T liquidus curve determined by Akimoto and
its intersection with the extrapolation of my curves for synthetic
armalcolite stability determined at lower temperature. This
7
indicates that rocks containing armalcolite of 0.5 Fe/Fe+Mg
ratio must have crystallized in the region bounded by the
whole rock liquidus and the curve separating the 3- and 2-phase
regions, which corresponds to a maximum depth on the moon of
about 350 km. Further,'single-phase armalcolite not coexisting
with ilmenite must have crystallized below the curve separating
the 1- and 3-phase regions. Crystallization may, of course,
have taken place at much lower pressures, and ilmenite plus
rutile could be formed by reactions other than the breakdown
of armalcolite, but these pressures should be the upper limit
for the pressure at the time of crystallization of armalcolite-
bearing lunar rocks.
Zirconium-bearing armalcolite consistently breaks down at
lower pressure than Zr-free armalcolite. This effect is less than
1 kb. at 12000C, and greater zirconia content would presumably
increase this effect. Thisplaces an upper boundary on the
pressure at the time of formation of the high-alumina non-mare
basalts. This data is consistent with a derivation of these
highly felspathic basalts from within the lunar crust; i.e.,
under less pressure than the more deeply derived mare basalts,
although it certainly does not prove this interpretation.
Further study is necessary to determine the distribution
of Fe, Mg, Ti, and Zr in the armalcolite breakdown assemblage,
and to see how armalcolite stability is influenced by
variations in oxygen fugacity.
9
Figure 1: P-T stability for armalcolite ((Fe 5Mg 5 )Ti205 )
and armalcolite containing about 1% ZrO2.
'1 phase = arm.
2 phases = rut. + ilm.
3 phases = rut. + ilm. + arm.
Piston-cylinder apparatus in Ag60 Pd40 capsules.
14
= Zr-ARMALCOLITE O
---- = Zr- FREE ARMALCOLITE
12- O
100
(K b)
.6
/O
O - 2 PHASES
4 - 3 PHASES /
.- l- 1 PHASE A
2 /
I I V I ..
800 900 1000 1100 1200
T(°C)
Figure 2: P-T stability of armalcolite prepared
in equilibrium with metallic iron.
.2-phase starting material (arm. + ilm.)
14
Fe-RICH ARMALCOLITE
12 - O- 2 PHASES (RUT.+ ILM) O
g- 3PHASES (RUT.+ILM.+ARM.)
--2 PHASES (ARM.+ ILM.) O
10-
.8 0
8- O *
P
(Kb) -
6- .
4
2
800 900 1000 1100 1200
T(°C)
Figure 3: Comparison of P-T stability of synthetic
armalcolite ((Fe Mg.5 )Ti2 05 ) with whole
rock liquidus curve determined by Akimoto
et.al. (1970) for an armalcolite-containing
Apollo 11 rock.
35
30
25
20
P
(Kb)
2 phases- -- -
10
5
5 L
1 phase
1 aim I I I I
1000 1100 1200 1300 1400 1500T(0C)
REFERENCES
Akimoto, S., M. Nishikawa, Y. Nakamura, I Kushiro, and T. Katsura
(1970) Melting Experiments of Lunar Crystalline Rocks.
Proc. Apollo 11 Lun. Sci. Conf. 129-133 Pergamon.
Haggerty, S.E. (1973) Armalcolite and Genetically Associat6d
Opaque Minerals in the Lunar Samples. Proc. Fourth Lun.
Sci. Conf. 777-797, MIT Press.
Hartzman, M.J., and D.H. Lindsley (1973) The Armalcolite Join
(FeTi2054MgTi205) with and without Excess Feo: Indirect
Evidence for Ti3+ on the "oon. (abs.) GSA Abstracts with
Programs, 653.
Steele, I.M.,(1974) Ilmenite and Armalcolite in Apollo 17
Breccias. Am. Min. 59 681-689.


Phase equilibria study of pseudobrookite type minerals

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