THE AMERICAN
MINERALOGIST,
VOL. 55, IANUARY-FEBRUARY,
19?O
THE SYSTEM MgO-GeOz-HrO
SrBpnBN R. LvoNr aNp Enursr G. Eurens, Departmentof
Mi.neralogy, The Ohio State Unioersity
Columbus,Ohio 43210.
ABsrRAcr
Compatibility relations within the system MgO-GeOrHzO have been investigated by
hydrothermal slmthesis. While no germanium-hydroxyl analogs of the humite minerals
were found, a new compound, Mg{GeO6.Mg(OH)r, alters previously reported tielines in
the system.
The system MgO-GeO2-HzO was examined as part of a larger study
involving stability relations of the humite minerals in the systems
MgO-GeO2-MgF2 and MgO-SiOz-MgF2. This system was briefly examined by Roy and Roy (1954) as part of a study on synthetic serpentines. They found the system MgO-GeOz-HzOto be somewhat analogous
to the system MgO-SiOz-H2O, studied earlier by Bowen and Tuttle
(1949), Roy, Roy and Osborn (1953) and more recently by Greenwood
(1e63).
The boundary system MgO-GeO2 was examined by Robbins and
Levin (i959), who found it to be partially analogous to the sysrem
MgO-SiOz ,having definite areas of immiscibility, and two intermediate
compounds Mg2GeOaand MgGeOe. The compound MgzGeOa was reported to have a congruent melting point at 1855+ 30oC, and MgGeO3
a congruent melting point at 1700t 20oC.The 1 : 1 germanate was found
to exist in two modifications. The low temperature orthorhombic form
stable below 1555oC,is isostructural with entsatite, and the higher temperature monoclinic polymorph probably is isostructural with clinoenstatite.Unlike MgSiOa,which melts incongruently at 1557oC,(Bowen
and Anderson, 1914), MgGeO3 melts congruently at 1700*20"C. The
compound MgrGeO6, synthesized by Robbins and Levin (1959), dissociates at 1495+ 10oC to periclase and germanium forsterite and has no
analogous compound in the silicates.2 Conversely, no protoenstatite
phase of the 1: 1 germanate was observed. Koelmans and Verhagen
(1959) synthesizedwhat might be an additional polymorph of MgGeO3;
their diffraction pattern did not correspond to the metagermanates of
Robbins and Levin.
The 2:1 orthogermanate exists in two crystalline modifications having
r Present address: Air Force Materials
Laboratory (MAMS), Wright-Patterson Air
Force Base, Ohio 45433.
f;
2 Attempts by the present authors to synthesize
this compound in the silicate system,
utilizing a BeIt apparatus at 600oC and 50 kbars, and a shear squeezer at 500"C and 30
kbars produced only MgO and MgrSiOr.
118
Il9
SYSTEM MgO-GDrHzO
spinel and forsterite structures (Goldschmidt 1931). Dachille and Roy
(1960) have determined the P-T curve relating germanium spinel to
germanium forsterite. The olivine structure is converted to the denser
spinel at about 806oCat atmospheric pressureand at 840oCat 20,000psi.
Roy and Roy (1954) performed the only experimental work on the
ternary system MgO-GeO2-H2O.They reported the synthesis of three
ternary compounds-germanium serpentine (Mg3GezOs(OH)+),germanium talc (MgaGe+Oro(OH)+) and germanium anthophyllite
(MgTGesOn(OH)t. Phase equilibrium relations in the system were
presented as a seriesof compatibility triangles from about 450 to 600"C
at 10,000 psi. Their data shows germanium serpentine decomposing at
520t15"C to germanium spinel, germanium anthophyllite, and H2O.
Germanium talc is reported to decomposeat about 600oC to germanium
anthophyllite, "x", and HzO. The compound "x" is reported as an
unknown phase in the border system MgO-GeOz.
Pnocrounr
Most of the experiments performed in this study used standard Stellite cold seal reactor
vessels (Tuttle, 1949) and either a Tem-Press Model HR-11304 four-unit pressure-temperature system, or equivalent. The samples, either with or without an excess of water,
were sealed in Pt or Au tubes, which were placed within the reactor vessels. The vessels
were brought to temperature in a nichrome-wound furnace; the desired water pressure
Taem 1. Couposrrrons rr.rrnn Svstpu McO-GnOrH:O
MoIe percent
Mixed Crystalline Phases
z
A
6
t1
c
9
10
1la
11b
T2
13
I+
15
16
6MgOf 8GeOzf3Mg(OH),
3MgaGeOo*Mg(OH)z
MgaGeOe.|Mg(OH)z
MerGeOo*4Mg(OH)z
12MgOf 2GeOrlMg(OH),
6MgOf 2GeOz*Me(OH)z
2MssGeOrtMg(OH)z
MgzGeOr*Mg(OH)z
3GeOg*MgrGeOo*2Mg(OH)z
4MgGeOr* 2GeOg*Mg(OH)z
2MgOf 4GeO:f Me(OH)z
2MgGeOa*2GeOzfMg(OII)z
6MgOf 8GeO:*Me(OH)s
MgGeOr
2GeOztMe(OH)z
6MgOtl2GeOzf Mg(OH):
2MgO*8GeOzf 5Mg(OH)z
Mso
GeOz
45.00
76.47
7t.43
61.54
81.25
70.00
62.50
60.00
50.00
4t.67
37.50
37.50
40.00
17.65
14.29
7.96
12.50
20.00
25.00
20.00
33.33
50.00
50.00
50.00
50.00
50.00
50.00
60.00
40.00
4J. /J
50.00
25.00
38.00
35.00
IIeO
15.00
5.88
14.29
30.77
6.25
10.00
12.50
20.00
16.67
8.33
12.50
12.50
6.25
25.00
5.00
25.00
120
STEPHEN R. LYON AND IiRNEST G. EHLERS
,MgO
Mg(oH)e
Frc. 1. Compositions in the system MgO-GeOrHzO
(S : serpentine; T: talc; An: anthophyllite).
for the reactor vesselswas achieved by a small air pressure pump. The sealed sample tubes
collapsed until the fluid pressure within was equivalent to the water pressure applied to
the reactor.
Sample temperatures were measured by placing either a Pt-Pt 10 percent rhodium, or
chromel-alumel thermocouple in a small well in the reactor vessel about + of an inch
from the sample. The precision is * l5oC.
Compositions used in this study are given in Table 1 by number, mole percent of
MgO, GeOz and HzO, and by the crystalline phases present in the mixture. Attainment of
equilibrium is difficult in many silicate and germanate systems; this may result from the
very rapid formation and presistence of metastable phases from highly reactive starting
materials (Fyfe, 1960); or alternatively, other synthesis reactions may involve such low
free energy changes that reactions do not take place in experimentally reasonable lengths
of time. A compromise between these two extremes must be reached in the selection of the
starting materials. fn this system, mixtures of fine-grained, crystalline phases were employed. As indicated in Table 1, MgaGeO6, MgzGeOr (germanium forsterite), Mg(OH)z
and MgGeO3 (germanium enstatie) are components of many mixtures. Although the
reaction rates of these materials are slower than the more commonly-used unstable gels
and glasses, a closer approach to equilibrium can be demonstrated. The reactant compositions are plotted in Figure 1, the composition triangle MgO-GeOrHzO.
The products were identified by X-ray difiraction, as the phases were almost always too
fine-grained for optical examination. The sample mounts for routine examination were prepared by packing the powder into a depression on a glass slide. Samples were examined
SYSTEM
MgO-GeOz-HzO
I2t
with a Norelco high-angle X-ray difiractometer using cu radiation and a Ni fiIter. For
ordinary determinations, the scanning speed was ro 20 per minute, and for precise work
]o
20 per minute with a chart speed of four inches per minute.
ExpBnrlmNrs
More than forty hydrothermal runs were made at three temperatures
-470,570 and 670oCat about 689.5
bars (10,000psi). Reacrionsin this
study were examined initially in order to determine whether hydroxyrgermanium members of the humite group could be synthesized. rt was
soon apparent that humites cannot be synthesized under these experimental conditions. Reaction products included a new compound in the
system in addition to the expected germanium analogs. Reaction data
are tabulated in Table 2.1 Products and equilibrium relations are summarized in the two compatibility triangles shown in Figure 2.
Reactions at 670"C indicate the presenceof an unreported compound
Mg+GeO6'Mg(OH)r. This compound forms easily when spinel and
brucite, or MgaGeo6 and brucite mixtures are reacted. compositions in
the vicinity of MgaGeo6.Mg(oH)r, when reacted,indicated the presence
of compatibility lines between this compound and HzO, MgzGeOa
(spinel), MgaGeO6and MgO.
The compound Mg+GeOo.Mg(OH), is usually too fine-grained for
optical characterization. Electron micrographs show most particles considerably less than 1prin diameter, sometimeshaving hexagonal outlines.
Particles large enough for optical examination have low first-order
birefringence with indices that fall between r.72s and 1.731 (white
light). The particles have a high density of anhedral to subhedral incrusions, with polysynthetic twinning occasionally observed. Uniaxial optical-charactercould not be demonstrated.
X-ray difiraction data is presented in Table 3. The pattern was corrected with a high-purity silicon internal standard. Electron diffraction
of hexagonal or pseudo-hexagonal platlets gave an apparent (0001)
centered pattern but graphical indexing on both hexagonal and tetragonal cells was unsuccessful.After 1000 hours, at 670"C (10,000 psi),
particles were still too fine for single crystal methods. Attempts to
synthesize larger crystals are continuing.
other runs at 670oc produced no other ternary compounds, and indicated that MgGeo3 (enstatite) and Mg2Geo+(spinel) are stable in the
presenceof excesswater. Roy and Roy (1954), at temperaturesabove
I To obtain a copy of Table 2, order
NAps Document#007r1from ASrS NationalAuxiliary Publicationsservice,c/o ccM rnformation Sciences,lnc.,22 west 34th street, New
York, New York 10001;remitting $1.00 for microficheor g3.00 for photocopies,in advance,payable to ASIS-NApS.
STT,PHEN R. LYON AND ERNEST G, EHLERS
Ge02
6700c.
MgGe03
Mg2Ge04
Mg4Ge06
Mgo
Hzo
Ge02
MgGe03
Mg2Ge04
4700and5700c.
Mg4Ge06
Hzo
Mg(0H)2
Mgo
Frc. 2. Equilibrium relationsin the systemMgO-GeOrHO'
(M:MgaGeOo.Mg(OH)z)
123
SYSTEM MgO-G&z-H.P
Teeln 3. X-nly DrrrnacrroN DArA lon McrGrOo.Mc(OH):
d(obs)
7.085
4.470
4.175
100
6l
o
3.490
3.275
3.041
2.925
2.899
2.777
2.743
2.705
2.581
2.567
2 .s 0 8
2.489
2.M3
2.414
2.341
2.266
2.257
2.218
33
6
11
z
J
A1
80
83
ll
36
25
33
8
6
6
IJ
33
5
2. 0 8 1
2.049
2.033
2. 0 1 8
1.905
1.872
r.799
t.ttJ
1.740
1.656
l.&9
1.6176
1.6048
| .5917
1. 5 5 3 2
1.5050
1.4786
1.4599
r.4207
|.3974
r.2320
1.1960
27
5
J
z
22
IO
tl
6
3
2
t7
8
1J
8
8
24
@
3
16
8
nn
* Height above baseline relative to the strongest peak.
nucleation and growth of the silicon anthophylilte, as seenby the studies
of Bowen and Tuttle (1949) and Greenwood (1963). If anthophyllite
exists stably in this system at 670"C, its compatibility relations with
other phasesin the system may be as indicated by the dashedlines in
Figure 2.
At 550oC, Mg+GeO6.Mg(OH), is stable. Similar compatibility relations are present, with the addition of a tie line to brucite, which exists
stably at this pressureand temperature. Roy and Roy (1954) indicate
coexistence of spinel and brucite, whereas in this study, crystalline
forsterite and brucite held at 600oCat 15,000psi, and 610oC at 19,000
psi, react to form spinel, Mg+GeO6.Mg(OH), and HzO. No tie line is
present between MgzGeOrand brucite.
The talc compositionat 570oCproducestalc, MgGeO3,and tetragonal
GeOzfrom mixtures of Mg(OH)r, GeO2and MgO, or mixtures utilizing
Mg(OH)r, GeOzand MgGeO3as reactants.Talc is indicated as stable in
study of Roy and Roy.
It appears that at and below 570oC, MgGeO3 may not be stable.
STEPHEN R, LYON AND ERNEST G. EHLERS
124
Mixtures near the composition MgGeOa crystallize other phase assemblagesin preferenceto MgGeOr; MgGeOa is not indicated on the diagram of Roy and Roy (1954) as their runs were all made with excess
water, and the possible stable area of MgGeO3 was covered by more
hydrated phasesup to at least 600oC.As MgGeOahas not been demonstrated to decomposeat these temperatures and pressures,the inferred
lack of stability of MgGeO3is necessarilyconjectural.
Also found at and below 570oC is serpentine.Roy and Roy (1954)
state a dissociationtemperature of 520*15oC for the Ge serpentine.As
the temperaturesin this system are read and calibrated to * 15oC or
better, no comparisonwith their data is possible.It should be noted that
neither formation nor non-formation of a phasefrom a batch of unstable
reactants proves equilibrium. The dissociation temperature of the serpentine should be further examined.
Examination of the system at 47OoCyielded the same compatibility
relations as observedat 570'C. No additional phaseswere observed.
CoNcrusrolrs
The system MgO-GeOz-H2Ois analogousto the system MgO-SiOzHrO in having structures analogousto serpentine, talc, enstatite, olivine'
stishovite and quartz. In addition, both systems do not contain any of
the humite-type minerals; apparently some MgF2 is necessaryin place
of Mg(OH), for successfulsynthesis.Anthophyllite is difficult to synthesizein both systems.The phasesMgaGeQeand MgaGeOo'Mg(OH)zhave
no known silicate analogs.
Acnrowr,nocwNt
The authors gratefully acknowledge the financial support of the National
Foundation under contract GP-3975.
Science
RrnBruNcns
Bowpn, N. L., aNo O. AwnansoN (1914) An investigation of the system MgO-SiOz.
Amer. f . Sei., Su. 4, 37, 488-500.
ewo O. F. Turrr-n (1949) The system MgO-SiOrHzO' Geol. Soc. Amer. Bull.6O,
439460.
DAcntr-m, F., Awo R. Rov (1960) High pressure studies of the system Mg2GeOrMg2SiO4
with special reference to the olivine-spinel transition. Amer. I- Sci' 258,225-246'
Fvrr, W. S. (1960) Hydrothermal synthesis and determination of equilibrium between
minerals in the subliquidus region. L Geol.68' 553-566.
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GnenNwooo, J. J. (1963) The synthesis and stability of anthophyllite.
317-351.
I. Petrology 4,
SYSTEM MgO-G0rHzO
125
Konr-umls, H., aNo C. M. C. VnnsecrN (1959)The fluorescenceof binary and ternary
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-,
aNo E. F. OssorN (1953)The system MgO-AlzOr-I[:O and influenceof
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Manuscribt recei.t:eil,May 20, 1969; accepted,
lor ?ublicat'i.on,October4, 1969.