Donpeacorite, (MnrMg)MgSi2O61 r n€\il orthopyroxene and its

Donpeacorite, (MnrMg)MgSi2O61 r n€\il orthopyroxene and its

American Mineralogist, Volume 69, pages 4724E0, 194
Donpeacorite,(MnrMg)MgSi2O61r n€\il orthopyroxeneand its proposedphase
relations in the systemMnSiO3r-MgSiO3-FeSiO3'
Enrcn U. PBrr,nseN.2LewneNce M. ANovrrz, nNo Enrc J. EsseNn
Department of Geological Sciences
University of Michigan, Ann Arbor, Michigan 48109
Abstract
occurs in a manganiferouspod in the
A Mn-rich orthopyroxene Mgl.alMns.5oCao.orSizOe,
marble units near Balmat, N. Y. The pyroxene coexists with triodite, tourmaline, ferrian
braunite, manganoandolomite, and hedyphane.Refinementof the crystal structure of the
pyroxene (spacegroup Pbca) showsthat as expectedvirtually all of the Mn is located in the
M2 site and the generalformula is therefore (Mn,Mg)MgSi2O6.BecauseMn is completely
ordered into the M2 site and comprises over 50 percent of that site, the mineral is a new
orthopyroxene. It has been named donpeacoriteafter Donald R. Peacor in recognition of
his work on pyroxenes and manganeseminerals. The unit cell parameters are a =
is biaxial
18.384(ll),b : 8.87E(7),
c : 5.226(3),2= 8,D (meas.): 3.36(l).Donpeacorite
negativewith 2V* : 88', Alc, a = 1.677(2),B : 1.684(2),y = 1.692(2).It is yellow-orange
with a vitreousluster and has an approximatehardnessof5-6 with perfect(ll0) cleavages.
The occurrence of donpeacorite and published data for metamorphic pyroxenes and
pyroxenoids permit modeling of a phasediagramfor the system MnSiOrMgSiO3-FeSiO3.
The diagram is dominated by extensive fields for solid solutions of orthopyroxene and of
pyroxmangite, with a small field inferred for kanoite (MnMgSizOo,C2lc clinopyroxene)and
a three-phasefield for ferroan kanoite, manganoanhypersthene,and magnesianpyroxmangite.
Introduction
SeveralMn-bearing minerals have been describedfrom
the manganoan pods in the Balmat, New York area
including tirodite (Rosset al.,1969), magnesianrhodonite
(Peacoret al., 1978),calcian kanoite, braunite,and hollandite(Brown et al., 1979;Gordon et al., l98l). Typically these minerals at Balmat form pink- to buff-colored
rocks which contrast strikingly with the enclosing rocks.
They may fluorescea deep red when exposedto ultraviolet light and thus have received attention well beyond
their abundance. Spectacular yellow-orange rocks containing an unusual Mn-rich pyroxene were collected on
the 2500level of the Balmat No. 4 Mine and provided for
our study by John T. Johnson and William delorraine of
the St. Joe Zinc Company,Balmat, N. Y. X-ray difraction studies and a structure refinement of the pyroxene
indicate that it is orthorhombic and completely ordered
with over half of the M2 site occupied by Mn. This
orthopyroxene therefore qualifies as a new mineral and
has been named donpeacoriteafter Dr. Donald R. Peacor
t Contribution No. 3E9from the Mineralogical Laboratory of
the University of Michigan, Ann Arbor, MI 48109.
2 Present address: Department of Geology and Geophysics,
The University of Utah, Salr Lake City, Utah 84112.
0003--0Mx84/0506-0472$02.
00
of the University of Michigan Department of Geological
Sciences, in recognition of his work with manganese
minerals,as well as with pyroxenesand pyroxenoids. The
name donpeacoriteapplies to the ordered orthopyroxene
of endmembercomposition MnMgSi2O6.Thus the mineral describedherein is DpseEnar.Both the mineral and the
name donpeacoritehas been approved by the I.M.A.
Commission on New Minerals and Mineral Names (A.
Kato, written connunication, 1982). Type material is
preserved at the Smithsonian Institution, Washington
D. C.
Mn-rich pyroxenes and pyroxenoids have recently received much attention (Peterset aL, 1977;Peacoret al',
1978;Albrecht, 1980;Albrecht and Peters, 19E0;Brown
et al., 1980:Brown and Huebner, l98l; Gordon et al.,
l98l). Brown et al. (1980)assembledthe then available
compositional data and inferred phaseequilibria at metamorphic temperatures corresponding to the amphibolite
facies for three faces of the RSiO3tetrahedron (R = CaMg-Fe-Mn). With the recent discovery of the unusually
Mn-rich orthopyroxene donpeacorite at Balmat, N. Y. it
is now possible to infer the general topology of a phase
diagram for the remaining Mn-Mg-Fe face of the RSiO3
tetrahedron. In this paper we describe this mineral and
evaluate the pertinent metamorphic phase equilibria.
472
473
PETERSEN ET AL.: DONPEACORITE
Occurrence and associations
The manganoanorthopyroxene described in this paper
occurs in manganese-richsiliceous marbles. Manganeserich pods are scattered within several of the mappable
siliceousmarble units at Balmat, N. Y. (Brown et al.,
1980).These marbles, which are common throughout the
Adirondack Lowlands (Engel and Engel, 1953) were
metamorphosedto the upper amphibolite facies (Buddington, 1939)during the 1.0b.y. old Grenville orogeny.
Recent estimates of peak metamorphic conditions at
Balmat yield 6.5-rl kbar and 650-+30'C(Brown et al.,
1978;Bohlen er al., 1980).
Donpeacorite occurs as l-3 mm interlocking grains
making up over 50 percent (modal abundance) of a
massivecoarse-grainedyellow-orange rock. The groundmass consists mainly of the fibrous Mn-Mg amphibole
tirodite (40 percent modal abundance),with minor tourmaline, ferrian braunite, manganoan dolomite, hedyphaneJike apatite and anhydrite. The tirodite, which is
closely associatedwith the orthopyroxene, is faint orange
in hand specimen and colorless in thin-section. The
subhedral orthopyroxene is pale buff in hand specimen
and faintly pink in thin section. It shows no evidence of
twinning or exsolution. Refractive indices were determined in oil on a crystal grain mount (Table l). The 2V
was measured on a universal stage using hemispheres
with refractive index equal to 1.65 and making no tilt
corr6ctions (Table 1).
Chemistry
The pyroxene, amphibole, tourmaline, braunite and
hedyphanewere analyzed using the University of Michigan ARL-EMXelectron microprobe utilizing wavelength
dispersive techiques. The operating conditions were 15
kV, 150 plA emission current and 0.015 pA sample
current. The standardsused were Marjalahti olivine for Si
Tablel. Opticalandunit cell parameters
ofdonpeacorite
DonDeacorite
B
Y
Y-q
2Vr(neas.)
a(8)
b(ff)
ctff)
v(R3)
D(calc)
space group
*All
**All
optical Propert'les
r.677(2)
1 . 6 8 42()
1.692(2)
0.015
8 8 (s )
Unlt Cell Paramters
r 8 . 3 8 41(l )
8 . 8 7 97( )
5.226(3')
8 5 3 . 1(27 0 )
3.403(2)
Pbca
Enstati te*
Table 2. Electron microprobe analysis of donpeacorite and
tirodite
Fonnulr
orld. t|llcht P$c.nt
orthoPJ'roranaSr
Sl02
AlZ03
l,ln0
il90
Fe04
C!0
ilrA0
Totrl
Sl0Z
AlA03
iln0
il90
Fe04
Ca0
lla20
Total
Sl02
Tl02
41203
reoa
tl90
tiln0
Cr0
ilazo
Kzo
55.12
0.23
18.f8
26.31
0.14
0.69
0.03
100.00
54.64
0.16
20,22
24,12
nd
0.82
nd
99.96
st
AI
l,ln
il9
Fe
Cr
l{a
0
riln
st
AI
iln
l,tg
1.983
0.010
0.563
1.411
0.004
0.027
0.003
5.989
0.28
2.009
0.007
0.630
t.322
F.
Ca
lla
0
xiln3
Tlrodlte2
55.20
sr
nd
TI
L.47
A ll v
Alvl
0.30
Fe2+
22.72
Irtg
9.47
tiln
4.77
Ca
1.18
l{a
nd
K
Ba
nd
L.26
OH
0.69
F
0.01
ct
-0. ?9
0
0.032
6.013
0.32
7.9{8
0.052
0.197
0.036
4.876
l.155
0.736
0,330
8a0
1.210
H2Q5
0.314
F
0.003
cl
22.479
0.F' Cl
t6.?8
Total
nd, not detected: lllorilrllzed to 4 catlons;
2ronnallzedas x+Y+2.15
rlth all l{a ln A sltei
3xm+tn/(ttn++tg);4All Fe assunedas Fe2+;
scalculated.
1.649
1.653
1.657
0.008
r25
18.214(4)n
8 . 8 1 8 (2 )
5 . t t 1( ? \
831.483
3.21
Pbca
optlcal drta fron Deer, Howlernd Zussman(1978).
unlt cell paramters from Hawthorneand lto (1977)
and Mg, Ingamells almandinefor Al, synthetic rhodonite
for Mn, ANU wollastonite for Ca, Irving kaersutitefor Fe
and plagioclase (Anaj for Na. Corrections for ZAF
effects were made using the program EMrADRvII (Rucklidge and Gasparrini, 1969).Two chemical analysesare
given in Table 2. The orthopyroxene is rich in manganese,
sample I having composition Mg1.a1Mnn.56Cao.orAlo.or
Si1.e6O5.ee
and sample 2 Mg1.32Mns.63Caa.s3Alo.e1Si2.s
O6.s1(Fig. 1). The tirodite has significant Ca replacing Mn
like those describedby Ross et al. (1969).The tourmaline
is intermediate between uvite and dravite (Uva6Dr5a)and
is much like the specimenwhose structure was refined by
Buerger et al. (1962).Semiquantitativeanalysesof braun-
PETERSENET AL.: DONPEACORITE
474
MgMnSirO6
FeMnSi205
Table 3. Principal diffraction lines for donpeacorite
Fe2Si205
lMg2Si2 06
Fig. l. Orthopyroxenequadrilateralillustratingthe donpeacoritecompositional
field andplot of donpeacorite
analyses.
ite and hedyphane show that the braunite is ferrian and
the hedyphaneis phosphorian.
X-ray crystallography
Donpeacorite and tirodite crystals were studied using
single-crystalX-ray techniques. Weissenbergand precission photographs indicated that donpeacorite is orthorhombic. The pattern of extinctions is consistent with
space group Pbca, the most commonly reported space
group for orthopyroxenes (Cameron and Papike, 1980,
l98l), and thus it is isostructural with enstatite and
hypersthene. Lattice parameters for donpeacorite, refined by least-squaresutilizing data from a lolmin. powder diffractometer pattern internally standardized with
quartz, are given in Table 1 and the diffraction data in
Table 3. Some of the tirodite displays a faint, light-blue
schiller of the kind commonly causedby submicroscopic
exsolution lamellae. Exsolution relations have been described in the manganoanamphiboles from the Balmat
areaby Rosset al. (1969).The presenceof submicroscopic exsolution lamellae in our samples was confirmed by
Weissenbergphotographs which showed a predominant
phase having space group P2lm intergrown with a much
less abundant amphibole, presumably with space group
Alm.
Since the standard chemical analysis does not permit
an unambiguous structural formula to be calculated, a
complete pyroxene structure refinement was undertaken
to determine if Mn is ordered into the M2 site. Intensity
data were obtained from a cleavagefragment measuring
approximately 0.08 x 0.09 x 0.19 mm mounted for
rotation about the c-axis on a Weissenberg-geometry
difiractometer. A Super-Paceautomated diffractometer
system was used with MoKa radiation monochromated
by a flat graphite crystal and measuredwith a scintillation
counter. A scan was made across each reflection and
backgroundwere measuredon each side. The intensities
of 1096 reflections were measured, of which 295 had
intensitiesbelow the minimum observablevalues, up to a
sine theta limit of 0.461, for /r = 0-24, k : 0-12 and / = 05. All intensities were corrected for Lorenz-polarization
and absorption efects (p(Moka) : 28.810 cm-r). A
modified version of the program ABSRrwritten by C. W.
Burnham was used for the absorption corrections.
The structure refinement was initiated using parameters for the structure of orthoenstatite eiven bv Haw-
lllo
d ( o b s .)
10
100
l0
10
4.03
3.24
3.18
3.09
2.96L
4.01,4.05
3,22
3.19
3.060
2.962
DU
9
9
9
4
2.896
2.847
2.724
2.551
2.531
2.896
2.848
2.725
2.551
2,533
10
8
J
2
5
5
9
6
11
1l
d ( c a l c .)
hkl
220, 2ll*
411
420, 221
500'
3?t*
510*
511*
42L*
131*
6ll*
2.510
2.516
202*
2 . 4 8 7 , 2 . 4 9 0 2.488,2.490,2.493 Ltz, 23t, szl
2.404
2.406
302
2.022
2,025
422
2.04L
2,040,2.041
141,820
2.071
7.9947
1.7420
1.4950
1.4794
1.4019
2.073,2.076
sr?, 721
C u K ar a d i a t j o n ; * P e a k su s e d i n r e f i n e m n t .
thorne and lto (1977).The program RFrNE2 (Finger and
Prince, 1975)was employed using the scatteringfactors of
Doyle and Turner (1968),and the weighting schemeof
Cruickshank(1965).During the early stagesofthe refinement the scalefactor and atom coordinates were refined,
while the occupanciesof Ml and M2 were held constant
with 100%Mg. In later cycles, refinement of occupancy
factors for Ml and M2 was alternated with cycles in
which coordinatesand/or anisotropic temperaturefactors
were refined. The final unweighted R-value was 5.6
percent for all reflections, and 5.0 percent for all unrejected reflections. Reflections with intensities below the
minimum observablevalue, and those with large individual R-values(R > 0.5) were rejected.Final atom positions
and temperature factors are listed in Table 4; selected
interatomic distancesand anglesare listed in Table 5; and
magnitudesand orientations of the thermal ellipsoids are
given in Table 6. The calculated and observed structure
factors are listed in Table 7.3
The final refinement showed an Ml site occupancy of
1.02(l) Mg (and -0.02(l) Mn) and an M2 site occupancy
of 0.47(l) Mg and 0.53(l) Mn. Thus the manganese
appearsto be completely ordered into the M2 site within
the sensitivity of the method. Comparisonof the suggested composition with that obtained by electron microprobe analysis (first analysis in Table 2) shows that this
result is in very good agreement.Analysis 2, which
contains slightly more Mn, was obtained on another
samplefrom the same locality. Small amounts of Ca and
Fe in the structure are approximately accounted for by
the Mn form factor and Al is accounted for by the Mg
form factor. The differencesfrom the actual analysesare
3 To receivea copy of Table7 order DocumentAM-84-241
from the businessoffice,Mineralogical
Societyof America,200
Florida Avenue,NW, WashingtonD. C. 20009.Pleaseremit
$1.00in advancefor the microfiche.
475
PETERSENET AL.: DONPEACORITE
Table 4. Final atom positions and anisotropic temperaturefactors (x l0-5)
Bzz
Btt
Bgs
\z
Bta
Bzs
l(s)
-11(4)
-7(4)
-10(4)
-10(10)
-rs(7)
-16(e)
-13(e)
3 (1 3 )
10(l0)
4(14)
20(20)
l0( 20)
40(2)
-30(20)
-30(10)
30(20)
20(20)
-80(40)
-10(50)
-30(s0)
M1
n2
siA
siB
0 . 3 7 s 11(
0 . 3 7 7 61(
0.27L?(L
0.4749(
1
( 2)
0.6543
( 1)
0.4801
0 . 3 4 1 31()
0 . 3 3 8 21()
0.872r(3)
0.3684(
2)
0.0484(
3)
0.7e4e(
3)
sl(5)
66(3)
s6(4)
4 5 3( )
2s0(20)
340(10)
220(l0)
220(r0)
550(70)
630(50)
520(60)
600(60)
0rA
02A
034
0.1833(2)
( 2)
0.309e
0 . 3 0 1 72()
0.3379(3)
0 .s 0 2 2 ( 3 )
0.22s2(4)
0.0407(
7)
( 6)
0.0467
0.822?(7)
68(e)
48(8)
72(8)
300(40)
280(30)
4r0(40)
610(r40)
870(140)
7r0(140)
01B
028
038
0. s632(
2)
0.4349(
2)
o.447s(z',)
( 3)
0.3402
0.4872(4)
( 4)
0.2050
0 . 7 9 8(r7 )
0.7028(7')
0.s902(7)
s8(8)
77(sl
64(8)
360(40)
330(40)
430(40)
4r0(140)
1190(
rs0)
820(
r30)
probably somewhat greater than the occupancy factors
suggest,but therefore well within the reasonablerangeof
compositional variation in this sample.
Comparison of the detailed structure data obtained for
donpeacorite with that for other (Mn,Mg) pyroxenes
Table5. Selectedinteratomicdistancesandanqles
siA-01A
siA-02A
siA-03A
si A-034'
(siA-o)
1.516(4)
1 . 5 0 0 3( )
l . 6 4 2( 4 )
1.5se4
()
L.629
srB-olB
siB-028
srB-038
stB-038'
(sia-o)
t.624(4)
1.s86(4)
1.670(4)
1.671(4)
r.638'
r,t1-0lA
r,r1-0lA'
titl-02A
]it1-0lB
itl-0lB'
!,t1-028
(lr-o)
2.036(
4)
2.r41(4)
2.202(4)
2.068(
4)
2.L87(41
2.049(
4)
2.114
it2-0lA
j42-02A
r42-034
itz-03A,
it2-0lB
]42-028
ti'|2-038
r,r2-038,
2 . 1 5 63()
2.r04(4)
2.33714)
3.s42( 3)
2 .l l e (3 )
2.044(4)
2.540(
4)
2.se4(4)
2,278
0lB-028
0lB-038
0lB-038,
028-038
028-038,
038-038,
2.740(5)
0lB-si B-028
0lB-si B-038
0 l B - s i8 - 0 3 8 ,
028-StB-038
028-St8-038'
038-Si8-038,
r17.2(2)
1 0 7 .(32 )
108.2(
2)
(2)
10e.3
r04.s(2)
1 0 9 . 71()
109.4
01A-0lA
0lA-02A
01A-0lB
01A-028
018-0lA
018-02A
018-18
018-028
01A-02A
02A-025
02B-0lB
018-01A
(3)
3.041
2.e78(5)
2.828(
6)
(5)
2.806
2.839(5
)
2 . 8 4(1s )
3.061(3)
3.030(
5)
( s)
3.001
(
2.e21s)
3 . 2 3 2s()
2.828(6)
2.95L
01A-Ml-01A
01A-Ml-02A
01A-Ml-018
01A-Ml-028
018-M1-0lA
018-M1-02A
018-Ml-018
018-Ml-028
01A-il1-02A
02A-l,ll-028
028-M1-018
018-r.,rl0lA
e 3 . 4 (l )
e l . 3 (l )
8 4 . 8 (I )
176.9(2)
8 4 . 4 (l )
172.e(r)
e 2 . 0 (1)
(o-n-o)
e s . 32()
e 5 . 4l()
e l . 8 (2)
94.8(2)
8 1 . 62()
104.6
01A-018
01A-028
01A-02A
01A-03A
018-028
0lB-02A
018-038
028-03A
028-038,
02A-03A
02A-038
02A-038
03A-038,
03A-038
03A-038,
2 . 8 2 8 6(
2 . 8 0 65
(
2.952(5
3 . 6 7 05(
3 . 0 3 5s(
(o-o)
2.733(3
2.997
01A-fil2-018
01A-r.,r2-028
01A-M2-02A
01A-M2-03A
018-M2-028
0lB-M2-02A
0lB-it2-038'
028-M2-03A
028-til2-038'
028-t42-038
02A-M2-03A
02A-142-038'
02A-r,r2-038
02A-li|2-038'
03A-lil2-038
8 2 . 61()
83.s(1)
87.s(1)
roe.1(1)
s 3 . 61()
8 4 . 51()
123.7(I )
r 1 4 . 9I()
58.1(1)
r04.4(l )
5 8 . 8r()
12e.
e(r )
84.s(l)
69.4(l)
7 4 . 3l()
03A-03A-03A
r 6 3 . 81()
(r'rz-o)
(o-o)
(o-o)
2 . 6 s 4s()
2.670(
s)
2.6s7(s)
2.s83(s)
2 . 7 3(35 )
2.673
2.841(s
3.032(s
3 . 6 9 7s(
2 . 5 8 35(
2.52s(s
3.084(
s
3.r38(5
3.084(
5
2.9s0(
s
(o-siB-o)
(o-r'rz-o)
038-038-038
01 2
1 4 5 . 0 3( )
s(12) -20(20't
3(4s)
ls0(50)
20(10) lo(30)
-20(10) -7(24') -2r0(60)
(Hawthorneand Ito, 1977;Gordonet al.,l98l) allows the
effects of this ordering to be observed. The (Ml-O) bond
length of orthoenstatite is 2.076A, while that for donpeacorite is 2.0844. If Mn is indeed completely ordered into
M2 (and therefore Mg in Ml), the (Ml-O) distancesof
donpeacorite and orthoenstatite should be quite similar,
as indeedthey are. As severalauthors (Hawthorn and lto,
1977; Cameron and Papike, 1980) have noted that the
(Ml-O) bond lengXhis strongly a function of the cation
radius, this comparison suggestsstrongly that Mg is the
dominant cation in the Ml site. The slightly larger (Ml-O)
distance may, however, indicate that some Mn or Fe is
presentin this site.
Comparisonof the (M2-O) distancesshows significant
differences between donpeacorite and other pyroxenes.
Table6. Magnitudeandorientationof the principalaxesof the
thermalelipsoids
Angle to:
RilS
Dlsplacilent
siA
stB
A axls
B axls
C axls
0.084(6)
0.0e3(4)
0.0ee(4)
76120)
162(221
7e(22,
7r(r2l
7s(24)
2s(1e)
24(14)
81(m)
u1(13)
0.089(3)
0.107(2)
0,117(2)
72(61
154(7)
108(8)
7714)
104(8)
19(7)
2r(6)
69(6)
e6(4)
0.082(4)
0.093(3)
0 . 1 0 23( )
7e(9)
L22lr4l
1o7lr2l
146(13)
117(14)
108(8)
0.085(
3)
0.090(3)
0.099(3)
32(15)
101(27)
60(10)
74(r7l
12e(r6)
136(16)
63(24)
42(19)
lr8(r5)
o.oe8(lo)
0.r08(7)
0.113(7)
103(18)
163(3e)
7e(s8)
6e(14)
84(57)
22(2ol
25(17')
106(25)
ro9(22)
0.086(8)
0.107(7)
0.111(8)
2r(14)
L07l22l
7e(271
r1l(15)
146(60)
65(70)
e2(15)
62(75)
28176)
0.100(8)
0.116(7)
0.129(6)
51122)
l3e(23)
78(l7l
86(12)
72(2rl
le(2r)
3e(2r)
54122)
104(16)
34(14)
2r(8)
ee(13)
I
2
3
0.074(12) 72114)
0.103(7) 162(ls)
86(16)
0.120(7)
89(7)
85(r6)
5(r6)
18(14)
t
2
3
0.104(7)
0.116(7)
0.141(7)
lr3(2s)
r54(e3)
77(Lrl
32(r4l
ro5(24)
6 2 (e )
r12(13)
68(14)
I
2
0.09s(8)
0.10s(7)
0.140(6)
68(34)
157(33)
e5(8)
?
60(8)
83(19)
31(7)
72(r4)
92(8)
3r(rr)
3e(221
68(2e)
r20(7)
476
PETERSEN ET AL.: DONPEACORITE
In order to limit problems due to variable M2 coordination, only average values for the six closest oxygens
(OlA, O2A, O3A, OlB, O2B, O3B) will be compared.
The (M2-O) distancefor orthoenstatiteis 2.1494, thar for
donpeacorite is 2.2lEA, and kanoite and manganoan
diopside yield 2.2994 and 2.495A, respectively. The
value for kanoite has been re-averaged from that in
Gordon et al. (1981)to account for only the six closest
oxygens. As these last two pyroxenes differ structurally
from donpeacorite (space groups P2lc and CZc, respectively), they are not strictly comparable,but should show
a similar trend. Comparison of the (M2-O) bond length of
orthoenstatite with that for clinoenstatite (2.1424, space
group P21lc, Ohashi and Finger, 1976) suggests that
kanoite is a reasonable analog for a more manganoan
donpeacoritein this respect. The occupancy reported for
the M2 site in a sample of clinopyroxene with exsolved
kanoite by Gordon et al. (1981)is 0.86 Mn, 0.14 Mg.
Extrapolating linearly from orthoenstatite and this kanoite to a stoichiometric-(1:l) Mn-Mg end-memberyields
(M2-O) equal to 2.3234. For the structural effects, we
predict a (M2-O) value for donpeacoriteof 2.239A, which
is only slightly larger than that actually observed. If the
kanoite value is corrected downward approximately
0.014 to correct for the structural differences, the predicted (M2-O) for donpeacoriteis 2.n44, which is still
larger than the measuredvalue. Hawthorne and lto (1977)
show that the relationship between (M2-O) and composition may not be exactly linear, and suggesta conection
basedon the degree of bond-length distortion A:
l6
a:r)
t(n,-R)1R1,
6 ,:,
where R is the mean bond length and R; is the individual
bond length. The value calculatedfor donpeacoriteis 5.86
x l0-3 which would-give an approximate correction
(their Fig. 5) of -0.08A. Using the above approximation
corrected for spacegroup structural effects, we predict a
(M2-O) bond length ot 2.2264 for donpeacorite, within
error of the measuredvalue.
A commonly discussedfeature of the pyroxene structure (Papikeetal.,1973; Hawthorn and lto, 1977;Cameron and Papike,1980,t98l; Gordonet al., 19El)is the 03O3-O3 angle, which describes the degree of rotation of
the tetrahedralchains from the ideal 180'conformation of
a straight chain. As the two tetrahedral chains in Pbca
pyroxenes are not symmetrically related, there are two
such angles in donpeacorite, 163.8ofor the A-chain and
146,0'for the B-chain.Comparisonwith other pyroxenes
(Cameron and Papike, l9E0) shows that the A-chain
rotation falls near the averagerotation reported, but the
B-chain angle is at the extreme upper end of the reported
range. The averagecation radius in the Ml and M2 sites
(Shannon and Prewitt, 1969) is approximatelv 0.75A.
Comparing this with the plot of average cation ionic
radius versus chain angle given by Cameron and Papike
(19E0,Fig. 28) the A-chain angle agreeswell while the Bchain is nearly 2o higher than the trend shown by other
orthopyroxenes.The scatter in the data suggeststhat 03O3-O3 angles are not a simple function of cation ionic
radius.
A similar approach cannot be used to study the relationship between cation ordering and the O3-O3-O3
angles. These angles are quite different in ortho- and
clinoenstatite and thus kanoite cannot be used to predict
the values for donpeacorite. In addition, as the tetrahedral chainscoordinateboth octahedralsites, their orientations should reflect an average cation property rather
than properties of the individual octahedra.
The (Mg,Fe) orthopyroxenesmay be used to model the
effect of cation ordering on the O3-O3-O3 angle. Burns
(1970)has noted that Fe should order into the M2 site in
orthopyroxene due to Jahn-Teller efects. The O3-O303 angle for the A and B chains may be plotted against
Fe/(Fe + Mg) or averageoctahedral cation radius for the
seriesorthoenstatite-orthoferrosilite(Fie. 2). The A-chain
angle increasescontinuously over this range, but the Bchain angle reaches a peak at about 50VoFe, and then
levels out and may even decreasetowards orthoferrosilite. This suggeststhat the B-chain angle is more sensitive
to the cation in M2 than the cation in Ml, and will
therefore reflect ordering into the M2 site.
The O3-O3-O3 angles of donpeacorite and a manganoan orthoenstatite synthesized by Hawthorne and Ito
(1977) are shown as a function of average octahedral
cation radius in Figure 2. While there are insufficient data
to be sure whether the (Mn,Mg) orthopyroxenes follow a
trend similar to that shown by the (Mg,Fe) orthopyroxenes,the trend suggestedby the three availablepoints on
this join at least suggeststhat more than average cation
radius affects the O3-O3-O3 angles.
Discussion
Donpeacorite at Balmat occurs in the amphibolite
facies, well down-gradefrom the orthopyroxene isograd
Mcon
0ctohadrol
Colion
Rolio
o72
oto
o.6e
o66
074
t7o r
o
165
a
n
o
rov
I r+s
I
n
B- Choin
t40
t35
oo
o2
o6
o4
o8
to
T#fr.
Fig. 2. O3-O3-O3 angle for (Fe,Mg) orthopyroxenes.
Sourcesof data: solid circles-Burnham et al. (1971);Dodd et al.
(1975);Ghose(1965);Ghoseand Wan (1973);Kosoi et al. (1974);
Miyamoto et al. (1975); Smyth (1973); Sueno et al. (1976);
Takeda (1972); Takeda and Ridley (1972). Open squareHawthorne and Ito (1977).Open circle-This paper.
477
PETERSEN ET AL.: DONPEACORITE
---:l:---fa\
--_"€;r___
\i\
\\ \
-mss
\
\'r
|
\ \'\r'
', \. .
|
\
,, \,| r \ . \ \ 'r, ....
r'\, \t\
t.
',. tt
psss \
\\ \\
\
oens
- \\
\
\\
\
\\
\
\ \
\
rzTr
'
'....
tt.
rr ,
t{n)r
t..ta
'.\..
!tza!\.
'tr.t..
'\'\
\\ .
\.
L--------\
i
!\,
\)..i,",i\
\
\
Rhs"
\
r..x.).
t..
\
Itr. ta
\
\\
''",.
r
\\
\\
\'"ll
.\
r
',
Xunsioo
Fig. 3. Two possible configurations of phase equilibria in the system MnSiO3-MgSiO3 consistent with lto's (1972) hightemperature phase equilibria and with our observations at metamorphic temperatures. Fig. 3a assumes that lto's manganoan
clinopyroxene shows complete solid solution to kanoite. Fig. 3b is drawn assumingthat Ito's clinopyroxene disproportionatesto
orthopyroxene and kanoite similar to pigeonite equilibria in the system CaSiQ-MgSiO3-FeSi03. The effect of Mn on the
orthopyroxene polymorphs is hypothetical but is chosen to avoid interfering with the data of Ito and ours at metamorphic
temperatures.
that marks the transition from the amphibolite facies to
the granulite facies (Engel and Engel, 1955).The presence
of donpeacorite in a marble, however, should not be
expectedto correspondto the appearanceof hypersthene
in a metabasite;the substitution of manganesein enstatite
or dilution of the fluid phase by CO2 in the marble may
well have locally extendedthe stability of the orthopyroxene * fluid assemblageat Balmat. Even there the occurrences of manganoanpyroxenes are extremely rare and
localities for tirodite (and "hexagonite") are widespread,
suggestingthat only locally were bulk rock and fluid
compositions compatible with the generation of these
pyroxenes.
The distribution of MnlN4g between pyroxene and
amphibole is of interest for reactions involving these
phases. Ignoring for the present intracrystalline cation
distributions, one may calculate the butk Kp:
KoGulk) =
(Mn/Mg)opx
0.630/1.320
= 2.015.
1.15514.876
Mr). Thus one would predict a Ki of 2.5 for a coexisting
tirodite.
MnMgSi2O6pyroxene and Mn2Mg5SiEO22(OH)2
At Balmat, Ca is also involved in these minerals' and
while the manganoanorthopyroxene takes little Ca, the
tirodite contains some 37%oCa in M4 (Table l). Thus Ca
displaces Mn in the M4 site of amphiboles while hardly
afecting the orthopyroxene and this increases the bulk
Ke in Mn:Mg. It therefore appears that the Kp of (Mn/
Mg) for pyroxene compared to amphibole is strongly
dependent on the availability of calcium for the amphibole.
The distribution coefficient of (Mn/I\dg)betweenthe M2
site of pyroxene and the M4 site of amphibole is also
instructive. Assuming all Mn in M4 of amphibole and M2
of pyroxene, as shown by the structural refinement,
(Mr/Me)S3. 0.63t0.32=
K^(lareesite): +=
(Mr/Mg)f;fr6
0.58i0.054
0.lE
This calculation suggeststhat the M4 site of amphibole
prefers Mn over Mg compared to M2 of pyroxene but the
The partitioning of Mn vs. Mg between orthopyroxene significant Ca in the amphibole will probably also affect
and amphibole may be rationalized in terms of their this Kp (large site).
crystal structures. In the MrrMg system, Mn clearly
No experimentaldata are yet available on the MgSiO3prefers the larger M2 site in pyroxene and the M4 site in
MnSiO3 binary at metamorphic temperatures, but Ito
amphibole, while Mg is partitioned into Ml in pyroxene (1972)has reported on experiments at 13fi)-1500'C. His
and Ml, M2, and M3 in amphibole.Thus the approximate experiments show successive fields of orthopyroxene,
upper limit of Mn:Mg expected in pyroxene is 1:l =
clinopyroxene, pyroxmangite and rhodonite from MgSiO3
M2:Ml and in amphiboleit is 2:5 = 2M+:(2Mr + 2M2 +
to MnSiOr, but the positionsof gapsbetweenthesefields
(Mn/Mg)amph
478
PETERSEN ET AL.: DONPEACORITE
present, and a three-phase field orthopyroxeness-pyroxmangitess-kanoitess must be located somewhere inside
the diagram. Only two or three kanoite occurrences are
known, and kanoite-donpeacorite pairs have not yet been
TP
reported
but the manganese-rich pods of the Balmat area
Pyumongite ss
may yet yield this assemblage.
The discovery of donpeacorite at Balmat requires some
minor revisions of the Brown et al. (1980) phase diagram
for the subsystem MnSiO3-MgSiO3-CaSiO3 (their Figs. 2
Pyqfer@il,E
and 4) with respect to their inferred MnSiO3 solution of
orthopyroxene. The field for orthopyroxene along the
join should be extended towards the
MgSiOrMnSiO3
OrthopyrcxeE
ss
clinopyroxene
field
such that the separation between the
o
oo
two fields is reduced to less than 20 mole Vo. Untike the
3 o-9$
-q.&:o'8-$----s-:J
CaMgSi2O6-Mg2Si2O6 system, the MnMgSi2O6-Mg2Si2O6
oo.--"FeSiOs
Enslolite
binary must show extensive solution in the orthopyroxFig. 4. Compilationof publishedanalysesfor the MnSiO3- ene and Mn must be much more easily accommodated
FeSiO3-MgSiO3
face of the RSiO3 pyroxene-pyroxenoid than Ca in the M2 site of the orthopyroxene structure at
tetrahedron.
P : 4-8kbar,T = 500-700'C
for mostof thepoints. metamorphic temperatures.
Pyrcxrcngil,o
(Rhodoitel
Sources
of data:FordandBradley(1913);
Henry(1935);
Tilley
(1937);Heitanen(193E);Yosimura(1939);Kuno (1947);Lee
(1955);Howie (1964);Momoi (t96a); Ewart (1967);Mason
(1973);Peterset al. (1973);Marek er al. (1975);Kobayashi
(1977);
Krogh(1977);
Petersetal.(1977);
€hopin(197E);
Deeret
al. (197E);
Murthy (1978);Schreyeret aI. (197E);
Jolnso-nand
Knedler(i979);Pavelescu
(1980);$ivaprakash
and Pavelescu
(1980);Fukuoka(1981).The solid circlesare rhe analyses
reportedin this paper.At the MnSiO3apexhalf shadedsquares
(!).represent rhodoniteanalysesin which Xsusio.< 0.07;
opensquares(-) representpyroxmangite
analyses.Coexisting
mineralsare connectedby solid lines and those inferred as
coexistingare connected
by dashedlines.
Acknowledgments
We would like to thank the geologicalstaff of St. Joe Minerals
Corporation Balmat-EdwardsDivision for collecting and providing the samplesexamined in this study. Microprobe funds were
provided by the University of Michigan. Thanks are due to the
statr of the University of Michigan Microbeam Laboratory for
maintenanceof the electron microprobe facility. We gratefully
acknowledge the help of A. Kato in critically reviewing our
submissionof donpeacoriteas a new mineralto the I.M.A. S. J.
Huebner and J. M. Rice are thankedfor their thoughtful reviews.
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deposit of the Kasa mine, Japan. Journal of the Faculty of
Science, Hokkaido Imperial University, Series IV, Geology
and Mineralogy, 4:313452.
Manuscript received, March 9, I9E3;
acceptedfor publication, January 2, BA.