American Mineralogist,Volume 66, pages5g6_591,lggl
crystal structure of a silica- and alkari-rich trioctahedral mica
R. M. HnzEN, L. W. FTNGER
GeophysicalLaboratory,2S0I Upton Street, NW
lllashington, DC 20005
nNo D. Vnron
Laboratoire de Pitrographie, (Jniversit| pierre et Marie Curie
4 Place Jussieu,75230 paris Cedex 05, France
Abstract
The crystal structureof a trioctahedralmica of unusual compositionfrom melilite-bearing
eruptive rocks has been determined(R : 3.020).This mica, which has excesssilica (si > 3j
and alkalis O Na + K > l), is the first known layer silicatewith octahedralsodium.in spite
of the complexity of the octahedrallayer, which includes Mg, Fe, Li, ca, Mn, Na, Ti, and
vacancies,there is no evidencefor cation ordering betweenthe two distinct octahedralsites.
Both siteshave the samemean M-O distance(2.077A) and the same electron density. The
structural formula of the mica, basedon chemicalanalysisand the structure refinement,is:
(h
n.nBao orrNao o,.)*tt(Mg, nrrFeo o3,Mn6 6*Cao -rTio orrLro zzsNao r r0Eo 078)vI
(Si3 3r2AL o42Fe3:46)rvO roF2.
Introduction
Velde (1979) described a suite of trioctahedral
micas of various compositionsfrom melilite-bearing
eruptive rocks.Of specialinterestis a group of silicarich micas, charactenzedby high alkalis and ferric
iron and low aluminum, in addition to signifisanl
amountsof Li, Ba, and Ti. Thesemiias are unusual
becauseX(K + Na + Ba) >> 1.0,Si >> 3.0,and l(Si
+ Al) << 4.0. Substantialdeviations from the ideal
biotite, (K,Na) (Mg,Fe)rAlSi3o'o(oH),,thereforeob_
tain. The principal objective of this study is to document the site distributions of cationsin this mica.
Experimental
Euhedral single crystals of a silica-rich mica
(sample Y253, Gragnani, 1972)from the type coppaelite,at Cupaello,Italy, were selectedfor X-ray investigation.Thesecrystalsare unzonedand display a
slight reverse pleochroism, characteristic of micas
w3-004x / 8| /0506-0586$0.200
quantitative determination could not be made. No
OH- was detectedby infrared analysisand the mica
thus has F/(F + OH) = l. It is not known if there is a
significant oxy-biotite component. Ion microprobe
analysisindicated 0.75+0.25 wt.VoLi,O (tourmaline
standard),and boron in only trace amounts.
X-ray ditrraction photographs of a crystal plate
140 x 130 x 50 pm revealedsharp diffraction maxima with none of the smearingor streakingof spots
characteristic of deformed crystals or disordered
stacking arrangements.Unit-cell parametersof this
silica-rich mica (Table 2) are typical for a one-layer
trioctahedral mica with potassiumin the interlayer
position (Hazen and Wones, 1972).
All X-ray reflections in one octant of reciprocal
space(sin 0/^<0.9) werecollectedon an automated
four-circle diffractometerwith Nb-filtered MoKq radiation (Finger et al.,1973\. All reflectionswere corrected for specimenabsorption (p, : 21.0 cm-') as
well as Lorcntz and polafuation effects. A total of
625 nonequivalentdata wereobserved(I > 2o). Trial
refinementsin spacegroup A, in which adjacenttetrahedra are symmetrically independent, and A/m,
in which all tetrahedraare equivalent,yielded similll
results. The A/m refinement yielded the residual of
586
Table 2. Unit-c€ll dimensions
Table l. Compositionand calculatedstructural formula of silicarich trioctahedral mica
Per
No.
No.
No.
44.43
o.47
trace
r'2og
1 1 .3 8
lili\
Tetrahedron
,.*,
J
0.49
4. 0 0 0
(')
(')
(")
o
15.9e
+15. 31
5.3290(9)r
9 . 2 3 0 0( 1 5 )
r 0 . 2 1 9 (11 1 )
9 0 . 0 0( 1 )
99.98(1)
9 0 . 0 r( 1 )
49s.0(1)
, (A)
.b (E)
c (A)
Eleclrons*
Charges
A1203
B-O-
Value
Parameterx
Unit-Cell
Slre
No,
wE, 7
s102
587
TRIOCTAHEDRAL MICA
HAZEN ET AL.: STRUCTURE OF SILICA- AND ALKALI.RICH
I
v
v ii3t
22.11
HnO
0.09
Ca0
0.07
T1O2
L,57
of unit-ce77
Paxameters fton difftactom(i'e'
triclinic)'
etet data was unconstrained
esd's'
represent
figures
+Patenthesized
xRefinmqx
Oc!ahedra
2.922
"-2"'
Na
20
0.84
Table 3. Refincmentconditions
o.ort \
Bao
0'62
Kzo
10.18
F5
4.5
(.0.1)
Hzoll
Alka1i
ilii ]
of
site
19.6e
+7.O2
efecttons
per
sixe
=
L
+Based on
is
!A-25
ion
Fez'/Fetprobe
weight
ptesent
SFluorjte
efectroniicroprobe
llsasea
on
infrared
dnknown.
wixh
anaT1sis
(nutuet
of
pet
atoms
x
sixe
3.0
3.0
625
0.6(+1.2) x l0-'
Weighted R (%)'t
R (7.)+
No, of observations
Extinction
Parameter
7 922
axonlc
nutuer
) .
+@tahedral
VaIue
ParameEer
97 '50
Totals
*NunTEr
+5,68
0,75
is
anaTgsis.
spectra
(>32),
arcunts
Calcuiations
analgsis
bg
vafue
standard'
xoilrmfine
Petcenx.
in signiticant
B'
ate
based
based
on
on
cited
* w e i s h t e dR = [ r w ( l r o l - l r " l ) 2 / z , r ! ] r / 2
jan
and
enil netuer
iR= rllrol - lr"lllrlr"l'
veitle
greatest significance,however, and the centrosymmetric spacegroup, as observedin previous refinementsof one-layerbiotites,was thus selected.All observedreflectionswere included in the refinementof
atomic positions,anisotropicthermal parameters'unconstrainedoccupancyfactors for all metal sites and
the fluorine site, and an isotropic extinction parameter. Conditionsof refinementare recordedin Table 3,
refinedpositional and thermal parametersin Table 4,
and observed and calculated structure factors in
Table 5. Least-squaresweights were derived from
standard deviations based on counting statistics to
which an amount equivalent to 2Eoof F has been
added.
Resultsand discussion
Interatomic distancesand angles(Table 6) and the
orientationsand magnitudesof thermal vibration ellipsoids (Table 7) are similar to those of previously
reportedtrioctahedralmicasin the phlogopite-annite
series (Hazen and Burnham, 1973; Bohlen et al',
1980).The 5.7" tetrahedral rotation angle, which is
factors,and number of electrons
Table 4. Refined atomic coordinates,anisotropict€mperaturefactors,equivalentisotropic temperature
per metal site in silica-rich trioctahedral mica
B
Brr
Bzz
P1l
Btz
p13
FZI
Equlv.
-0.0000(1) 0.ooo7(1) -o.ooo1(1) 0.87(1)
0.s749(1) 0.1667(1) 0.225s(1) 0.0082(2) 0.0022(1) o.oo23(1)
r
0.71(4)
o
0 . 0 0 0 8( 2)
0
0 . 0 0 6 3 ( 5 ) 0 . 0 0 1 2 ( 2 ) 0 . 0 0 2 6( 2 )
r/2
L/2
O
Ml
0 . 7 8( 3 )
o
0.0007(2)
o
rl 2
0.0061(4)0.0019(1)o.oozs(1)
0 . 8 3 3 7( 1 )
o
vrz
2.40(3)
o
o . 0 0 1 5( 2 )
0
0 . 0 2 2 7( 5 ) 0 . 0 0 5 8 ( 2 ) o . o o 5 7( 1 )
K000
0
.
0
0
0
3
(
2
)
2.55(4)
-0.0036
(4) 0.0012(3)
0.8198(3)0.23s5(2) 0.t672(2) 0.0236(8)o.oo97(3) o.oo42(2)
01
2.61(6)
0
0.001r(5)
0
0 . 1 6 6 9 ( 2 ) 0 . 0 3 4 7 ( 1 30) . 0 0 6 6 ( 3 ) o . o o 4 1 ( 3 )
0
02
0.5264(6)
(
1
)
1
. 0 3( 3 )
0
.
0
0
0
1
(
2
)
o
.
o
o
o
o
0.0006(2)
0 . 6 2 9 8 ( 3 ) 0 . 1 6 7 0 ( 2 ) 0 . 3 9 0 1 ( 1 ) o . 0 0 9 6( s) o . 0 0 2 6( 2 ) o . o o 2 7 ( 1 )
03
1.19(s)
o
o . o o o 7( 3 )
o
F
0 . 3 9 9 2 ( 2 ) 0 . 0 1 1 3 ( 8 ) 0 . 0 0 3 s ( 2 ) o . o o 2 7( 2 )
0
0.1336(4)
*Irum-ber of
efectrans
is
tefined
for
each
cation
site
based
on
el-ectton
densitg.
No consttaints
are
used'
No. of
Electrons
15.0(2)
rr.7(2)
rr.7 (2)
19.3(2)
8'3(2)
HAZEN ET AL.: STRUCT,URE OF SILICA- AND ALKALI-RICH
TRI)1TAHEDRAL
MICA
Table 5. Observedand calculatedstructurefactors (x l0)
L
OBS
L
OBS
-60L
I
I98
2 756
3 300
4786L
5 155
6 432
't
253
I
r02
9 39?
l0
330
-1
I92
772
29s
I53
442
267
98
393
319
-40L
L
2
3
4
5
6
1
8
I0
L2
13
72't
94L
793
126
34I
995
79
151
353
83
309
-2
0
I
694
2 356
3 399
4 673
5 439
6 1015
7 1187
8 262
9 159
I0
629
II
IOO
L2 250
14 405
744
96I
820
L27
366
I0I8
'17
L52
361
t5
288
713
345
401
572
440
1015
1197
277
172
628
98
248
40r
|
8
I0
tI
12
13
14
400
294
1074
5I4
IIOI
310
)t)
555
643
396
500
244
3r7
t t7
663
4I8
498
240
328
136
20L
0 1100
I
998
2 867
3 33s
4 1328
5 397
6 374
7 108
I
589
9 130
1L 499
12 489
t3
85
I
2
191
703
J
d5v
11L
0
|
2
3
4
5
6
7
l0
rr
365
407
950
393
58
43
320
324
18r
188
366
40'l
945
388
74
32
32L
325
I'19
191
t3
118
125
II28
1005
878
340
1335
406
374
115
681
I29
498
482
89
3lL
I
502
5
L70
o
2o6
502
397
173
294
132
r93
9I
128
I94
101
L
0
281
I
]J6
3
4
5
182
9'1
24L
284
2
539
3
I43
4
181
5
8667576
24L
7
266
I
7't
10
tI6
L2
988
10 126
0 r93
r6567
27288
4 180
5 r93
-1
2
3
4
5
6
L79
205
239
355
366
102
8't
I
2
4
5
6
7
8
9
10
lt
3
5I2
348
3I5
26't
153
460
500
r85
138
330
239
277
240
99
7IL
-o
z
38083
4 2t6
5 110
8 186
9 202
10
93
-4
I82
L
2I3
lll
180
2LO
33
2L
I
175
2 I75
3 247
4 386
59497
't
68
8 2r7
96757
r0
L02
11
89
12 156
13 I20
I
2
5
6
9
I0
370
3L2
130
209
133
169
80
261
153
I27
209
372
196
L47
208
355
187
14I
23L
346
359
87
96
166
J
50v
4
245
244
45
258
213
84
159
t1'l
404
43L
z
Jo3
3
4
5
6
7
8
9
lt
I75
L'16
242
393
68
22L
87
73
L67
I20
I
I 1553
2 474
36759
4 265
5 672
6 458
'1
l-20
I
574
9 519
t0 119
It
388
12 463
tor
37
156
695
160
223
150
691
I55
2L3
o
26z
ztt
7
224
2I7
375
360
z
)z)
5fb
3
4
5
6
't
642
202
tl2
790
I99
253
38I
84
634
2I0
86
782
I92
256
3'15
90
8
10
Il
I tI54
2 L048
3 907
4 340
5 1340
6 393
7 366
8 106
9 684
r0 132
11
76
12 507
13 491
14
86
264
658
4s4
1I7
571
588
96
378
462
1132
10Il
875
338
1335
405
373
II2
678
126
25
492
479
87
0
69
L 544
2 578
4 L75
s6t
7 I77
858
10 I5I
rr
L33
6l
530
570
L76
55
364
r84
82
I37
137
a
3
4
5
6
/
8
I0
11
L2
44L
0
208
L
ZOt
6
4
0 280
1 160
2 101
39891
49085
5 208
/oJ
651
164
235
84't
tt5
639
155
228
853
oof
b59
245
348
Il8
I30
23''
344
I24
134
06L
205
Z5L
3 I35
4 r05
5 361
6 204
7 160
8 r13
99894
131
r05
356
209
r49
113
L
275
158
82
0 1569
L 478
Z
3
4
5
6
7
I
9
10
II
L2
OBS
0 30r
I5466
2 I74
3 452
4 276
s tI2
6 4I4
7 348
242
90
422
207
197
I48
83
301
I73
458
29L
100
415
334
59L
0
2
3'r2
II5
r05
158
-4IO
L
2
3
5
I
I44
519
107
284
269
230
'12
230
r01
106
IAJ
0 232
I 103
2 420
3 205
6 202
7 I47
8 101
IO L47
IL L42
JL
bI
268
655
466
I18
579
598
97
38'l
47I
64
I
3
4
67
357
t97
89
L42
-210
'17
159
251
83
I48
163
L
2 I40
3 187
56895
7 224
I
I07
128
190
231
I34
ttL
010
0
1
5
145
2r0
130
-4
8
I6635
3 I72
4 146
7 I2I
8 24'1
-2
8
t
149
2 16I
3 317
4 115
56792
'1
2L3
8 156
9
79
11
97
1567
485
274
67't
463
t25
574
578
I07
385
464
a1
L
L
0
8t
L 362
2 203
38899
4
79
5 I5I
6 250
88077
98251
L
1550
484
L
I7L
-46L
398
4r7
367
94
90
28I
134
548
520
95
276
253
2I5
2L9
LZ
-66L
04L
0
I
OBS
39L
369
3I3
136
2I4
L27
173
55[
o 207
I
218
3798s
4
93
5 23I
78794
246
I?I
8 2r5
975
11 163
12 179
L
5I7
354
328
262
15s
469
608
183
L25
337
2
L
24L
-J
0 I88
18741
272s8
OBS
J ) L
70
268
164
120
196
3L
-)
L
-24L
ozL
51L
o 233
r 266
46681
5 239
7 r03
OBS
-44L
42L
184
702
841
r71
110
70
39s
190
90
L92
160
80
4 L'r6
5 r18
659
7 402
I
I87
't9
9
11 187
12 150
14
79
9
IO
11
OBS
IL
L
00L
1 443
2 275
3 1053
4 483
5 IO94
6 3L2
L
145
207
146
L
I72
143
lt6
24L
L
146
I47
323
105
2L4
163
8s
LO2
2 270
3 143
47878
6 I49
't
r97
99480
I49
305
3t8
52
L't 4
L79
28L
r
291
2 3r2
497
188
7
8
1I0
tr3
97
6f
L
199
203
Z
L)J
I)J
3
4
5
6
102
'17
tlo
249
80
85
116
257
4IO
L
0 I00
r
2r5
28442
3939r
I
3
4
1.24
r82
IL2
r7534
2 247
3 253
s tt4
7 L97
I11
310
tI7
I52
207
r
-31I
138
296
273
!44
2IO
08L
I
I31
2 296
36753
4 L57
6 30I
't
3t6
8
72
9 I70
II
L79
L
0 157
18677
2 226
3
99
5 133
110
201
L
107
189
105
234
247
I24
205
L
158
224
r05
l3r
HAZEN ET AL: STRUCTURE OF SILICA- AND ALKALI.RICH
s89
TRIOCTAHEDRAL MICA
Table 5. (continued)
L
OBS
CAIC
L
-z
40L
L
2
3
4
s
6
8
9
L0
629
r54
228
834
648
234
341
103
139
631
t4'7
222
855
653
231
344
117
r34
60L
t
2
3
4
5
5
168
345
206
I0I
323
398
169
352
196
94
330
390
-.7 IL
29394
4 Is't
5 r57
-5
165
160
1L
r
2
83
r20
J
16)
4
346
I
6
I
9
O f
lr9
223
167
78
110
L82
350
75
L26
222
L82
I L
r
2
255
L36
4
384
]
IIf
6
L2L
7 r82
I 352
r0
58
1r
87
r
5
0
12
256
136
636
394
113
13r
r77
362
67
a2
150
1
2
3
4
5
6
7
I
9
10
lr
12
13
14
L
OBS
z
404
422
24t
185*
176
328
6L4
336
250
190
139
290
I52
92
OBS
'144
o
376
r
2 4t7
3 690
4 458
5 1034
6 1199
7 268
171
I
9 645
r0 100
11 260
13 415
72 0
346
404
614
449
1031
1209
216
r73
639
92
252
405
33L
435
897
638
9
194
409
140
rr
200 r98
L2
13
99
IIO
105
II3
22
L
o 407
r 420
283
3 311
5 434
7
I
10
rt
106
3r8
20L
292
400
424
86
306
443
LL2
3r5
208
29r
0 48r
15462
2 275
3 6'1'1
4 467
5 r07
6 59'l
't
59'l
14
I
9 401
461
r0
11
92
0 622
2t2
r
29594
3 784
4 t'12
5 248
7 370
8 r22
0
478
27L
656
46'l
rll
5'15
59 3
93
389
467
65
627
207
'786
184
257
374
97
-I
269
134
r37
fJJ
I
9
L94
L57
9'1
LZ)
1r8
69
96
3lr
r04
104
10
t 1r 1 8 8
2 158
L2
'19
t 31
CAIC
200
360
202
173
62
299
294
t0
r1
100
153
relatedto the relative sizesofoctahedral and tetrahedral layers, is also typical of other trioctahedral
micas.
It is important to resolvethe distribution of cations
among the four different metal sites (the interlayer
site, the tetrahedral site, and two symmetrically distinct octahedralsites).The structurerefinementwith
unconstrained metal occupancy factors, combined
with chemicalanalysis(Table l), are sufficientto derive a reasonable,if not unique, structural formula
I,
5'l
111
5
982
A21
129
378
LO42
L44
361
72
302
156
3?r
r7
301
46L
I
2
3
4
5
6
7
8
340
273
169
484
625
20r
143
354
331
264
150
488
625
186
r30
352
66L
163
363
522
r09
53
84
81
224
28L
23L
60
218
2'15
228
101
99
90
100
126
281
90
r00
r09
L49
277
62
154
r52
r88
148
65
198
350
658
200
173
29'l
305
114
160
0
I
3
5
6
155
2OL
116
237
158
147
I81
L02
247
152
/O
6
8
r0
11
143
317
203
7 160
8 2L3
88
LO
91
II
3
-L'I
2
3
5
7
8
447
459
188
309
r72
OBS
L
OBS
311
L
I
2
226
98
2L9
104
L
2
3
4
244
'lL
275
L29
247
29
276
140
012
L
o
r
3
4
5
720
163
236
279
244
724
158
223
290
235
0
359
370
378
90
r02
48L
l5L
t29
r4'1
199
155
86
L25
236
1r5
s33
308
L26
160
r43
3r4
0 986
r
I3r
2 L23
3 353
4 1035
5L
156
312
0
L
3L
r38
25?
100
196
L42
74
225
r
2 r10
3 548
4 310
5 L2'1
5 155
7 L25
8 309
97663
74
10
93
t2
-64L
9
OBS
I,
26L
28994
3 r30
4 255
6 r0r
8 178
9 130
'17
t0
53L
'l
CAI'
OBS
-55L
o2L
0 418
19395
2 895
3 639
4
55
59795
6 194
7 4II
87048
10 rso
L
13L
L
397
426
248
238
t1'l
30r
622
330
255
r95
t 31
295
139
100
CArc
148
310
r98
156
2r'l
74
75
-59L
I
2
A
5
364
234
269
r37
356
22'1
268
13r
-39L
I 80r
2 206
4 164
5
JbJ
6
7
8
9
200
IIl
333
400
-1
9
L 527
2 648
3 210
493
'199
5
5 189
9
I0
3'7'l
'16
1
0
I
2
3
4
5
6
7
I
9
803
r99
173
364
199
98
334
395
294
394
L
530
64'7
2tr
88
803
L92
265
384
97
9L
371
69
34L
269
159
488
629
190
139
342
372
23
339
266
r64
495
630
189
I3I
353
L
444
46r
193
312
L6'l
for this unusual mica. A crystal-chemicalconstraint
conmon to all micas is that there must be exactly
four tetrahedral cations per ll oxygens.Of the cations present (Table l), only Si, Al, and Fe'* commonly enter tetrahedralcoordination in micas.Magnesium has been observedin four coordination in
iron-free synthetic mica (Tateyama, 1974),but tetrahedralmagnesiumhas not beenreportedfrom natural micas,nor has it beenobservedin any mica with
>(Si + Al + Fe3*)> 4. Furthermore,the refined elec-
590
HAZEN ET AL.: ST:RUCTURE OF SILICA. AND ALKALI-RICH
TRI)CTAHEDRAL
MICA
Table 6. Interatomic distancesand bond ansles
Distance
(l)
Angle
(')
Distance
Tetrahedron
T-01
T-OI
T-O2
T-03
Mean T-0
L . 6 5 3( 2 )*
r . 6 5 4( 2 )
1 . 6 s 5( 1 )
L . 6 5 7( 2 )
1 .6 5 5
01-o1
01-02
or-02
ol-03
01-03
02-o3
Mean 0-O
2 . 6 78 ( L )
2 . 6 7 7( 3 )
2 . 6 7 7( 3 )
2 . 72 3( 2 )
2 . 72 5( 2 )
2 . 72 9( 3 )
2.70L
r-r[2]f
T-T
Mean T-T
3.076(1)
3 . 0 7 8( r )
3.077
Interlayer
Ml Octahedron
0l-T-01
01-T-02
01-T-02
o1-T-03
ol-T-o3
o2- T-03
108.16(7)
1 0 8 .0 8( 1 3 )
u0.74 (8)
1 0 8 .0 6( 1 3 )
1 1 0 .7 s( 9 )
1 1 0 . 9 s( 1 0 )
L 0 9. 4 6
M1-03[ 4 ]
M1-F[2]
Mean M1-O
2 . o 9 3( L )
2 . 0 4 4( 2 )
2.O77
0 3 - 0 3[ 2 ]
0 3 - 0 3[ 2 ]
o3-F[4]
o 3 - F[ 4 ]
Mean O-O
2 . 8 3 4( 3 )
3 . o 74 ( 3 )
2 . 76 7( 2 )
3 . o 75 ( 2 )
2.932
(adj acent
)
o3-I11-03[2]
o3-M1-o3[2]
o3-r..il-F[4]
03-M1-F[4]
M e a nO - M - O
( a d Ja c e n t )
03-n1-03
F-MI-F
( o p p o s i t e)
94.80(8)
8s.20(8)
96.04(5)
83.96(5)
90.00
180.00
180.00
M2 Octahedron
trz-o3l2l
\12-03l2l
r12-Fl2l
Mean M2-0
Site
K-ol[4]
K-o2[21
Mean K-O
(lnner )
3 . 0 2 2( 2 )
3.020(3)
3.02r
0 3 - 0 3[ 2 ]
03-o3
o 3 - o 3[ 2 ]
o3-F[2]
03-F[2]
03-F[2]
K-or[4]
K-o212l
Mean K-O
3 . 2 8 2( 2 )
3.282(3)
3. 2 8 2
F-F
Mean O-0
(adjacent)
^ (K-o)
o.26L
2 . 0 9 2( 2 )
2 . 0 9 4{ L )
2 . 0 4 4< 2 )
2.O77
2 . 8 3 s( 3 )
3.082(3)
3 . 0 74 ( 3 )
2 . 7 6 7( 2 )
3 . 0 74 ( 3 )
3.084(3)
2 . 70 7( 4 )
2.955
03-t'12-o3
0 3 - M 2 - 0 3[ 2 ]
o3-M2-O3[ 2 ]
o3-r12-F[2]
o3-M2-F| 2 l
o3-rq2-Fl2l
F-M2-F
Mean O-M2-0
8 5. 2 7( 9 )
8s.26(6)
9 4. 4 9( 6 t
8 3 . 9 3( 7 )
9 6. 3 3( 7'
9 6 . 0 6( 6 )
8 2 . 6 6( 9 )
9 0 .0 0
(adjacent)
03-M2-03
03-M2-F
Mean O-M2-O
(opposite)
L79.66(9)
L77.83(7)
L 7 8 . 75
xParenXhesized
+Bracketed
figures
represent
esd's.
figures
represent
nujxiplicitg.
Table 7. Magnitudesand orientationsof thermal ellipsoids
* Patenxhesi
Angle
tus Displacenenr (L)
&is
wlth
respect
to:
84(4)
'l;[li]
r1
f2
0 . 0 9 7( r ) *
0.107(1)
0 . 1 1 0( 1 )
89(6)
r58(12)
1t-2(72)
6( 5 )
8 7( 6 )
9 s( 4 )
r1
r2
.3
0.071(5)
0.093(4)
0.115(4)
90
r 7 8 ( 7)
9 2 ( 7)
0
90
90
f7
f2
r3
0.091(3)
0.092(3)
0.114(3)
90
17s(5)
9 5( s )
0
90
90
90
8s(5)
5(5)
r1
r2
r3
o . 1 7 7( 2 )
o.172(2)
0 180(2)
73(9)
90
17(9)
90
0
90
2 7( 9 )
90
1 1 7( 9 )
r1
r3
0 . 1 4 5( 3 )
0.168(3)
0.218(3)
83(5)
7 4 7( 3 )
s8 (3)
r1
r2
r3
0 . 1 4 6( 5 )
0.168(4)
o.223(4)
8 9( 2 )
90
7(2)
r1
f2
r3
0 . 1 0 7( 3 )
0.110(4)
0.118(3)
83(17)
139(17)
50(16)
r1
f2
r3
0.118(4)
0.123(4)
0.128(4)
7 2 < r 7)
90
18(17)
zed
f igures
represent
esd,s
83(4)
1 2 1( 3 )
1 4 8( 3 )
e0
'311i
1 7( 6 )
72(6)
93(2)
90
0
90
ilt"
101(2)
9(27)
8 1( 2 1 )
9 0( 1 0 )
,13[i;]
90
0
90
2 7 ( 7 7'
90
1 1 7( 1 7 )
tron density of the T site is significantlygreaterthan
that of Si, Al or Mg. It is assumed,therefore,that all
silicon plus aluminum, aswell as0.646atomsof Fe3*,
occupythis site.The total number of electronson the
tetrahedralsite(l5.le- basedon refinedelectrondensity) and the mean T-O bond distance(1.655A)from
the structure refinement,conform well with this assumedtetrahedralsite occupancy.
A secondcrystal-chemicalconstraint of all micas
is that therecan be no more than 1.0interlayer cation
per I I oxygens.In this mica the sum of (K + Na +
Ba) is l.ll, indicating that a significant fraction of
thesecationsare not in the interlayer position. All K
* Ba, as well as 0.013atomsof Na, are assignedto a
fully-occupied interlayer site. The total number of
electrons(19.3e-)and the mean inner and outer K-O
distances(3.021 and 3.2824, respectively)from the
structure refinement are consistentwith this assign-
rso(17) ment.
All remaining cations must occupy the two octahedral sites.This necessaryconclusionleads to one of
the most complexsite compositionsin any known silicate. The resulting averageoctahedral layer compo-
HAZEN ET AL.: STRUCT:UREOF SILICA- AND ALKALI-RICH
TRIOCTAHEDRAL MICA
591
ber of octahedralcations will decreasefrom 2.92 to
2.85.
This unusualmica servesa specialrole in the crysMg, nrrFeoo'Mno *uCa" -5TL u88LLrrrN&o ,,oDo-,
talTuation history of the melilite-bearing eruptive
The remarkableMl and M2 sitesthus have cations rocks in which it is found. It appearsto conc€ntrate
with alkali metals,including pbtassium,sodium, and lithof valence4+,2+, and 1+, as well asvacancies,
(Shannon,
ium, in an environment that is deficient in aluminum
cation radii ranging from 0.61 to 1.02A
the
and thus underscoresthe adaptability of the mica
1976).The total refined number of electronson
mean
structureto a wide range of compositionallimits.
two sites.Ml and M2 arc both I l.7e-, and the
Al'
Ml-O and M2-O distancesare both 2.0774Acknowledgments
though the complexity of the octahedralsite chemisThe authorsgratefully acknowledgeS. W. Lonker, D' Rumble
try precludesa unique solution to Ml-M2 cation disIII, and H. S. Yoder, Jr., for their constructivereviewsof the mantribution, there is no evidencefor cation ordering, uscript. Bruce Velde performed infrared analysis of the mica' This
consistentwith previous refinementsof both igneous work was supportedin part by NSF Grant EAR79-19768'
(Hazen and Burnham, l9'13) and metamorphic(BohReferences
ler- et c/., 1980) biotites, as well as synthetic
BaLiMgrAlSi3O,oF2(McCauley and Newnham, Bohlen, S. R., Peacor,D. R., and Essene,E. J. (1980) Crystal
sition. basedon three octahedralcationsper ll oxygens,is:
r973).
The resulting mica, though complex, may be representedas composedprimarily (65Vo)of the ferriphlogopite end member, KMgrFe3*SirO,oF2.Additional componentsinclude 23Voof the magnesium
lithium mica taeniolite, K(LiMg2)SLO,.F2,plus minor amounts of the phlogopite-annite seriesand a
Ti-mica. Note that the calculatednumber of titanium
atoms approximately equals the number of octahedral vacancies,as observedin many other biotites
(lJrazer-and Burnham, 1973).The only previously
unknown component of this mica is approximately
4Voof an end member with Na in the octahedral
layer.
Analytical uncertainties for fluorine, lithium, and
octahedral Fe'*/Fe3* result in small but significant
uncertaintiesin the structural formula. For example,
l07o oxy-biotite substituting for fluoro-biotite could
increasethe total number of cationsfrom 7.92 to the
ideal value of 8.00. Refined electron density of the
fluorine site (8.3 e-) is smaller than the 9.0 e- value
expectedfor a pure fluoro-mica. This electron deficiency may imply the presenceof an oxy-biotite component or undetected(OHf. Uncertaintiesin lithium
content may also affect the structural formula. If total lithium is 0.50insteadof 0.75 wl.Vo,then the num-
chemistry of a metamorphic biotite and its significance in water
barom€try.Anerican Miaeralogist 65, 55-62.
Finger, L. W., Hadidiacos,C. G., and Ohashi,Y. (1973)A computer-automated,single'crystal,x-ray difractometer. Carnegie
Institution of WashingtonYear Book 72,69+699.
Gragnani, R. (1972) 16 vulsenili melilitiche di Cupaello (Rieti).
Rendiconti della Societa Italiana di Mineralogia e Petrologia
28, 165-189.
Hazen,R. M., and Burnham, C. W. (1973)The crystal structures
of one-layerphlogopite and annite. American Mineralogist 58,
889-900.
Hazen, R. M., and Wones, D. R. (1972)The effectof cation substitutions on the physical properties of trioctahedral micas'
American Mineralogist 57, lO3-129.
McCauley, J. W., and Newnham, R. E. (1973) Structure refinement of a barium mica. Zeitschrift fur Kristallographie 137,
360-361.
Shannon,R. D. (1976)Revisedefective ionic radii and systematic
studies of interatomic distancps in halides. Acta Crystallographica A32, 751-767.
Tat€yama,H., Shimoda,S. and Sudo,T. (1974)The crystal structure of synthetic Mgrv mica. Zeitschrift fur Kristallographie
r39,196-206.
Velde, D. (1979)Trioctahedralmicas in melilite-bearingeruptive
rocks. Carnegie Institution of Washington Year Book 78' 468'
475.
Manuscipt received,August 19, 1980;
acceptedforpublication,January 26, 1981.