Ferri-Annite from the Dales Gorge Member Iron-Formations, Wittenoom Area, Western Australia

American Mineralogist, Volume 67, pages 1179-1194,1982

Ferri-annite from the Dales Gorge Member iron-formations, Wittenoom area, Western Australia

TeresHr MrvRNor

Department of Geology Indiana University Bloomington, Indiana 47405

exo Suuxo MrveNo Institute of Geoscience The University of Tsukuba Ibaraki 305, Japan

Abstract

Ferri-annite, a new, naturally occurring mica with a representativecomposition of K1.s2 (Mg1s3Fe42|) (Fe?.teAlg zesio.oz)Ozo(OH)a, probably expressedby the quadrilateral components annite, phlogopite, ferri-annite (K2Fea+Fd+Si6O20(OH)4),and ferri-phlogopite (KzMgoFei*Si6O2o(OH)4),is described from the Wittenoomarea, WesternAustralia. The mineral occurs as flaky to tabular grains or as massive aggregatesof fine acicular grains near or within a riebeckite-rich zone of bandediron-formation, the Dales Gorge Member of the Hamersley Group. It coexists with hematite, magnetite, quartz, ankerite, stilpnomelane, and riebeckite. The ferri-annite can be chemically subdividedinto two groups, A and B. The group A variety contains 4to7 wt.VoAlzOr and has light reddish brown (:X) to pale yellow green (: Y,Z) pleochroism, and the B variety, with the lowest Al2O3content (l to 2 wt.Vo),has brownish red (:X) to pale greenishbrown (: Y,Z) pleochroic colors. The latter variety generally contains about l0 wt.Vo morc Fe than the former. (001) cleavage is perfect. Twinning is frequent. Cell parameters(a, b, c, B, V) of the mica (group B) were calculated as 5.402(6),9.237(4), 10.306(7)A, 99'16'(10), and 507.54(6fA3 using X-ray powder diffraction analysis. Formation of much of the ferri-annite appearsto be the result of potassium enrichment in stilpnomelaneconcomittant with the formation of associated riebeckite.

Introduction iron-formations resulting from magmatic condi- a mica has not beenfound in Veres et al. (1955) and Wones (1963a) have tions. However. such (1960) described the synthetic iron mica (ferri-annite), iron-formations of magmatic origin. Foster tetrahedral ferric K2Fe3"(Fe3+Si3)Or0(OH)2,and the phaseequilibria showed that biotites containing found in alkaline rocks and of ferri-annite have been examined by Wones iron are frequently pegmatites. to (1963a).The crystal structure of this mica was Such biotites, however, contain6 15 Al2O3than the ferri-annite of this study. determined to be a trioctahedral one-layer mica wt.Vomote Fe-rich and Al-poor mica (hereafter (1M) by Donnay et al. (1964).The mica synthesized Naturally referred to as ferri-annite) occurs in the very low- by Wones (1963a)was formed at conditions of grade of the Dales Gorge Member, oxygen fugacity between those of the HM and WI iron-formation Group, Western Australia. Mineral as- buffers, at 400-850"C.and at 1035and 2070 bars Hamersley involving ferri-annite (Fe-rich mica) (:Pn,o + Pn). He suggestedthat mica with a semblages described and their petrologic signifi- composition such as ferri-annite might be found in have been cancehas been discussedby Miyano (1982). The purpose of this paper is to describe the rPermanentaddress: Institute of Geoscience,The University mineralogy, chemical composition, and cell param- ofTsukuba, Ibaraki 305, Japan. eters of the ferri-annite. 0003-0MX/82ll I l 2-l 179$02.00 tr79 I 180 MIYANO AND MIYANO: FERRI.ANNITE FROM IRON.FORMATION

Occurrence,paragenesis, and optical properties light reddish brown (:X)

Flaky to tabular mica grains with a diameter of l0 a : 1.653-'- 0.005 to 40 g,mare dispersedin chert-rich mesobandsthat v : 1.691-'-0.005 are closeto or within a riebeckite-richband or zone 'ya :0.038 - 0.044 (Fig. 1A). They coexistwith quartz, hematite,mag- 2V* :0o - 10" netite, stilpnomelane,riebeckite, and ankerite, and are oriented nearly parallel to the sedimentary brownish red (:X) structure. Massive aggregatesof fine-grained ferri- a : 1.677-f 0.005 -r annite commonly form thin, lenticular and discon- Y : 1.721 0.005 tinuous bands up to about 0.2 mm thick, alternating Td :0.045 - 0.052 with iron-oxide microbands (Fig. 1B). Stilpnome- 2V* : small lane is commonly replaced by ferri-annite where they coexist (Fig. 1C). Textures suggestthat the bands now occupied by ferri-annite may originally Chemical composition have been stilpnomelanebands. In some occur- The chemical composition of the ferri-annite was rences. however. the mica is found as veinlets in obtained by electron microprobe, and is shown in chert (Fig. lD); suchveinlets originate in stilpnome- Tables 1,2, and 3. Table I showschemical compo- lane bands. sitions of flaky to tabular grains of ferri-annite The mica appearsto be later in formation than the (SampleZ, anal. I to 3; DE, anal. 4 to 6 ; GE, anal. 7 other silicatesexcept riebeckite.Textural relations to 9) and fibrous to acicular grains which form between ferri-annite and riebeckite are complex. massiveaggregates (Sample MA, anal. l0 to 12),in Both minerals occur in a similar manner around or various assemblages.Table 2 representschemical alongmagnetite grains or bands(Fig. 1E, F), where analysesof fibrous to acicular grains, forming mas- the mica is flaky and coarser than the needle-like sive aggregates, of mica with the lowest Al2O3 (fibrous) riebeckite. It may be that Al releasedfrom contents (SampleMB). Both mica types (samples stilpnomelaneduring replacementby riebeckite has MA and MB) commonly form lenticular and discon- been reprecipitated as ferri-annite close to riebeck- tinuous bands, but the MB type is more coarsely ite bands (Miyano, 1982). However, prismatic crystallizedand makesthicker bandsthan type MA. grains of riebeckite commonly cut acrossboth ferri- Table 3 representsaverage chemical compositions annite and fibrous riebeckite bands. Fibrous rie- of the mica in both occurrences. beckite (also crocidolite) is much more abundant The composition of the mica can be describedby than prismatic riebeckite. If there were two stages K2O, MgO, FeO, Fe2O3,Al2O3, and SiO2 (other of formation for riebeckite, the mica may well be components are minor and total less than one contemporaneous with the first stage of fibrous percent) and can be chemically subdivided into two riebeckiteformation. groups, A and B. Group A contains 4 to 7 wt.Vo Ferri-annite occurs in two colored varieties under Al2O3and 28 to 33 wt.Vo FeO (total Fe), and group the microscope,one with light reddishbrown (:4 B 1 to 2 wt.Vo Al2O3 and 38 to 43 wt.Vo FeO (total to pale yellow green (: Y,Z) pleochroism, and the Fe) (Tables1 to 3). The former showslight reddish other with brownish red (:;1 to pale greenish brown to pale yellow green pleochroism and the brown (:Y,Z) pleochroic colors, which are very latter brownish red to pale greenish brown pleo- similar to those of synthetic ferri-annite of Wones chroic colors. The chemical difierences between (1963a,p. 583).The latter variety is generallyfine- ferri-annite and other micas such as biotite and grained and constitutes massive aggregateswhich phlogopite, are depicted in Figures 2 and 3, with may resembleferristilpnomelane. The former (with total iron of all micas recalculated as FeO. The light reddish brown to pale yellow green color) ferri-annites contain much less AlzOg than biotite often occurs as flaky to tabular grains or aggregates. and phlogopite. The ideal compositions of micas on In the flaky grains,(001) cleavage is perfectbut not the join annite (KzFe?*Alrsi6o20(oH)4)-ferri-an- as well-developedas in biotite. Twinning is fre- nite (K2Fe?*Fel*Si6O2o(OH)a)show that the K2O quent. Becauseof the limited amount of material, content offerri-annite is usually less than that of the only approximate refractive indices could be ob- idealformula (Fig. 2). Associatedstilpnomelane, an tained. There are: Fe-rich phyllosilicate can also be divided into two f

Fig. l. Someoccurrences offerri-annite. (A) Aggregatesofflaky to tabulargrains offerri-annite in a chert-richband. Grey to dark coloredportions with high index, showingtwinning, are ferri-annite.Sample GE. Polarizedlight. (B) Massiveaggregates of fine, aciculargrains of ferri-annite(mica, grey). Black parts are hematiteand magnetite.Sample MB. Polarizedlight. (C) Ferri-annite (mica)in a massivestilpnomelane band. Black portionsare hematiteand magnetite.This stilpnomelaneis enrichedin potassium. SampleGE. Polarizedlight. (D) Veinlet of ferri-annitein chert. Black euhedralgrains are magnetite.Flaky grainsare disseminated alongthe veinlet.Sample GE. Polarizedlight. (E) Needlelikeriebeckite and flaky ferri-annite(grey) around euhedral magnetite grains (black)in a chert-richband. Black irregulargrains are hematite.Sample DE. Polarizedlight. (F) Needlelike(fibrous) riebeckite and flaky to tabularferri-annite (mica) are intergrownwith quartzalong a magnetiteband. Small black dots in the lower right cornerare hematite.Samole Z. Polarizedlieht. MIYANO AND MIYANO: FERRI.ANNITE FROM IRON-FORMATION

Table l. Representativeelectron microprobe analysesof ferri-annite from the Dales Gorge Member iron-formations

l2 3 4 5 6789 10 11 12 wt. t zl15 zI3I 227I DE3 DEI 8 DE2O GE328 GE34O GE352 MA368 MA373 MA379

sio2 38.01 39.02 35.79 39.33 38.93 39.69 38.56 39.r3 3'7.7t 36.49 35.76 36.20

AI2O 3 4.7r 5.04 5.40 5.86 6.37 I.89 L,79 1.92

TrO2 0.05 0.00 0.00 0.04 0.00 0.00 0.00 0.02 0.00 0.00 0.01 0.00 Cr203 0.01 0.00 0.0I

FeO* 29.68 29.rr 33.80 28.95 30.52 29.79 30.43 3r. 33 30.34 40.29 40.80 39.80

MnO 0.o2 0.00 0.01 0.o2 0,09 0.03 0.00 0.05 0.00 0.00 0.01 0.03 NiO 0.07 0.04 0.00 '1 M9o 12.03 12.50 11.70 L2.72 L2.t7 t2.55 1I.70 10.81 L2.48 .3-7 7.50 7.85

Cao 0.04 0.00 0.01 0.03 o.02 0.00 0.0I 0.01 0.01 0.01 0.02 0. 01

Na2O 0.06 0,09 0.00 0.04 0.04 0.02 0.09 0.00 0. 36 0.0 I 0.01 0.04

Kzo 8.2r 1.49 8.30 8.53 8.32 8.57 8.38 8.35 8.30 7.80 7.73 7.56

Total 94.37 93.83 94,93 95.03 94.80 95.69 94.57 95.56 95.57 93.93 93.63 93.4r

Fe.O.** 6.28 6.L4 9.67 6.?1 7.66 7.05 6.77 5.'t6 7.07 rt.57 12.30 tI.76

FeO 24.03 23.59 2s.10 22.92 23.62 23.44 24.34 26.L5 23.98 29.88 29.7 3 29.22

TotaI 95.00 94.45 95.90 95.7L 95.56 96.39 95.25 96.r4 96,28 95.09 94.86 94.59

cations on the basis of 22 oxygens

ei 6.0'78 6.216 5.80s 6.205 6.198 6.234 6.L67 6.2L5 5.9'70 6.1s5 6.070 6.I2L

A1 I.167 I.048 1.015 0.999 0.884 0,932 I.0I8 1.097 I.188 0.376 0.358 0.383 3+ Fe 0.755 0.736 I.180 0.796 0.918 0.834 0.815 0.688 0.842 L.469 L.572 L.496

t 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000

Ti 0.006 0.000 0.000 0.005 0.000 0.000 0.000 0.002 0.000 0.000 0. 001 0,000

Cr 0.001 0.000 0.001

Fe-' 3.2L4 3.143 3.405 3.024 3.I46 3.079 3.256 3.414 3.115 4.21s 4.221 4.L32

Mn 0.003 0.000 0.00r 0.003 0.oL2 0.004 0.000 0.007 0.000 0.000 0.00r 0.004 Ni 0.009 0.005 0.000

Mg 2.868 2.969 2.829 2.992 2.A89 2.938 2.790 2.560 2.945 r.853 1.898 L.979

I 6.101 6.tt7 6.236 6.024 6.047 6.O2L 6.046 6.043 6.L20 6.068 6.L2L 6.I15

0.007 0.000 0.002 0.00s 0.003 0.000 0.002 0.002 0.002 0.002 0.004 0.002

Na 0.0r9 0.028 0.000 0.0I2 0.012 0.006 0.028 0.000 0.110 0.026 0.003 0.013

K r.o/5 L.)zz L. tLt L.7),7 1.690 I.7L7 1.710 L.692 L.616 L.679 L.674 1.63I

I I.701 1.5s0 L.7I9 I.'134 1.705 L.723 L.740 L.694 1.788 r.707 1.68r L.646

so2i 1po2*a6n 0.528 0.sr4 0.546 0.503 0.52L 0.5I2 0.539 0.s76 0.519 0.695 0.690 0.676

*All Fe as Feo. **estimated from tetrahedral ferric iron (see text). The ratio te2+11re2++ug) in the bottom line' the same as in Tables 2 and 3, is calculated after subtraction of tetrahedral ferric iron from total FeO. Samples Z, DE, and cE: flaky to tabular grains (group A), sample l4A: massive aggregates of acicular grains (group B). Sample DE was analyzed at the Geological Survey of Japan (G.S.J.), and the others at the AnaLytical Center of the University of Tsukuba (A.C.U.T.). 1. Mt-tln-Qt-Mica. 2. Hm-Mt-Mica. 3. Rk- Mi-ca=Stil*Mt-Hn. 4. Rk-et-Mica-Mt-Hm1An. 5. et-Mica-An-Hm-Mt. 6. Mica-et-Mt-Hm1Rk. 7, Mica-et-Mt(mica veinlet, Fig. lD). 8. Qt-Mica-Mt-An. 9. Mica-et-stit-Mt-Hm. I0. Mica-Rk-Hm. tl. Rk-Hm-Mica-etjMt. L2. An-Qt-Mica-Hm1Mt. Mineral abbreviations (through the text)r An=ankerite, Hm=hematite, Ho-hornblende, K-feI=K-feldspar, Mica=ferri-annite, Mt=magnetite, Qt=quartz, Po=pyrrhotite, Py=pyrite, Stil=stilpnomelane. MIYANO AND MIYANO: FERRI-ANNITE FROM IRON-FORMATION I 183

Table2. Representativeelectron microprobe analyses of ferri-annitewith the lowestAl2O3 content from the DalesGorge Member

T2 345 678 wt. t M8479 r'r8480 MB484 IrtB485 M8487 MB488 M8491 ytB492

sio2 35.8I 35.28 a< o? a< o? a< 1< 35.08 35.41 35.57

AI203 L.46 r.3I 1.36 1.19 L.46 1.33 1.38 1.36

TiO2 0.00 0.04 0.00 0.03 0.01 0.01 0.03 0.03

cr20 3 0.00 0.03 0.05 0.02 0.00 0.07 0.00 0.00 FeO* 4L.45 42.09 4L.43 4L.65 4L.82 42.4t 42.03 4L.69

Iltno 0.00 0.00 0.00 0.03 0.01 0.00 0'.07 0.09

Nio 0.03 0.11 0.05 0.00 0.00 0.09 0.08 0.00

M9o 7.25 7.0I 7.05 7.38 7.L4 7.2L 7-.2L 7.L6

CaO 0.05 0.03 0.02 0.04 0.00 0.r0 0.06 0.07

Na2O 0.00 0.0r 0.o2 0.00 0.00 0.03 0.02 0.04

K2o 8.Ir 8.16 8.rr 8.L2 8.28 7.76 7.87 7.91

Total 94.17 94.07 94.02 94.43 93.88 94.09 94.L5 93.92

* Fe2O3* 12.83 13.40 L2.7L L3.24 t3.23 13.70 r3.34 13.08

FeO 29.90 30.03 29.99 z9. tJ 29.9L 30.08 30.03 29.92

Total 95.45 95.41 9'1.29 95.20 95.46 95.50 95.23

Cations on the basis of 22 oxygens

si 6.071 6.0I7 6.103 6.O79 6.005 5.976 6.018 6.053

AI 0.292 0.263 0.272 0.237 0.294 0.267 0.276 0.273

Fe3+ l.aJt L. tzv L.625 1.684 r.701 L.757 1.706 L.674

T 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000

Ti 0.000 0.005 0.000 0.004 0.001 0.001 0.004 0.004

Cr 0.000 0.004 0.007 0.003 0.000 0.009 0.000 0. 000

Fe2+ 4.240 4.2A3 4.26L 4.203 4.273 4.285 4.268 4.259

0.000 0.000 0.000 0.004 0.00r 0.000 0.010 0.0r3

Ni 0.004 0.015 0.007 0.000 0.000 0.oL2 0.01r 0.000

M9 1.832 t.782 1.785 1.8s9 I.8r8 1.83I L.A27 1.816

E 6.076 6.089 6.060 6.073 6.093 6.r38 6.L20 6.092

0.011 0.005 0.004 0.007 0.000 0.0r8 0.0r1 0.013

Na 0.000 0.003 0. 007 0.000 0.000 0.0I0 0.007 0.013 r.706 L.7L7 K !:E_4 r.775 L.757 1. 751 r.804 L.687

I L.765 I.783 1.768 1.758 1.804 1.7rs L.724 L.743

te2+1Fe2++ug 0.698 0.706 0.705 0.693 0.702 o. 701 0. 700 0. 701

*AII Fe as Peo. **estimated from tetrahedral ferric iron. Analyzed at A.C.U.T. Massive aggregates of ferri-annite' coexisting mainly with nagnetite and hematite, and a small amount of quartz, ankerite, and riebeckite (see Fig. lB). I 184 MIYANO AND MIYANO: FERRI.ANNITE FROM IRON-FORMATION

Table 3. Average electron microprobe analysesofferri-annite

I 2 5 DE GE MA MB wt. 8 (r8) s.D. (r8) s.D. (24L s.D. OsI s.D. t15) S.D.

sio2 38.50 0.7 7 38.47 0 .6 3 38.44 0 .77 35.23 0. 31 35.49 0. 28 33. 33

AI2O 3 5.43 0.48 5.00 0.59 5.84 0.49 1.83 0.rr r.42 0.15 TiO2 0.02 0.02 0.03 0.04 0.02 0.02 0.01 0.01 0.01 0.42

Cr2O3 0 .02 0. 03 0 ,02 0.02 '11.98 Fe203* 6.84 0.96 7.70 I.00 6.65 0.52 0.29 13.25 0.53 L4.77

FeO 23.62 0.70 23.26 0.43 25.L5 0.89 29.57 0.54 29.77 0.31 39.86

MnO 0.03 0.04 0.04 0.03 0.03 0.04 0.02 0.o2 0.03 0.03

Nio 0.02 0.04 0.04 0.04

Mgo 12.05 0.44 12.45 0. 39 rr.50 0.66 7.56 0. 31 7.24 0. 18

CaO 0.04 0.04 0.03 0.03 0.01 0.o2 0.02 0.o2 0.05 0.03

Na2O 0.06 0. f0 0.06 0.04 0.15 0.18 0.06 0. r0 0.02 0.02

K2o 8.46 0.45 8.28 0.43 8.30 0.13 7.87 0.2r 8.33 0.27 8.71

Total 95.09 0.72 95.32 0.56 96.09 0.39 95.15 0.70 95.67 0.72 96.67

cations on the basis of 22 oxygens

si 6.154 6.136 6.111 5. r15 6.024 6. 000

At 1.023 0.940 r.094 0.354 0.284 -?+ Ie- 0.823 0.924 0.795 r.521 L.692 2.000

I 8.000 8.000 8.000 8.000 8.000 8.000

Ti 0.002 0.004 0.002 0.001 0.001

Cr 0.003 0.003

)L Fe'' a 1E? 3.102 3.344 4.L74 4 .225 5.000

Mn 0.004 0.005 0.004 0.003 0.004

Ni 0.003 0.005

qn, Mg 2.872 z-tov 2.725 1 L.832

t 6.04r o.u/r 6.075 5.080 6.07r 5.000

0.007 0.005 0.002 0.004 0.009

0. 0r9 0.019 0.046 0.020 0.007

K L. I Z) 1.585 1.583 r. 804 2.000

I ; L.709 L.73L L.719 1. 820 2.000

F"2+ yFe2++Mg 0.524 v. )Lz u. f f,r 0.687 0.698 1.000

*estimated from tetrahedral ferric iron. Number in parentheses is the number of analytical points. S.D. standard deviation (1o) . Including chemical compositions listed in Tab1es I and 2. 6z Ideal composition of ferri-annite . MIYANO AND MIYANO: FERRI-ANNITE FROM IRON.FORMATION l 185

A1203 ported by Veres et al. (1955)and Wones (1963a). a40 Fe2* and Fe3* cannot, however, be distinguished by electronmicroprobe analysis. Assuming that all Fe3*, Al, and Si of the mica are locatedin tetrahedral sites totalling 8 cations, the minimum amount of Fe2O3can be estimatedas shown in Tables I to 3. As a result, the number of octahedralcations ranges from 6.0 to 6.2 (closeto ideal)and the Fe2*/(Fe2*+ Mg) ratios of the micas are a relatively constant value. The number of silicons is usually greater than 6.0 and the number of cations in the interlayer positions commonly less than 2.0. The Si values greater than 6.0 and the sum of the octahedral cations may be related to the general deficiency of of mico olongonnile- 'ferrionnite' cations (K,Na,Ca) in the interlayer positions.This join deficiency, however, is unlikely to have much effect 60 70 80 90 t00 on either the ratio of Fe2*/(Fet"+Mg) in the octa- CoO-l No2O* K2O FeO'+MnO + MgO hedralsites or on Fe3*/(Fe3*+Al) in the tetrahedral Fig. 2. Graphical representation of chemical analyses of sites. biotite, phlogopite,and ferri-annite. Micas A and B refer to ferri- The compositional variations of the ferri-annites annites of group A and B, respectively. The data for biotite and of this study may be expressedby four end-mem- phlogopite are taken from Deer et al. (1962).The area enclosed bers: annite (K2Fee*Al2Si6O2o(OH)4),ferri-annite by a dashedline representstrioctahedral micas with tetrahedral (K2Fee*Fel*Si6O20(OH)4), phlogopite (KzMgo ferric iron of Foster (1960). FeOx indicates all Fe in micas recalculatedas FeO. Solid line with An markingsshows the ideal AI2Si6O20(OH)4),and ferri-phlogopite (KzMgo composition along the annite (An 100)-ferri-annite(An 0) join. Fel*Si6O20(OH)+),in terms of the ratios of Fe2*/ (Fe2++Mg;and Fe3+/(Fe3*+Al;as shownin Figure 4. The variations are generally limited within rela- groups,A and B, by usingthe ratio Fe*/(Fex + Mg) tively narrow rangesof the ratios of Fez*l1Fe2n + where Fe* indicates total Fe. Such ratios for ferri- Mg) and Fe3*/1Fe3*+Al).The one exceptionis the annite in groups A and B are nearly the same as Fe3*/1Fe3*+Al) ratio of mica sample DE, which thosefor stilpnomelanein the respectivegroups as rangesfrom about 0.4 to 0.7. As seenin Figure 4, shownin Figure 3. Accordingto Miyano (1982),the Fe/(Fe+Mg) ratio increaseswith decreasing/e,, which implies that stilpnomelane in group A may reflect higher,fo, conditions than that in group B. This same ratio in ferri-annite, however, seems to be independentof variations of fo,, as judged from its occurrencewith both hematite and/or magnetite. The number of ions per formula unit, recalculated on the basis of 22 oxygens and assumingno tetrahedral Fe, shows a cation of deficiency in the tetrahedral sites(less than 8 by 1 to 2 cations)and a cation excessin the octahedral sites (greater than 6 by I to 2 cations). This means that the tetrahedral Al- position is in part filled by ferric iron. Such a stilpnbmeloneB substitution has been proposed for some natural biotites by Foster (1960).Her micas, with tetrahe- 40 50 70 80 90 Mgo FeO'* MnO dral ferric iron, are plotted in Figure l. Steinfink (1962)determined the crystal structure of a natural Fig. 3. Graphical representationof chemical compositions of phlogopite, phlogopite (ferri-phlogopite) biotite, ferri-annite, and stilpnomelane. FeO* iron-rich with some indicatesall Fe in micas recalculatedas FeO. Groups A and B of ferric iron located in the tetrahedral sites. Synthetic ferri-annite and stilpnomelaneshow the distinctive ratios of Fe/ ferri-annite with tetraheral iron also has been re- (Fe+Mg). I 186 MIYANO AND MIYANO: FERRI-ANNITE FROM IRON.FORMATION

Table 4. Representativeelectron microprobe analysesof stilpnomelane(group A) coexisting with ferri-annite and/or riebeckite

I 2 J4 f,ot 8 9r0 wt. E 2262 2265 2259 2268 z258 8447 8423 8432 8424 8426

sio2 47.61 46.30 47.28 45.82 46.32 49.83 47.87 49.62 49.45 48.79

AI203 4.19 4. 53 4.34 5.05 5 .25 4. 3t 4. 51 4.39 4.44 4.25

TiO2 0.02 0.02 0.00 0.0I 0.00 0.00 0.03 0.02 0.00 0.01

Cr2O3 0.00 0.0r 0.00 0.00 0.00 0.00 0.01 0.00 0. 00 0.00

FeO* 26.45 26.92 25.88 25.9I 24.60 24.I0 25.I5 25.45 24.27 24.67

MnO 0.I0 0.I2 0.09 0.08 0.25 0.00 0.03 0.01 0.00 0.00

Nio 0.00 0. 00 0.01 0.00 0.00 0.00 0.01 0.05 0. 00 0. 00

Mgo 9.43 9. 85 r0.87 10.87 11.05 9.50 9.65 9.54 9.67 9.70

0.0I 0.03 0.02 0.01 0.01 0.04 0.05 0.01 0.03 0.04

Na2O 0.04 0.00 0.15 0.00 0. II 0.00 0.51 0. 13 0.54 I.49

K2a L. t) J.I) 4.94 5.82 7.03 2.I5 2.88 3.81 5.46 7.20

Total ;; 93.58 93.s8 94.62 89.93 90.70 93.13 93. 86 96. Is

cations on the basis of 22 oxygens

si, 7.477 7.276 7.249 7.086 7.087 7.666 7.437 7.513 7.477 7. 338

AI 0.523 0.724 0.75I 0.9r4 0.913 0.334 0.563 0.487 0.523 0.662

I 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000

0.253 0.t-L5 0.033 0.008 0.0 34 0.447 0.263 0.295 0.268 0.091

'fa 0. 002 0.002 0.000 0.000 0.000 0.000 0.004 0.002 0. 000 0. 001

Cr 0.000 0.001 0. 000 0.000 0.000 0.000 0.001 0.000 0.000 0.000

F"2* 3.474 3.538 3.3I9 3.3s1 3.148 3.I0I 3.258 3.223 3.069 3.103

Mn 0.013 0.016 0. 012 0. 010 0.032 0.000 0.004 0.001 0.000 0.000

Ni 0.000 0.000 0.00r 0.000 0.000 0.000 0.001 0. 006 0.000 0. 000

Mg 2.208 2. 308 1.+6) Z.)Ub z.azL z.!t9 z.z5a z.Lto z.L6U Z.Lt> ------_ -- IA 5.950 s.980 l.df,u ).6/) ). /J) 5- tZt 5.776 5.704 f.lI/ ).J/u

0.002 0.005 0.003 0.002 0.002 0.007 0.008 0.002 0,005 0. 005

0.012 0.000 0.045 0.000 0.033 0.000 0.154 0. 038 0.158 0.434

K 0, 35t 0.632 0.966 1.148 I.372 0.422 0.571 0.73e I.U)J I. JUI

LD 0.35s 0.637 1.014 I.150 L.407 0.429 0. 733 0.776 tr* ,*^

'A+TB 6. 315 6.6]-7 6.864 7.025 7.L42 5.155 6.509 5.480 6.733 7.tgL

Fe*/Fe*+Mg 0.511 0. 605 0.572 o.572 0.s55 0.587 0.594 0.597 0.585 0.588

*Al-1 Fe as FeO. Analyzed at A.C.U.T. Sample Z O Eo 5):StiI(brown)-Mica-Mt-Rk(+Ho)-et. rt is possible that some compositions are a mixture of stilpnomelane and ferri-annite (see text). Sampte B (6 to I0): Stil(pale green)-Hm-Qt-An-Rk-M-t. Note variable K2o contents. 1: From Miyano (1982) . MIYANO AND MIYANO: FERRI-ANNITE FROM IRON.FORMATION I 187

Table5. Representativeelectron microprobe analyses of stilpnomelane(group B) coexistingwith neitherferri-annite nor riebeckite

I 2 3 6 7 I wt. t J2 88 J299 J295 J286 J298 FH3 FH4 FH5

sio2 45.02 45.42 45.36 44.9r 44.75 46. 08 45.76 46.34 A120 4.O2 4.59 4.77 4.65 4.9L 5.16 5.17 5. 7r 3 Tio2 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0. 08

FeO* 33.5 2 32.54 32.33 32.97 32.29 32.82 32.42 32.19

MnO 0.2r 0.05 0.04 0.06 0.03 0.00 0.r3 0.10

Mgo 4.49 4.73 5.0r 4.75 5.03 3.76 4.00 3.89

CaO 0.23 0.09 0.02 0.01 0.04 0.03 0.o2 0.07

Na2O 0.24 0.42 0.48 2.03 1.43 0.23 0.23 0.19

K2o 2.32 3.02 3.66 4.64 5.r3 r.44 I. 39 r. 49

90.05 90.86 9r.67 94.02 93.6r 89.52 89.r3 90.06

cations on the basis of 22 o:

5t- 7. 39r 7.364 7. 310 7.176 7.165 7.48r 1.458 7.446

A1 0.609 o. 636 0.690 O.824 o. 835 0.519 0.542 0. 554

T 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000

AI 0.169 0.24r 0.216 0.052 0.091 0. 46I 0.451 0,527

Ti 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.0r0

Fe2+ 4.602 4.4L2 4.357 4.406 4.324 4.456 4.4L9 4.326

Mn 0.029 0.007 0.005 0.008 0.004 0.000 0.018 0.014

Mg 1.099 1.143 t.204 1.131 r.201 0.910 0.972 0.932

5.899 5.803 5.IdZ ).f,vl 5.620 5. 834 5.861 5. 809

Ca 0.040 0.016 0.003 0.002 0.007 0.005 0.003 0.012

Na 0.076 0.r32 0. r50 0.629 0.444 o.072 0. 073 0. 059

K 0.486 0.625 0.752 o.946 1.048 0.298 0.289 0.305

XB 0.602 0.773 0.905 L.577 L.499 0. 375 0. 365 0.376

EA+'B 6.501 6.576 6.687 7.7r4 7.Lr9 6.209 6.226 6.185

Fe*/Fe*+Mg 0.807 0.794 0.784 0.796 0.783 0.830 0.820 0.823

*AI1 Fe as FeO. Sample J (1 to 5) was analyzed at A.C.U.T. and sample FH (6 to 8) at G.S.J. Sample J: Stil-An-Mt-PY. SamPIe FH: Stil-An-K-Fel-Py-Po. I 188 MIYANO AND MIYANO: FERRI-ANNITE FROM IRON.FORMATION

'ferri-onnite' Annite x2rel+ atrsi6o2o(oH)4 Fe3+/(Fe3++At) x, refi+r e3r+si.oro{o H )o 20 40 60

80 80

60 60 ol + + de o, O) -9 lr o +< E (\l fl40 40

oz EDE 20 AGE 20 .MA XMB

KzMge Al2Si5O2g(OH)a mole o/o K2Mg5Fe!+si6o2o(oH)4 Phlogopile 'ferri-phlogopite' Fig. 4. Quadrilateral representationof ferri-annite. one may identify one group of the ferri-annites as the octahedral sites, it is treated as ferrous. Figure 5 annite and the other as ferri-annite, becausethe shows a more realistic representationof the chemi- Fe2*l(Fe2* +Mg) ratios of both groups are grearer cal composition of the ferri-annites than does Fig- than0.5. ure 2 and also plots not only all ferri-annites but After having subtracted the calculated ferric iron stilpnomelaneas well. content from total FeO, the ferri-annite compositions may be depicted in terms of cations in the X-ray crystallography interlayer (CaO + Na2O * KzO), octahedral ((FeO) + MnO + MgO), and tetrahedral(Al2O3) positions, Becauseof fine grain size of the massive aggre- as shownin Figure 5 (A, B). If any iron is presentin gates and the limited amount of material, neither MIYANO AND MIYANO: FERRI-ANNITE FROM IRON.FORMATION

Al2o2 net. Three samples,GE, MA, and MB, taken from this type of layer, were carefully separatedunder the microscope.Samples GE and MB include very small amounts of magnetite, ankerite, and riebeckite. A small amount of ankerite is present in sample MA. However, hematite and quartz could not be completely removed from samples MA and MB because small grains of both minerals are present within the layers. The samples were analyzed by X-ray powder (FeKa). (a : )'i--)- techniques Using silicon 5.4307A)as an internal standard, five to eight d-spacingswere measured on a diffractometer. The X-ray powder given in Table 6, and were indexed with a / rMA(..nito data are following unit-cell parame- MB(mico sf:Xr" monoclinic setting. The ters were determinedon sampleMB: a : 5.402(6), 95 100 b : 9.237(4),c : 10.306(7)A, : 99'16'(10),and (FeO)+MnO+MgO F 507.54(6T43,by meansof a least squaresmethod (program uNlcs RsLc-3)using all reflections except At203 113) listed in Table 6. Theseparameters are compa- rable with those of annite (Eugster and Wones, 1962),ferri-phlogopite (Steinfink, 1962), and ferri- annite (Morimoto and Donnay, 196l; Wones, 1963a)(see Table 7). Table 7 shows,however, that the b dimension of the ferri-annite is remarkably smaller than that of other Fe-rich micas. The parameters of b and p are rather similar to those of

Al2O3

'-t ..\ ^ \--r St ilpnomelone siltA i \. (other n ./--) iron-formolion) slii (oiher iron- formotions)

80 95 100 CoOtNo2OtK2O (FeO)+MnO+MqO

Fig. 5. Graphical representationofchemical variations offerri- annite and stilpnomelane.(FeO) means that FeO for mica does not include ferric iron estimated from the tetrahedral site occupancy, but all Fe in stilpnomelane is expressed as FeO. Figure A shows the variations of the two minerals in several occurrencesof the Dales Gorge Member. Figure B depicts the variation of stilpnomelanein other iron formations (dashedlines) for comparison. Note the wide variation of K2O content of stilpnomelane.

80 85 90 95 r00 singlecrystals nor concentratesofferri-annite could CoO*No20*K2O FeO'*MnO+MgO be separatedfor X-ray investigation. Accordingly, a -- DolesGorge Member (this study) band relatively rich in ferri-annite within banded ----- Morrolvlombo lron Formotion (Klein E Gole,19Bl) was used for this purpose. Such a Sokomonlron Formotion(Klein, 1974; Klein I Fink,1976) iron-formation - Archoeoniron-formotions (Gole, t98O) layer usually contains small amounts of impurities - - - Gunflinilron Formolion (Flo.on I Popike,1975,1978) such as hematite, magnetite, quartz, ankerite, and Fig. 6. Graphical comparison of chemical variations of riebeckite. Magnetite was removed by a hand mag- stilpnomelanefrom various iron-formations. MIYANO AND MIYANO: FERRI.ANNITE FROM IRON-FORMATION

Table 6. X-ray powder data for ferri-annite (FeKa)

GE MA MB Ferri-annite (ll Ferri-annite' ( 2 ) hkl d(R) r* d(R) r* d(R) r* d(R) r** d (A) r*** I ( INT)

001 10.139 VS 10.163 Itt 10.159 VS 10.r80 100 r0.163 100 100

Lr2 3.686 Vw 3.683 w 3.684 M 3.736 5 3.72I 7 9

003 3.379 M 3.379 14 3.394 5s 3. 388 L7 z) fr3 2.903 wl 2.905 vw 2.950 6 2.959 tr 8

200 2.646 VW 2.666 VW 2.673 4s 2.674 24 I3

133 | 2.044 t2.O28 6 w I 4 Lr.o* '2I 204 .026 5

006 r.696 M 1.699 l0 L.694 4 I

242 I.600 w 1. 611 1 r

060 1.539 w 1.539 W I.539 vW L.567 20 L.567 o f,

GE, Ml\' and MB: this study (trln filter; SIit 10, 0.3, lo; scan speed 0.25 o/mi-ni chart speedt 20nun,/min; T.C. 2; scale 800 cps). (I): Wones (1963), powder data. (2): Morimoto 4nd Donnay (1961), Donnay et aI. (1954), Borg and smith (1969) singte crystal- r*: rerative intensity: vs=very strong, s=strong, M=mediumt w=weak, vw=very weak. I**: I/To on arbitrary sca1e. I***' peak intensity from the simulated diffractometer trace. r(INT): relative integrated intensity normalized to a maximum value of I00.

Table 7. Cell parametersand refractive indices offerri-annite and related micas

I z J

-phlogopite annite ferri ferri-annite ferri-annite ferri-annite o a (A) 5.391 5. 35 q A1 5.430 5.402 (10.01) (10.0r) (10.011 (10.002) (!0.005)

b (8) 9.348 9.29 9.40 9.404 9.237 (!0.004 ) (10.02) (10.01 ) (10.003) (10. 004) o c (A) 1n ?1 " 10.41 10.33 r0. 341 r0. 306 (r0. 02) (r0.02) (r0.01) (r0.006) (10.007) B 99042' l0ooo' 1000lo' 10004 ' g9016, | (115' ) (i10 ) (115' ) (110'I (110' )

r.oz) I. /UJ I.677 (10.002) (10.00s) (10.005)

r.o>r I.748 L.72L (10.00 2) (10.003) (10.00s)

1. Eugster and Wones (f962), AnFe6, por^'derdata. 2. Steinfink (L962), (K9.9Mng.fludg(Fesi3)o1g(oH)2, single-crystaL data. 3. Morimoto and Donnay (1961t, xre3(Fesia)oi6(orla, single-crystal data. t. Wones (1963), KFe3(Fesi3)o1g(oHi2, poiaei-aatal 5. This study, sample ttB, powder daEa.- MIYANO AND MIYANO: FERRI-ANNITE FROM IRON-FORMATION I l9l biotite(a:5.30, b:9.21, c: 10.16A,andp: Stilpnomelone 99o18';Donnay and Ondik, 1973). (deo'= 11.99a, According to Wones (1963b),the b dimensionof synthetic biotite on the phlogopite-annitejoin de- creaseswith decreasingFe/(Fe+Mg) ratio, andwith increasingoxygen fugacity. Synthetic ferri-annite used for the determination of cell parameters was formed in the stability field of wtistite and fayalite (probablybetween the MW and WI buffers;Wones, 1963a;Donnay et al., 1964).Because ferri-annite usually coexists with hematite and magnetiteand commonlycontains MgO (Fe*/(Fe* + Mg) : 0.75- 0.78, group B), the b dimensionmay be shortened more than that of pure ferri-annite. On the other hand, Donnay et al. (1964)show in their structural model that the length of the K-O bond of ferri- annite restricts the length of b more severelythan either d, (tetrahedral metal-oxygen distance) of do (octahedral metal-anion distance), and that the K- O distance of the mica is surprisingly large (:3.05A). If we assumethat the samevalues for d and do as those of Donnay et al . (1964)for the ferri- annite,our b dimensionl:9.237A) givesthe normal K-O bond lengthof 2.83A.

Origin of ferri-annite Becauseof the intimate associationof ferri-annite and stilpnomelane,the chemical composition of the 6789 mica may well be related to that of the stilpnome- Degrees2 I lane. Especially,the similarity of the ratio of Fe*/ Fig. 7. X-ray reflections(d-, in A) of stilpnomelaneand ferri- (Fe* + Mg) between the two minerals is remarkable annite(CuKa). as alreadymentioned. In one sample (Z) a feni-annite layer grades transitionally into a stilpnomelanelayer, and thin found in stilpnomelanesclose to a fibrousriebeckite fibrous riebeckite and magnetitebands (0.2 to 0.3 band (Table 1, sampleB), where the X-ray powder mm thick) are present between the two layers. diffraction pattern shows no extra peak for mica. Magnetiteoccurs in both layersbut hematiteonly in Generally, stilpnomelaneswith high K2O contents the mica layer. The stilpnomelanelayer consists (>3 wt.%) havecommonly been reported from iron- mainly of aggregatesof interlocking laths of stilpno- formations with a large amount of riebeckite (La- melane and partly of Fe-rich mica as judged from Berge, l966a,b; Trendall and Blockley, 1970:' the X-ray powder diffractionpattern (Fig. 7). How- Ayres, 1972;Klein and Gole, 1981;Miyano, 1982; ever, the chemicalanalyses of the laths do not show also seeTables 4 and 5). The variation of the KzO the typical compositionof either stilpnomelaneor content of stilpnomelanein the Dales Gorge Mem- ferri-annite because they contain a higher K2O ber is shown in Figure 5,{. Comparisonwith those content than "normal" stilpnomelane whereas the of other iron-formations is made in Figures 58 and other oxide contents are similar to those of stilpno- 6. These figures show that stilpnomelane of the melane(Table 4, sampleZ, anal.2 to 5). Becauseof Dales Gorge Member is more alkali-rich than that of the presence of X-ray reflections for mica, it ap- other iron-formations. A similar tendency is found pears that the composition may be equivalent to a in riebeckite-containing assemblagesin the Marra mixture of stilpnomelane and ferri-annite. As Mamba Iron Formation (Klein and Gole, 1981)' shown in Table 4, the K2O content rangesfrom 1.8 Miyano (1982)proposed that ferri-annite (and also to 7.0 wt.Vo. Such a range in K2O content is also some riebeckite) may replace stilpnomelaneas a' n92 MIYANO AND MIYANO: FERRI-ANNITE FROM IRON-FORMATION

o Z, mico x MB, mico o Z, stilpnomelone o MA, mico Z, stilpnomelone o J, slilpnomelone " (tronsifionol ) A FH, slilpnomelone A B, slilpnomelone

o o .l' ao oo o t a o a OA oe c' s.o o o4 E ^ca o e-^a N o o,A.. z. A^ o" a + 4.o o A N cA )< ga o AAA -oo o "s"

oo 2.O zn- oP oa A A L r.o

GroupA GroupB

o.4 0.5 0.6 0:7 0.8 0.9 t.o Fe*/(Fe* t tuts) Fig. 8. Compositionsof stilpnomelaneand ferri-annite in terms of K2O + Na2Ocontent (molVo)and Fex/(Fe+ + Mg) ratio. The area below the dashed line shows the K2O content of "normal" stilpnomelane. In sample Z, the K2O content (group A) increases continuously from that of "normal" stilpnomelaneto that offerri-annite. MIYANO AND MIYANO: FERRI-ANNITE FROM IRON.FORMATION I 193 result of interaction with alkali-bearingsolutions, mica may have formed from stilpnomelane by en- which may also have been responsible for the richment of potassiumduring metamorphism.Be- formationof riebeckite.This interactionappears to causeof cation deficiencyin the tetrahedralsites if accompanyenrichment of alkaliesin stilpnomelane tetral Fe is excluded,the mica compositionmay be prior to replacement by ferri-annite. However, expressed by quadrilateral components: annite, someparts of the DalesGorge Member rocks seem phlogopite, ferri-annite (K2Fe?*Fe]*Si6Oz0(OH)4) not to have been affected by such alkali-bearing and ferri-phlogopite(KzMgoFel* Si6O20(OH)4). Cell solutions,because the K2O content of stilpnome- parametersof the mica are, however, somewhat lanein such rocks is "normal" as shown in Table 5 different from those predicted from the quadrilater- (sampleFH, anal. 6 to 8). Using chemicalanalyses al components.The diferences may be due to the listed in Tables l, 2, 4, and 5 and preliminary conditions of formation of the mica. Further refine- analytical data on stilpnomelaneand ferri-annite, ment and characterizationof this mica must wait this featurecan be illustratedas a function of (KzO until pure, crystalline material is available in suffi- + Na2O)mole percentagainst the variation of Fe*/ cient amounts. (Fe* + Mg) as in Figure 8. In the caseof sampleZ (group A), the KzO(+NazO) content of stilpnome- Acknowledgments lane increases continuously toward that of ferri- This work was begunwhile one of the authors(T.M.) was a anniteat a constantratio of Fex/(Fex+ Mg). Figure post-doctoral fellow in 1976 of the Association of Japanese Academic Promotion at Tokyo University of Education' Part of 8 alsoshows mica and stilpnomelanein other occur- this study was supported by a Grant-in-Aid for Encouragement rences,including those in group B. No stilpnome- of Young Scientists (574326) of the Ministry of Education, lane with a high ratio of Fe*/(Fe* + Mg) was found Scienceand Culture of Japan. Further funding was provided by to be associatedwith ferri-annite in this study. NSF grant EAR 80-20377to C. Klein. Electron microprobes and at However, sampleJ containsstilpnomelane of group were used at the Analytical Center of Tsukuba University the Geological Survey of Japan. T. M. is grateful to A. Sasaki B and it has a wide variation of K2O content as and K. Okumura at the Geological Survey of Japan and to N. shown in Table 5, probably as a result of action of Nishidaof TsukubaUniversity for their help in obtdiningmicro- alkali-bearingsolutions. The X-ray powder diffrac- probe data. We thank Y. Suzuki, S. Sueno,and M. Kimata for tion pattern of this stilpnomelaneshows only stilp- obtainingX-ray powderdata; Messrs. W. H. Moran, R. T. Hill, photography of the illustra- nomelanepeaks. This stilpnomelanemay be regard- and G. R. Ringer for drafting and Thea Brown for the typing of the manuscript;and C. group tions;Mrs. ed as a precursor of the ferri-annite of B Klein and A. Kato for their critical and constructive reviews (samplesMA and MB), becausethe variation in which considerablyimproved its content. K2O content from stilpnomelaneto mica is similar References to that demonstratedfor sampleZ (Figure8). It is of interestthat the stilpnomelaneof samplesB (group Ayres, D. E. (1972)Genesis of iron-bearingminerals in banded Gorge Member, Ha- A) and J (group B) which contain little ferri-annite, iron formation mesobandsin the Dales mersley Group, WesternAustralia. Economic Geology, 67, showsa relatively higher Na2O content than stilp- t2l4-12f3. nomelanethat coexist with ferri-annite.It appears Borg, I. Y. and Smith, D. K. (1969)Calculated X-ray powder that much of the ferri-annite may have formed from pattems for silicate minerals. Geological Society of America, stilpnomelaneby secondaryenrichment of potassi- Memoir 122. Donnay, J. D. H. um at a fixed Fe*/(Fe**Mg) ratio, and that Fe2* Donnay, G., Morimoto, N., Takeda, H., (1964)Trioctahedral one-layer micas. Acta Crystallographica, to Fe3+in order to fill the may have been oxidized t7 , r369-lf8r. cationdeficiency in the tetrahedralsites. It is proba- Donnay,J. D. H. and Ondik, H. M. (1973)Crystal Data. Deter- ble that such potassiumenrichment and oxidation minativetables 3rd ed., Vol. 2, InorganicCompounds. U. S. may have been brought about by alkali-bearing Department of Commerce, National Bureau of Standards (Nsnps). solutions,but the presenceof "normal" stilpnome- Eugster,H. P. and Wones,D. R. (1962)Stability relations of the lane shows that such enrichment has not been femrginousbiotite, annite.Journal of Petrology,t,82-125. uniform throughoutthe entire member. Foster, M. D. (1960)Interpretation of the composition of trioctahedral micas. U. S. GeologicalSurvey ProfessionalPaper 354- Conclusions \ B, ll-49. (1981) and petrologyof Ferri-annite, from a very low-graile metamorphic Klein, C. and Gole, M. J. Mineralogy parts of the Marra Mamba lron-Formation, Hamersley Basin, iron-formation,the Dales Gorge Member of West- Western Australia. American Mineralogist, 66, 507-525. ern Australia, is definedon the basisof mineralogi- LaBerge, G. L. (1966a)Altered pyroclastic rocks in South cal, chemical,and X-ray powder data. Much of this African iron-formation. Economic Geology, 61, 572-581. tl94 MIYANO AND MIYANO: FERRI.ANNITE FROM IRON.FORMATION

LaBerge, G. L. (1966b)Altered pyroclastic rocks in iron-forma- Veres,G. I., Merenkova,T.8., and Ostrovskii,I. A. (1955) tion in the Hamersley Range, Western Australia. Economic Synthetic pure iron hydroxyl mica (in Russian). Akademie Geology,61,147-161. Nauk SSSRDoklady, l0l, 147-150. Miyano, T. (1982)Stilpnomelane, Fe-rich mica, K-feldspar,and Wones, D. R. (1963a) Phase equilibria of "ferriannite", hornblendein bandediron-formation assemblagesof the Dales KFer*2Fe*3SirOro(OH)2. AmericanJournal of Science,261, Gorge Member, Hamersley Group, Western Australia. Cana- 581-596. dian Mineralogist,20, 189-202. Wones,D. R. (l%3b) Physicalproperties of syntheticbiotites on Morimoto, N. and Donnay,J. D. H. (1961)Crystal structureof the join phlogopite-annite.American Mineralogist,48, 1300- synthetic iron mica. CarnegieInstitution of Washington year 1321. Book 1960-1961,214-215. Steinfink, H. (1962) Crystal structure of a trioctahedral mica. American Mineralogist, 47, 886-896. Trendall,A. F. and Blockley,J. G. (1970)The iron formationsof the PrecambrianHamersley Group, Western Australia. Geo- Manuscript received, October 12, I98l; logicalSurvey of WesternAustralia. Bulletin l19. acceptedfor publication, June 18, 1982.