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 compo- nents 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, stilpnome- lane, 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.