Ameilcan Mineralogist, Volume 59, pages 82M29, 1974
AenigmatiteFrom Mt. Edziza,British Columbia,Canada Krnzo Yecr, Departmentof Geologyand Mincralogy,Hokkaido Uniaersity, Sapporo, Iapan
eNo Jacr GonnoN Sournrn Geological SartteY ol Canada, V ancouuer' Brilish Columbia, C arwda
Abshact
Aenigmatite is a ubiquitous constituent of the highly fractionated, peralkaline end members of the Mt. Edziza volcanic complex in northern British Columbia. In comendite lavas and tephra it occurs as phenocrysts with aegirine, aegirine-augite and arfvedspnite, whereas it is confined to the groundmass of trachytes. This is attributed to different for's during the intra- telluric and gtoundmass stages of crystallization of comendite and trachyte. Electron microprobe analyses indicate that both groundmass and p\enocrystic aenigmatite havp similar compositions that are close to those of aenigrnatite from o-ther localities' The absence of intermediate compositions between aenigmatite and rhoniteusuggests that solid solution between the two minerals is lacking or limited by a wide range gf immiscibility'
Geological Setting with explosive dischargeof pur,niceand ended with short, thick Mt. EAziza (Fig. 1) is the largest in a group of upwelling of vispousfEva as domes and of Mt. Edziza volcanic centersalong a north-southzone of normal flows. The latter comiriqe the edifice the surface faults that cut diagonally across the Cordillera of proper, which rises 1500 meters above composite northwestern British Columbia (Souther, l97O). of the basaltic shield as a steepsided p summit cratet I l/2 The entire complex, which covers an area of 2,0@ dome surmountedby circular was confined square kilometers, includes a broad spectrum of km in diameter. Stage three activity periphery of the central alkaline and peralkaline rocks ranging from picritic to satellitic vents around the flows qf olivine-rich stage- alkali olivine basaltthrough trachyteto sodicrhyolite dorne.Effusion of blocky shower of (comendite). The first flows issued from a cluster three lavas was followed by a 5ingle all activity to an end. of vents south of the present crater of Mt' Edziza rhyolitic pumice that Brought in Late Miocene time and give K/A agesof abotrt PehograPhY 9 m.y. (Wanless,'personal communication). Erup tions continued periodically throughout Pleistocene A full treatment bf ttre petrochemistry of the and into Recent time. The youngestdated flow has F/rzrr;a lavas is being prepared by Souther. In a Cr. age of 1340 -F 130 years B.P. (Souther, summary, the basalts have the 5imple mineralogy 1967) and a still younger, but undated pumice fall typical of alkali olivine basalts: moderately zoned is estimatedfrom its state of preservationto be less labradorite, clinopyroxene,olivl4e, and titaniferous than 500 yearsold. magnetite. The mineralogy pf the trachytes and Three principal stagesof eruption are recotnized rhyolites is characteristicof perplkaline rocks, and (Souther, 1972). Each stagebegan with the volumi- includes such soda-rich minerafpas aegirine-augite, nous outpouring of fluid alkali olivine basalt and aegirine, arfvedsonite,and aenigmatite' No ortho' culminatedwith eruption of a relativelysmall volume pyroxene was found in any of more than 300 thin of more acid magma. The basalt formed flatJying iections examined. Average valges of 70 modal in shields associatedwith pmall composite cones and analyses and 90 chemical analyses are shown local piles of sub-glacialtuff-breccia and pillow lava, Table 1. whereas eruptions of trachyte and rhyolite began In general the stageI basalts are ophitic to sub- 820 AENIGMATITE FROM MT, EDZIZA, CANADA 821
ophitic, with laths of plagiocla$e(An7o to Ansn), granules of olivine (Fo,:) and titanomagnetiteen- closed in deep purple titanaugite. No transitional rocks intervene between qtageI basalt and stage I comendite (sodic rhyolite). The latter forms ash- flows, ash falls, and lava do4es; all of which con- tain abundant phenocrystsof quaftz, sanidine, and anorthoclase and sparse phenocrysts of aegirine- BRIIISH augite, aegirine, and aenigm3tite.Stage II basalts commonly havg 3n intergranular texture in which subhedral to euhedral plagloclase, strongly zoned cot.uAlBtA titanaugite, olivipe, and titaniferous magnetite all occur as both groundniassdnd phenocrysticminerals in a glassy matrix. They $rade through trachy- basalt-in which titanaggite is rimmed with aegirine- augite-to trachyt6, in' which aegirine (associated with aenigmatitband arfvedsonite) is the principal pyroxene. Stpge III baqalts are commonly porphy- ritic, with phenocrysts of plagioclase, titanaugite, and abundantolivine in 6'glassyor finely crystalline Frc. 1. Index map. matxix highly chargedwith magnetitegranules. The
Taslr l. Average Modal and Chemical Analyses of Mt, Edziza Lavas
No. of Speclrnens Stage I Stage I Stage 2 Stage 2 Stage 3 Basalt Comendite BasaIt Trachyte Pasalt 10 4 10 5 5 Plagioclase (An5s-An79 ) 54.4 46.0 'l_l Plagioclase (Sodic) 35.4 'I 53.7 Anorthoclase 4 q trace Sanidine 1-1 L2.6 l-l .---. Quartz 'l_l Titanaugite 17.8 29.8 24.2 Olivine 5.2 2.2 15.3 Magnetite '?-_! 2.6 9.8 0.5 Aegirene-augite 'i_l r.2 ':::" 9.4 Aegirine 2.O 4.4 Arfvedsonite trace 1.9 Aenigmatite 1.1 L2.5 Glass :-: 'lo i:; 8.5 f,.1- r6. 5 Alteration products 5.9 5.2 3.5 1. t 0.9 sio2 46.2 74.8 46.5 64.L 48.6 TiO2 3. 06 0.16 2.88 0. 45 2.tI A1203 16.4 12.T 15.4 L3.7 17.3 Fe2O3 3.0 3.0t 6.2 8.5* 2.2 FeO 9.4 n.d. 5.9 n.d. 9.2 MnO 0.I8 0. 04 0. 21 0.18 0.I5 Mgo 3.9 0.4 4.6 0.7 7.L CaO 8.3 0.3 9.8 L.2 Na2O 8.4 3.3 5.0 2.7 s.7 3.5 Kzo L.4 3.9 0.7 Hzo 4.5 1.3 1.3 n.d. I.7 n.d. 0'6 Pzos 0.73 n.d. 0.3r coz 0.49 2.5 n.d. 0.7 n.d. 0.2 *indicates total Fe calculated as Fe2O3. 822 K. YAGI, AND I. G. SOUTHER it final, phase III rhyolitic -pumice is completely ubiquitous constituent of the trachytes, where vitreous. may form up to 15 percent of the rock' and occurs The aenigmatite-bearingtrachytes and rhyolites sporadicallyas a minor constituentof the rhyolites' of Mt. Edziza are considired to be the late frac- In holocrystalline rhyolite lavas, the aenigmatite asso- tionation products of a parent alkali olivine basalt forms tiny (*1 mm) intergranular euhedra plagioclase,an- magma with the initially high content of TiOz and ciated with quartz, sanidine, sodic magnetite' Fe5 and unusually low VtgO that is characteristic orthoclase, aegirine-augite,aegirine, and crystal of the Edziza. shield basalts. The trachytes and The aenigmatite commonly exhibits sharp be rhyolites themselveslie close to the boundary be- boundariesagainst aegirine which may, in turn, (Fig.z A andB)' tween alkaline and peralkalinelavas as definia by partly surroundedbyarfvedsonite as dis- Shand(1951) withmolecular (NazO l- K2O) equal This same assemblageof mineralsis found to or slightly greater than Al2Os. Occurrence of Aenigmatite Aenigmatite is confined, on Mt. Edziza, to tra- chytes and rhyolites of stageI and stage II. It is a many comenditeflows is chargedwith tiny, oriented
of aenigmatite (black) partly Frc. 2. photomicrographs of aenigmatite-bearingrocks from ly'rt. Ed:aza A. Euhedral crystals (dark gray). From stage I comendite flow' B' enclosed by aegirine. Aegirine crystal at left is partly surrounded by arfvedsonite stage I comendite lava dome' Euhedral crystals of aenigmatite (black) completely surrounded by a narrow rim of aegirine. From glass. From chilled base of lava flow' D' C. Microlites of aegirine (longer of two crystals) and aenigmatit" io .tug" I comendite II trachyte' This section was used for Interstitial aenigmatite(black) and aegirine (dark gray) in the vitreous groundmassof stage microprobe analysisof groundmassaenigmatite (Table 4, sample 2)' AENIGMATITE FROM MT. EDZIZA, CANADA
Fto. 3. Electron micrographs of aenigmatitephenocrysts from stageI comendite ash. Thesecrystals are from the sample used for optical and X-ray study (Tables 2 and 3) and for chemical analysis(Table 4, sample 1). A. Crystals of aenigmatite afteisepara- tion with methyleneiodide. B. Close view of single aenigmatite crystal showing well developed(100) and (010) faces. C. Det;il of crystal face (100) showing lenticular voids. Note that the voids are oriented along both cleavagedirections, one in the plane of the photograph and the other at approximately right angles to it. D. Close view of lenticular voids parallel to (0r0).
microlites of plagioclaseand slightly larger, acicular comprising radiating acicular crystals of feldspar, crystals of aegirine and aenigmatitewhich appear aegirine, and aenigmatite, obviously formed after to have crystallized simultaneously(Fig. 2 C). the flow was essentiallysolid. In holocrystalline trachytes, aenigmatite rarely It is noteworthy that aenigmatiteis found mostly forms phenocrysts. It occurs with aegirine and as phenocrystsin the comenditesand as a ground- arfvedsonitein a groundmassof ragged,intergrown mass constituent of the trachytes. The genetic sig- anhedralcrystals that are interstitial to the feldspar nificance of this is discussedlater. laths (Fig. 2 D). Glassytrachyte from the chilled base of one flow contains plagioclasephenocrysts Optical Properties up to 2 mm long and smaller microlites of aegirine Due to its small size, groundmass aenigmatite and aenigmatite.Locally the glass has spherulites was not usedfor the determinationof optical proper- 824 K. YAGI. AND T. G. SOUTHER ties. The properties listed below were measured TlsrB 3. X-Ray Powder Data for Aenigmatite on phenocrystsfrom a'non-weldedash fall of highly Mt. Edzizq Kolc Mt. Edzizq Kolq expandedcomendite pumice. The aenigmatiteforms dA I eirairait prisms, 1. to 4 mm in length, with well elongated 8.1I 8.il vs0) 2.073 25 2,073 w developedfaces on (100), (010), (110), and 7,43 W 2.010 25 2,007 MW 6.38 W t5 1.973 Ww (110), and perfect cleavageparallel to (010) and A,84 30 4.82 \\' I .948 t5 r.947 WW 1,41 20 4,40 W l.9ll 1.912 \^V (100). In thin sectionsof phenocryststhe cleavage 4.20 25 4,20 w 1.802 wr'B 3.7@ 30 3.704 MW 1.772 Ww traces appear as einuous or undulatory dark bands 3.491 25 3.484 W 1.732 20 1,731 wB the electron microscope,are seen to 3.1s2 100 3.144 VS(2) 1,678 l0 1.680 w which, under 2.940 45 2,939 M5 1 Aaa be lenticular voids aligned along both cleavagedi- 2,813 25 2,8M VW | .6lJ 25 1.627 M 2,754 25 2.753 \ v l.6ll t.6r0 w (Fig. have averagedi- 75 2,70s s(3) 1.590 ww rections 3 C, D). The voids 2.707 1/ 2.655 20 2.658 WW l0 1.560 \^ mensionsof about 0.5 x 5.0 microns. They may 2.546 60 2.545 S 1.518 20 1.514 w 2.453 20 2,442 \ v I .495 25 L498 MW be analogous to the pores in "light wolframite" 2.117 35 2.413 M 1.480 25 1.454 MW (Escobar et al, l97I) which are attributed to in- 2.345 15 2,347 WV 1.478 20 2.307 15 2.310 w 1.471 corporation of water and gassesduring crystalliza- 2.'t93 l0 2,197 VVw I .46S 1.168 MB 2,120 60 2,119 MS 1.445 35 tion. Similar voids were no observedin the ground-
mass aenigmatite. Mt. Edzizo: Pres€nlsludY Optical pr,opertiesof the analyzed phenocrystic Kolo: Keley ondMcKie (1964) aenigmatiteare given in Table 2. Tlte optic plane is parallel to (010). Both groundmassand pheno- McKie (1964) made a careful X-ray analysis of crystic aenigmatitehave very strong absorption and aenigmatiteand determinedthe unit-cell dimensions' are almost opaque in the Z direction. Dispersion is The results of the present study are in good agree- strong with r ( u and, in some sections,an ano(na- ment with their values, except for a few peaks of lous bluish-$ey interference color is noted. The weaker intensity. From the shape and dimensions refractive indices are very high and were measured of the unit-cell, the pattern of aenigmatite is un- by the immersion method, using standard glass indexable.Kelsey and McKie determinedonly three powders in methylene iodide with dissolved phos- peaks with larger d-spacingsas follows: 8.11 A, phorous and sulfur (Sueno, 1933). Because of 001 and 010; 7.43, 011; 6.38, 111. Of.these, the its strong pleochroism and high refractive indices second and the third peaks are missing in the the error of measurementwas high. aenigmatiteunder investigation. Kelsey and McKie (1964) suggestedthat aenig- X-Ray Analysis matite is triclinic, and its structure is based on sili- X-ray powder spectrographsof aenigmatitewere cate chains of pyroxene type crms-linked by Tia- obtained at 30 kV, 2O mA, scanning speedof l/2 and Fe2*.More recently Canillo et aI (1971), in deg.20 per minute, scalefactor and a time constant their detailed study of the structure of aenigmatite, 8. An external quartz standard was used for cali- showedthat its silicatechains are of sapphirinetype' bration. The average d value from four determi- composedof pyroxene-like chains, and that forma- nations is given in Table 3, column 1. Kelsey and tion of twins, frequently oherved in volcanic aenig- matite, is interpreted as due to the quenchingeffect of crystallization under rapid cooling. Phenocrystic Aenigmatite Ttsr-n 2. Optical Properties of the was analyzed by Mr. from Mt. Edziza The F;dz:a;aaenigmatite A. G. Plant of the Geological Survey of Canada, PLeochrolsn using a Materials AnalysisCompany electronmicro- at 20 kV and with a specimencur- c 1.800 r 0.005 x 1lght yellowlsh brown probe operated B 1.813 r 0.005 v reddlsh brown rent of 0.035 JrA. Three phenocrystsfrom stage I - black y 1.88 t 0.01 z deep reddish brown comendite and ten grains from the groundmassof Each eAZ 43" Absorptlon X
(sirr.e4Alo.06)>rr.ooono oo, I Mt, Edzizo (pherccryrt) (this study, 2 Mi. Edzizo (goundmss) (this study) 3 Pont.lleriq (Cormichoal.1962) 4 Kolo (Kelrcyond McKie, 1961) 5 Greenlond(K€lseyond McKic, both of which are in good agreementwith the gen- l 4) 6 NondeworfAbbdt, l95n 826 K, YAGI. AND T. G. SOUTHER
reported occurrenceof this mineral in basalticr,ocks. There aenigmatite locally and selectively replaces ilmenite lamellae in ilmenomagnetiteformed simul- taneouslywith or later than sodium-richclinopyrox- enes.All these relationshipssupport very late stage formation of aenigmatite. Experimental studies of aenigmatite have been carried out in severallahratories. Ernst (1962) formed "Ti-free aenigmatite" at the lo2 of wustite- magnetite buffer, but not at that of the fayalite- magnetite-quartz buffer. The synthesized crystals properties very similar to those of natural Fe2'*Ti1* have and this clearly indicates that Ti is Frc. 4. Relation between Fee* and Fea -F Tin* cations in aenigmatite, unit formula of aenigmatites. not essential. Ernst tentatively gave the formula Na4Fe2*7Fe3*6Si12O'a2to this Ti-free analogue, fol- Abbott (1967) in the aenigmatite-bearing trachyte lowing Fleischer'sformula, but this should now be from Nandewar, New South Wales. He attributes expressedby NaaFe'z-sFe3*+SirzOaoafter Kelsey and the low TiO2 content of groundmass aenigmatite to McKie's formula. Formation of "Ti-free aenigma- extraction of Ti from the liquid by precipitation of tite" raised the question of whether or not there is titanomagnetite or other Ti-bearing minerals in the a solid solution series between Na+Fe2*roTr,zSirsOro phenocrystic stage of crystallization. This explana- and NtuFe'*eFe3*+SirzOao.Figure 4 indicates a re- tion is consistent with the petrography of the Ednza ciprocal relationship between the number of Fest rocks, many of which contain early formed pheno- cations and the total (Fe2. plus Ti) cations in the crysts of both titanomagnetite and titanaugite. Also, unit formula. This, and the fact that low Ti in the the chemical formulae suggest that Ti is replaced groundmassaenigmatite frorn Hziza is compensated by Fe3* in the groundmass aenigmdtite. by increasedFe3*, supports the possibility of a solid In the syenites and rnonzonites of Morotu, Sak- solution seriesbetween the two above-mentionedend -F halin, Yagi (1953) described a complex zonal members through replacement of Fe'z- Ti = growth of aenigmatite, aegirine-augite, arfvedsonite, 2Fe"'. The Ti content of aenigmatiteis, however, and aegirine in that order from the core to the rather limited, rangingbetween 1.50 and 1.90 cat- margin. Similar zoning is not so well developed in ions per unit formula in all but a few analyses the Edziza rocks; however, the order of crystalliza- (Fig. 5 ). This meansthat the replacementof Fe'z.* tion (aenigmatite, aegirine-augite,arfvedsonite) de- Ti = 2 Fe3*is valid only for a limited range of Ti, duced from partially mantled crystals, is the same. and only aenigmatitesmoderately rich in Ti content The presence of feldspar, aegirine (or aegirine- microlites in obsidian from 1i augite), and aenigmatite 2.5 the chilled margins of aenigmatite-bearing flows sug- gests that the crystallization of these three minerals began simultaneously.Subsequent mantling of aenig- matite euhedra by aegirine-augite and arfvedsonite indicates that crystallization of the latter two min- erals continued after crystallization of aenigmatite. Carmichael (1962) found an antipathetic relation between the amount of aenigmatite and that of fayalite and ilmenite, suggesting that aenigmatite may form by the reaction of fayalite and ilmenite with sodium-rich liquid. Although no evidence has sr ru! been found to indicate this reaction, Lindsley and 60 65 ?.0 75 60 85 90 Haggerty (I97I) described an interesting occur- Fe2+ rence of aenigmatite in the pegmatoid of Picture FIa. 5. Relation between Ti and Fe'z*cations in unit formula Gorge basalt of Oregon, U.S.A., which is the first of aeniematites. AENTGMATITE FROM MT. EDZIZA. CANADA do form in nature, even though a Ti-free analogue been found in various mafic to intermediatealkalic has beenformed in the laboratory. Thereforemagma rocks as a primary mineral or as an alterationprod- with a relatively high Ti content is consideredessen- uct of amphiboles.Rhondite has been regarded as tial for the formation of aenigmatitein nature. isostructural with aenigmatite,because of similari- Following Ernst's experiments, Thompson and ties in optic properties and crystal morpho,logy. Chisholm (1969) synthesizedaenigmatite of ideal- Fleischer ( 1936) suggestedthat rhonite is iso- ized composition. The conditions wete as follows: morphous with aenigmatitethrough substitution of Aenigmatite was formed within 48 hours frorn the CaAl for NaSi and that the two minerals are end oxide mixtures at 700oC, pnzo: 1 kbar and fe, = members of a solid solution series of plagioclase iron-wustite buffer. From the occurrenceof aenig- type.Cameron, Carmen, and Butler (1970) recently matite in the pantelleritesfrom Pantelleria,Nicholls discussedthe substitution between aenigmatiteand and Carmichael(1969) indicatedthat aenigmatite rhonite and, following Fleischer's suggestion,ex- may be stable at oxygenfugacities even three orders pressedintermediate membersby the formula of magnitudeabove the Ni-NiO buffer, but Lindsley (I971) experimentallyshowed that aenigmatitewill [Ca, -"(Na, K),]4 Y12(Al1 -,Si")4Si8O40. dissociateinto assemblagesof acmite, titaniferous The amount of Si calculatedby this formula is, magnetite, and quartz under the lo2 of Ni-NiO however, always greater than that present, from buffer. When these decomposed materials were which they concludedthat if rhonite and aenigmatite hydrothermally heated under the lo2 of. fayalite- do form a solid solution series, then they should magnetite-quartz buffer, aenigmatite was again have a more complex substitution than CaAl for formed. From these results Lindsley (1971) esti- NaSi. mated that the stability curve for aenigmatiteshould In order to investigatethe relation between the lie between the fo, values given by Ni-NiO and two minerals, all existing chemical data, including fayalite-magnetite-quartzbufier curves at 75O"C Ernst's Ti-free aengimatiteare plotted in Ca-Na-K and 500 bars. The fact that aenigmatitecan form and Fe-Mg-Ti diagrams (Figs. 6 and 7), in which from its anhydrous starting materials in evacuated Fe is the total of Fe2* and Fe8.. The diagramsindi- silica-glasstubes at atmospheric pressure suggests cate that aenigmatite and rhonite have their own that 16" rather than total pressurehas the greater fields, each of limited compositionalrange, and that influence on the stability of aenigmatite. there are no intermediatecompositions. Incorpora- Although we have no direct determinationof 111, tion of Ti-free aenigmatite does not materially valuesin these alkalic rocks, Fudali's (1965) esti- change the compositional range of aenigmatite. mation is relevant to the present discussion.Fudali found that lo2 rangesfrom L0-eto 10-6.5atmosphere for basalts and andesitesof the Cascadesat their liquidus temperatures and that there is a strong tendency for the lo2 to increase with increasing SiO2 content. From these data it is reasonableto consider that f s, increasesfrom trachyte to comen- dite and that it also increasesfrom the intratelluric to the groundmassstage within the samemagma as crystallization proceeds.Therefore, the presenceof aenigmatitein the groundmassof trachyte and in the phenocrystsof comenditesuggests that the fer's at these stagesare just suitable for the formation of aenigmatite,whereas lo2's are either too low in the intratelluric stageof trachyte or too high in the groundmassstage of comendite.
Relation between Aenigmatite and Rhonite Nq Rhonite was originally described by Soellner Frc. 6. Flot of aenigmatite and (1907) frorn southern Rhon, Germany, and has diagram. 828 K. YAGI, AND I, G. SOUTHER
Cameron, also of the Geological Survey of Canada, who made the scanning electron photographs; and Mr. S. Terada of Hokkaido University, who prepared the diagrams. References Ansorr, M. J. (1967) Aenigmatite from the groundmass of a peralkaline trachyte. Am. Mineral. 52' 1895-190l. C,luenox, K. L., M. F. C,rnrulx, rxp f. C' Burren (1970) Rhonite from Big Bend National Park, Texas. Am. Mineral. 55' 86+-874. CeNxrrr.o, E., F. l'/lttzzt, J. H, FeNc, P. D. Roonsox' eNo Y. Osve (1971) The crystal structure of aenigmatite. Am. Mineral. 56, 427446. Cmrvrror.lrr., I. S. E. (1962) Pantelleritic liquids and their phenocrysts. Mineral. Mag. 33, 86-1 13. ERNsr, W. G. (1962) Synthesis, stability relations, and occurrence of riebeckite and riebeckite-arfvedsonite solid solutions. l. Gebl. TO' 6E9-836. Escorln, C., H. CTpDRESDNER,P. Krrrr, lxo I. Duuren M9 Ti (1971) The relation between "light wolframite" and common wolfraniitp. Am. Mineral.56' 489-498. Frc. 7. Plot of *"tu5h3: rhonite on Fe-Mg-Ti Flxcen, L. W. (197.2) The uncertainty in the calculated ferric iron conteni of a microprobe analysis. Carnegie Inst. Wash. Yew Book,71' 60M03. M. (1936) The formula of aenigmatite.Am' I. Tomita (1934) describeda zonal structurecom- Ftelscnen, Sci. 12, 323-34E. prising a rhonite core grading outward through FuDALI, R. F. (1965) Oxygen fugacities of basaltic and rhoqite-aenigmatiteto an aenigmatitemargin in the andesitic magmas. Geochim. Cosmochim. Acta,29, 1063- teschenite from Oki, Japan. Conversely, rhonite 1075., cores separatedby sharp boundarigsfrom aenigma- GossNnn, B., rNo F. SprBLsenoER(1929) Cbemische und an Silikaten: Ein tite rontgenographische Untersuchungen margins in the monlonites and syenites from Beitrag zur Kenntnis der Hornblendegruppe. Z. Kristol- Morotu, Sakhalin(Yagi, 1953), suggestthat the two logr.12, lll-142., minerals must, if they foim a solid'solution series, Gnulwrecrx, H., eNo H. A. Secr (1972) Rhonit aus einem have a wide range of immiscibility. This is further Melaphonolith vom Puy de Saint-$andoux (Auvergne). Mitt. lt' 17-38. supported by the lack of zoning in relatively large Tsch. Mineral. Petrogr. H., exo D. McKIB (1964) The unit-cell of phenocrystsand also by the similarity composition KBrsEv, C. in aenigrnatite. Mineral. Mag. 33, 986-1001. of phenocrystic and groundmassaenigmatites from Kosrvreve, E. E. (1930) Trans, Museum Min:eral. Acad. Mt. Edziza. .tci. URSS,4, E7. Grunhagenand Seck (1972), in their study of Ln*osrnv, D. H. (1971) Synthesis and preliminary results rhonite from metaphonolite of Puy de Saint- on the stability of aenigmatite (Na'Fe'TiSLOn). Carnegie Inst. Year Book,69, 188-190. indicated possibilities: llash. Sandoux, the following two lNo S. E. HeccenrY (1971) Phase relations of ( 1) There is a miscibility gap between aenigmatite Fe-Ti oxides and aenigmatite: oxygen fugacity of the and rhonite. (2) Though a complete sbriesof solid pegmatoid zones, Carnegie Inst. Wash. Year Book, 69, solutions is thermodynamically stable, conditions 278--284. favorable for their formation do not exist in natural MniorlNov, F. P., ,rxo L. I. ArlNm'eve (1965) Aenig- syenite of the Eastern Sayan Moun- magmas. possibility matite from alkaline They considerthe second more tains. pokl. Akad. Nauk. SS'SR, 165' 444446 (Chem. likely in view of complex substitutionsbetween the Abstr. 64, l@32). two minerals. Experimental study of the composi- NrcHoLs, J., eno I. S. E. Cenrvrrcrreur.(1969) Peralkaline tions of pure rhonite and thoseintermediate between asid liquids: a petrologic study. Contrib. Mineral. Petrol. aenigmatiteand rhonite under various conditions is 20,268-,94. (1951) John Wiley problem. Snexp, S. I. Eruptioe Rocks, 4th ed. expectedto thiow light on this and Sons, New York. 488 p. (1907) ein neues aenigma- Acknornlgdgments SonuNnn, J. Ueber Rhonit, titahnliches Mineral und uber das Vorkommen und die The authors acknowledge the assistance of Mr. A. G. Verbreitung desselben in basaltischen Gesteinen, Nezes 'the Plant of Geological Survey of Canada, who did the lahrb. Mineral. Beil. Bd. 24, 475-547. microprobe analysesof Mt. Edziza aenigmatite; Mr, B. E. B. Sourrrsn, I. G. (1967) Cordilleran volcanic study, 1966. AENIGMATITE FROM MT. EDZIZA. CANADA 829
In, Report of Activities, Ed., S. E. Jenness, Geol. Suns. TuoupsoN, R. N., eNo J, E, Cnrsnorrrl (1969) Synthesis Can. Pap. 67-1A,89-92. of aenigmatite.Miheral. Mag. 37,253-255. (1970) Volcanism and its relationship to recent ToMtte, T. (1934) On kersutite from Dogo, Oki Islands, crustal movements in the Canadian Cordillera. Can. I. Japan, and its magmatic alteration and resorption. Earth Sci.7, 553-568. l. Ehanghai Sci.Inst. Sect.2,1,99-136. (1972) Physical evolution of Mt. Edziza volcanic YrcI, K. (1953) Petrochemical studies on the alkalic rocks complex, Northwestern British Columbia, Volcano. Soc. of the Morotu District, Sakhalin. Geol, Soc. Am. Bull. Iapan, Bull. Sec.Ser.17, 107-108. 64, 769-EtO. SueNo, T. (1933) On the use of standard glass powders in refractive index determinatioa. Am, Mineral. 1:E.42l- Manuscript receiaed, lune 25, 1973: accepted 430. for publication, March 27, 1974.