American Mineralogist, Volume 60, pages 566-573, 1975 Armalcolite-Ti-Phlosopite-Diopside-Analcite-Bearing Lamproites from Smoky itutte, GarfieldCounty, Montana Dlutrtlr Vnlnn Laboratoire de Pbtrographie, Uniuersitb de Paris VI, 4, place Jussieu, 75230 Paris Cedex 05, France Abstract Lamproitedikes and plugs occurringnear Jordan,Montana, contain highly unusual mineral assemblagesof three major types:(l) sanidine,armalcolite, and alkali amphiboles (potassiumrichterite and potassiumriebeckite); (II) armalcolite,Ti-phlogopite, diopside, olivine (altered),and sanidine;(III) armalcolite,Ti-phlogopite, diopside, olivine (altered), analcite,and glass.These rocks are low in Al2Os,rich in TiOr, and extremelyrich in KzO. They may be relatedin origin to the TiOz-and KzO-richultramafic rocks associated with the Montana diatremes. Introduction surroundedby type I-a fine grainedassociation of sanidine,armalcolite, and alkali amphiboles,with The Smoky Butte intrusiveshave beendescribed abundantinclusions of sedimentaryrocks. by Matson (1960),where pertinent field data and a MineralogicalDescriptions summaryof previouswork can be found. The out- Sanidine crops of thesepotassic lavas are located8 milesdue west of Jordan (Montana),between Smoky Butte The only feldspar found at Smoky Butte is Creekto the north and thejunction of Big Dry Creek sanidine.It is presentin typesI and II. In type I it and Lone TreeCreek to the south.The nearesterup- representsthe greatestpart of the rock, occurringas tive rock, a nepheline-haiiynitealn<iite, has been poorly-defined,very smallcrystals. In type II it is pres- describedin Winnett (Ross,1926; Powell and Bell, ent as long euhedralcrystals with squarecross sec- 1970)90 milessouth and westof Smoky Butte. A tions, often forming radial clusters(Fig. l). These numberof kimberliticbodies and associatedalkaline crystals often show hollow cores filled with glass intrusions(characterized by forsteritephenocrysts in which are quite similar in their morphologyto the a groundmassof melilite,monticellite, nepheline, Ti- quench crystalsof plagioclasedescribed by Bryan phlogopite, perovskite, and opaques) have been (1972)in submarinebasalts. They do not show any reportedfrom north centralMontana (Hearn, 1968) compositionalzoning. 100miles west of the area consideredhere. The sanidinewas determined from a typeII sample The Smoky Butte outcrop extendsnorth-south for to be sodium-poor,with a high iron content;the 1.9miles, mainly as thin dikesand small plugs which percentageof iron feldsparmolecule is closeto l0 metamorphosethe Paleocenesediments of the Tul- percent.Titanium wasdetected with the microprobe. lock memberof the Fort Union formation(Matson, The compositionof the sanidineis similar to that 1960).No extrusiverocks have been found. reported by Carmichael(1967) for sanidinesin a The samplesstudied are from a quarryon thesouth jumillite from Spain.The iron contentof the Smoky side of Smoky Butte. Three major rock types Butte sanidineis significantlybelow that reportedin (Matson, 1960)are present,Type lll-composed the Leucite Hills feldspar(up to l87oiron feldspar; of armalcolite, Ti-phlogopite, diopside, olivine, Carmichael,1967) (Table l). analcite,and glass-forms the narrowcenter part of a non-symmetricplug surroundedby type II, which is Amphiboles similarin compositionto typeIII but for thepresence Table 2lists representativemicroprobe analyses of of sanidineand the absenceof glass.Type II is in turn amphibolesfound in type I lamproite.They belong 566 LAMPROITES FROM SMOKY BUTTE. MONTANA 567 for the mostpart to the potassiumrichterite group, but someare potassiumriebeckites. Potassiumrichterites have been reportedfrom a number of lamproites(Prider, 1939; Hernandez- Pacheco,1965; Carmichael, 1967) and from certain minettes(Velde, 1965; Nemec, 1973); the host rocks for both of theseoccurrenced have very low sodium contentsand hyperalkaline characteristics. Potassium richteriteshave also been reported in nodulesfound in kimberlites(Erlank and Finger, 1970; Aoki,1974). Experimentalwork by Huebnerand Papike(1970) hasshown that complete solid solution exists between sodium and potassiumrichterite end-members at I kbar waterpressure, between 775 and 850"C. Ftc. l. Phlogopitephenocrysts and clustersof skeletalsanidine The potassiumrichterites from Smoky Butte are crystalsin a type II lamproite. almostcolorless in thin section,being slightly pinkish alongB and very paleyellow along 7. This unusual trivalentiron contentof lunar armalcoliteis thought optical characteristiccan probably be related to to be zero, on the groundsthat the mineral is in their relativelylow iron contentcompared to other equilibrium-or at leastcoexists with-native iron. reportedmembers of the series.Potassium richterite The mineralfrom Smoky Butte has low FerO,and appearshere as a late crystallizingphase, occurring high MgO and TiO, contentcompared to most ter- mostlyin vugs.Sometimes it is rimmedby a thin zone restrialpseudobrookites (Smith, 1965; Rice, Dickey, of very strongly pleochroicamphibole (purple to and Lyons, 1971;Anderson and Wright, 1972).It dark prussianblue) towards the interiorof the vug. has, consequently,the closestcomposition to the Analysesreported in Table2 indicatethat this zone lunar armalcolite thus far reported in terrestrial has the compositionof a potassicriebeckite. This rocks. It could aptly be named ferri-armalcolite, showsthat potassiumriebeckite can in fact existin rather than magnesium-richferro-pseudobrookite. nature.However, the rarity of hyperalkalinepotassic Figure 3 shows the position of lunar armalcolite, rocks, and the usual presenceof calciumin these Smoky Butte ferri-armalcolite,and Hawaiian rocks,can explainwhy potassiumriebeckite has not pseudobrookiteon a (Ti,Si)Or-(Fe,Mg,Mn)O-(Al, yet beenreported. Its occurrenceas rims on crystals Fe'+r)O3diagram (after Smith, 1965).The cell in vugs suggeststhat it could have formed by data, obtainedfrom an X-ray powder diffractogram precipitationfrom the vaporphase, similar to certain usinga programwritten by C. W. Burnham(1962), occurrencesof acmite (seefor exampleSmith and Lindsley,1971). Tluls l. Microprobe Analyses of Sanidine, Armalcolite, Ti-Phlogopite, Diopside, and Glass from Armalcolite Types II and III Lamproites The name armalcolite(Anderson et al, 1970)has Sanidine Atulcolite Ti-Phlogopite Diopside GLass been proposedfor a pseudobrookite-likemineral cenEet Edge with thegeneral formula Fe2+o.uMge6Ti2O6 found in a sio2 65.20 38.33 40.79 54.72 55.93 numberof samplesbrought back by the Apollo I I Al203 16.53 11.40 8.89 0.17 8.87 mission.Pseudobrookite is presentin most samples Ftzo3 2 -54 g.zo* from the FeO 20.28- :.za* 3.59 5.53 SmokyButte area. In typeIII rocksit occurs Mgo 11.68 19,18 17.25 t7 .56 5.25 as euhedralcrystals 0.03 by 0.3 mm (Fig. 2). These CaO o. o1 o.o2 0.o3 o.78 crystalsare not opaque,showing a faint purplishtint Na2O 1.14 o.28 0.34 0.30 1.10 K2o 14.49 10.52 10.52 4.14 in thin sectionand a weak birefringence.Table 3 MnO 0,15 gives the chemicalanalysis for the pseudobrookite lio o.33 61.56 2 11.21 I 1.O5 0.99 2.21 from type III rock. The structuralformula is: Ct203 + + (Til.s0Sio.0aXAlo.oaCro.o2Fe8o.ro)(Fe2 o.rrMgo.ur)Ou Total 100.28 102,06 96.83 98.13 100.29 93.11 Its generalcomposition corresponds to that reported for lunar armalcolite, except for the presenceof x indicates total Fe calculated as Fe2o3 or Feo *x totaL includes 4.7O7, iqnition loss,detemined on trivalent iron in the Smoky Butte specimen.The a sMlI sanple of 568 DANIELLE VELDE TABLE2. MicroprobeAnalyses of Alkali Amphibolesfrom In lunar rocks (Haggerty et al, 1970) or in Type I LamproiteStructural Formulas Computed Hawaiian basalts (Anderson and Wright, 1972) followingCzamanske and Wones(1973) pseudobrookiteis alwaysmantled by ilmenite,unless it occursin anotherphenocryst. In the SmokyButte Potassic Richterites Potassic RiebeckiEes intrusives,microprobe analyses for Fe, Mg, Ti, Cr, to be homogeneousand un- si02 53.19 s3.35 50.59 49.90 and Mn show armalcolite At^o^ o.47 0.22 o.04 o.05 zoned(a representativemicroprobe analysis is listed FeO 3.66* 4.09* 29.50* in Tablel). Mgo 19,56 20.12 t,5r 2.67 The place of armalcolitein the crystallizationse- CaO 8.87 1.29 0.06 0.10 in- 'l quenceis of importance.In the Smoky Butte 6.41 ,o4 Na2O 3.90 3.87 trusives,armalcolite is foundincluded in clinopyrox- K2o 3.96 4,16 4.76 4.7L in somemica phenocrysts,but neverin the Tio2 3.45 3 . 43 4. 50 enesand olivines. This indicates that it crystallized after Total 91.O7 96.52 94,49 91.ffi olivine, and before some (if not all) of the Ti- pressure, si 7.626 8.014 1.973 phlogopite.In lunar rocks at atmospheric AL o.080 0.037 o. oo9 armalcoliteis the first phaseon the liquidus,followed Ti o.294 o.287 o. 018 (Akimoto et a/, t (teL.) 8.O00 8.000 8.014 8.OOO by clinopyroxenesand then olivine AI o.oo7 1970).This same relation was also reported by 0.439 o.492 4.O44 3.941 with 6.7to l0 Mg 4.180 4.3L4 0.356 o,622 O'Hara et al (1970)for compositions Ti o.078 o. o84 o.536 o.428 percentTiOr. At pressuresabove 20 kbar, no Fe-Ti > (M.-M^ ) 4.691 4.890 4.947 4.99r It oxidesare formed. Further, the Mg/Fe ratio of ar- 7.362 t.124 o.009 o. 017 NA 0. 638 o.876 1.988 1,983 malcolite decreaseswith decreasingtemperature, > (Ir4) 2.OOO 2.000 1.997 2.OOO from 0.81at l200oCto 0.59at ll50'C at I bar;at Na 0.445 o.204 o.197 I125"Carmalcolite is replacedby ilmenite(Akimoto K o.724 o.764 0.963 o.959 et al, 1970).The Smoky Butte armalcolite,with a ) (A) r.169 o.968 o.963 1.156 Mg/Fe ratio of 0.60,would then havecrystallized at * indicates total Ee calculaced as FeO the lower pressureand temperaturerange of its stability in the chemicalsystem in which it is found. are quite comparableto thosefor lunar armalcolite or for syntheticarmalcolite (Anderson et al, 1970). Analcite Valuesfor Smoky Butte are a : 9.749(9)A; b : Analciteoccurs as euhedral microphenocrysts (Fig. 10.026(12)A; c : 3.73s(8)A; V :365.0s7(791)A3; 4) quite similar in aspectto the leucitein the Wyom- for lunararmalcolite they area:9.743(30) A;6 : ing wyomingites (Carmichael, 1967).