AmericanMineralogist, Volume61, pages405408, 1976 Sanidinefrom the Mesa Falls Tuff, Ashton,Idaho AsHurrBesu'nNr Cser.rEs J. Vtre.t-tnNo I ndianaUniue rs ity, Bloomingt on, I ndiana Abstract K-feldspar crystalsfrom different horizons and localitiesin the Mesa Falls member of the Yellowstone Tuff, in the vicinity of Ashton, Idaho, are unzoned and unaltered.Calculated formulae of representativecrystals, based on electron-microprobeanalysis, permit classifica- tion of a[ as sanidine,near Oru* Ab* An,.uin the ternary feldspardiagram. X-ray diffraction studiessuggest a uniformity of cell dimensionsand high albite-high sanidineseries structural state.Optical data are in agreementwith this classification.The data lead to the conclusion that all sanidinephenoclasts in the Mesa Falls Tuff appearto have crystallizedin the magma chamber during a single epoch prior to eruption. Eruptions from the magma chamber occurred in pulses to produce the different horizons in the tuff' Introduction YellowstoneGroup, of Pleistoceneage (Christiansen and Blank, 1972, p. 86). The phenoclast-bearing Alkali feldsparcrystals constitute the major pheno- layer is as much as 15 m thick in the type locality, a clast component of an airfall crystal-richlayer in the road cut on U.S. Highway 20 and 9l (seeHamilton, Mesa Falls Tuff, the middle member of the Yellow- 1965,p. C5, fig. 3). There it restson a thin layer of stone Group (Christiansenand Blank, 1972,p. B3). loess. It crops out in severaladditional but smaller The unit crops out in the vicinity of Ashton, Idaho, road cuts along the highway, is presentin the cliffs at and mechanicalerosion continually affords an abun- Mesa Falls on the Henry Fork, and is exposedin a dant supply of crystalswhich accumulateat the break quarry about 6.8 km north of Ashton as well (Chris- in slope in several easily accessibleoutcrops (e.9. tiansenand Blank, 1972,p.87). Locationsof individ- along U.S. Highway #l9l). Microscopicexamination ual samplesare given in the appendix. shows that the feldsparcrystals are mainly sanidine. The sanidine crystals are white to colorless,sub- They are unaltered and unzoned and readily ame- hedral to euhedral and reportedly attain a length in nable to optical and chemical study. This and the excessof I cm (Christiansenand Blank, 1972,p. B6). lack, until recently2,of any detailed information in The crystals selectedfor study are clear and trans- the literature on the material from this occurrence parent, and despitea whitish dusty appearanceon the prompted us to study the sanidine.We haveprepared surface of some, microscopeexamination failed to this note on the basisof eight samplescollected and reveal any alteration products. Refractive indices kindly provided us by Mr. David Doherty of Wayne were determined for several crystals from each State University. sample. ln addition to the crystals selectedfor refrac- The airfall tuff tive index determination,eight moderate-sizesingle- feldspar phenoclasts were handpicked at random The zone which contains the sanidine crystals is from eachof the original samples.Each selectedgrain part of the Mesa Falls Tuff, the middle member of the was divided into two parts, one to be usedfor X-ray powder diffraction and the other for electronmicro- I Present address: Harvard and Smithsonian Astrophysical probe analysis. Observatory, 60 Garden Street, Cambridge Massachusetts02138 'Recentfy, (1974) Friedman el al , have studied the oxygen OPtical data isotopic composition of the sanidine crystals from the volcanic rocks in Yellowstone National Park. Among the results they The selected crystals were crushed and sized to presented are two determinations on sanidine crystals from the - 100 + 120 mesh and studiedby immersion method' The values for 6'"0 given for the Mesa Falls Tuff dated at 1.2 m.y. The refractive index of the matching liquid was im- two are *4.0 and *4.1 respectively.A K/Ar age of 1.2 m.y. has of the Abbe refractome- been determined by J. D. Obradovich (written communication to mediatelychecked by means Christiansenand Blank. 1972). ter (sodium light source) and adjusted,where neces- A BASU AND C J. VITALIANO sary, to p 25"C. The index,determination of which Geology, Indiana University,was followed.Details presented some slight difficulty, was checkedin each of the procedureare given in Klein (1974). instanceby using severaldifferent orientations in ad- Average analysesfor each of the eight samplesare dition to the nearly centeredBxo figure.The optical given in Table I along with the chemicalformulae data are given in Table l. and the calculatedmolecular proportions Or, Ab, and An. The latter valueswhen plotted Electron probe microanalysis on a ternary diagram cluster closelyin the sanidinefield near the The portion grains of the set aside for chemical border betweensanidine and soda sanidine(see also analyseswere mounted with epoxy in small sleeves, Barth, 1969,p. a4). carbon coated, and analyzed with an srrc Auto_ probe. For each sample,several random single spots X-ray study wereanalyzed in order to obtain an averagecomposi_ X-ray data were obtained by meansof a phillips tion. An acceleration potential of 15 kV was used goniometer using Ni-filtered CuK-radiation. X-ray with a sample current of about 0.022uA. and the powder diffractograms were obtained from each effectivebeam diameter was about l-2 microns. A samplewith three oscillatoryscans at Vr" 20 per min- counting time of 10 secondswas used for each ele- ute from 60' to 20" 20 using a chart speedof % inch ment for the nine element analyses.Data were re- per minute. Each corrected peak positiona of the ducedon an pDp-ll on-line computerwith the cEo_ feldspars,if unaffectedby interfering linesand agree- wHIZ program (Finger and Hadidiacos, 1971,1972), using B factors calculated as per Bence and Albee s Pure semiconductorgrade (1968)and a factorsfrom Albee and Rav fi970) for crystallinesilicon metal (as supplied by Jarrel Ash, JM 424, spec-impurity (3 ppm) was used as an the standardsused. The general pro..dur" at the rnternal standard. Its unit cell parameter (D. R Waldbaum: electronmicroprobe laboratory of the Department of personalcommunication) is 0.543093+ 0.000008nm at 25"C TAsLr l. Optical propertiesand electronmicroprobe analysesof Mesa Falls Tuffsanidine IP-1 tP -2 1P-3 IP -ll 1l,f-4 TLP-6 TT.p-O -25 (ave. (avg. ILP of 3) of 2) (avg. of (avg, ( )+ 3) ol JJ avg. of ) (avg. of 3) (avg. of 3) (awg. of 3) si02 oo. J4 65.26 66.28 oo. I o 66.a7 66.28 A1203 18.jz 67.36 18.50 18' 5r r8.36 18. 5l+ 18.1+1 r8.76 Fe0* 0.13 0 .08 0,13 o. oB 0.01 o. oB 0.09 0.08 Mso 0.00 0.01 0.00 0.01 0.01 0.02 a.o2 0.0t Ca0 a .24 0. 33 0.31+ o .26 a .22 0.31 o .35 Na20 3. 8l+ !. 09 3.98 J.yo 3.92 3.97 Kzo 1o ' 3r 10.09 3.93 10.32 to.r6 10. Ll+ 9.9r .97 Mn0 0.05 0.00 9 0.00 0.00 0.01 0.02 0.03 0.01 Ti02 0.05 0.01 0 .02 0.01 0.01 0.01 0.03 0.01 Total 99.\8 99 .37 99.\' 99.L' 99.II 99 .66 99.50 100.47 a r,5t67 i . )a lo 1 .5168 r .5182 1 . tt85 r .rLBz 1.519r B r.5203 I .5203 \.52A)+ r.j225 L , v I <cr 7 r2).9 r .5220 1.5208 I .52I)1 t.)zlo t.>z5o r ,r226 L,j225 2v(-) 1To r .522r 9o Bo loo r6o 41" I eo Nunbers of io basls of 8 si 3.01 3.01 3.0l 3.01 3.00 3.02 A1 o .99 o .99 3.O2 o .99 o.99 1 .00 I .00 0.98 0.99 Ca 0 .01 0,02 0.02 o.42 0 .0r 0.01 0.01 Na o .3)+ 0.02 0 .36 0 ,3, o .35 0.35 K 0 .50 o o'3)+ ,59 0 .50 o ,59 a.19 o .59 o .57 Mole !ercent Or 5I 6I 6I 61 6I Ab JO 37 10 3T 37 38 38 37 An I 2 2 2 I 1 t_ 2 xFe recafcul ated as FeO SANIDINEFROM THE MESA FALLS TAFF, ASHTON,IDAHO ing within 0.03" 20 betweeneach scan,was averaged tions derivedby Luth (1974,revised,1975, personal for the three scansand tabulated.Miller indiceswere communication) also show that thesefeldspars have assignedaccording to Wright and Stewart(1968) and highest structural states with a mean Abc (Stewart Borg and Smith (1969).Usually more than 20 unique and Ribbe. t969) of 0.53 + .03. As might be ex- reflectionswere obtained for each sample. pected,maximum disorderis evidencedfrom the esti- A relatively precise method of approximating mated Al occupancyof the 7'(0) site in all the crys- Al/Si order-disorder involves the estimation of the tals (26.l%o L l.3Vo,Table 2). relative structural state of an alkali feldsparfrom its refined unit cell parameters(see Smith, 1974,for de- Discussion tails). Luth (1974) developeda set of equations in- volving the b and c cell edgesand the a* and 7* Optical properties, chemical compositions, and angles(reciprocal cell) to estimatethe proportion of structural states of the sanidine crystals in all the Al in the various T sites.We have used theseequa- eight samplessuggest that they are almost identical. tions to obtain an estimateof AllSi order-disorderof Individual oxides of the major cationsvary within * the sanidinephenoclasts from the Mesa Falls Tuff. 1 percent, refractive indices do not vary more than Unit cell parameteiswere refined by a least-squares 0.0024:and standard deviation of percentagesAl oc- method using a program written by Burnham (1962), cupancy at the ?"(0) site is only 1.3.All the crystals and modified by Waldbaum (1973,personal commu- studied are high-sanidineand the Or:Ab ratio of nication).