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Chemical Investigation of Pyrope Garnets in the Stockdale Kimberlite

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Chemical Investigation of Pyrope Garnets in the Stockdale Kimberlite American Mineralogist, Volume 60, pages 675480, 1975 ChemicalInvestigation of Pyrope Garnetsin the StockdaleKimberlite Intrusion,Riley County,Kansas FnnNz Rosrt, EsnnHd\noBnnnMlNN, huo GroRc Atrlrsnugn Fachrichtung Mineralogie der Uniuersitiit des Sqarlandes, Saqrbrilcken, Germany Abstract The color variationsbetween 4l separatepyrope garnet fractions from the Stockdale kimberliteare causedby differingchromium concentrations.Two main seriesmay be dis- tinguished.Series I showsa widecolor range from paleorange to dark violet,corresponding to CrrOrcontent from 0.2to 13wt percent;the cell edge increases from ll.5ll to 11.647A. SeriesII is characterizedby light pink or redcolors, CrrO, values below 2.5 wt percent,ahd TiO, contentsgreater than 0.5 wt percent.The largercell edgesin SeriesII are causedby higher content of Fe3+, which is proved by reflectancephotometry and Mcissbauer spectroscopy.The quantitativedistribution of chromium in the Stockdalegarnets is representedin a variation diagram. Introduction the calculatedformula induced Brookins and others Many kimberlitescharacteristically contain pyrope to reinvestigate the red garnet. Brookins' (1967) garnetswhich originate from garnet peridotite and analysisof red pyropeyielded SiOr,44.l l;TiO2,0.27; pyroxenite xenoliths, or are geneticallyrelated to AlrO3, 21.50; FerOt, 1.37; CrrO3,2.77; FeO,7.55", kimberliteitself. Investigationson pyrope garnets MgO, 20.57; CaO, 4.64; MnO, 0.37. Rosa and from kimberlitesby Nixon, Knorring, and Rooke Brookins (1966)determined ao to be ll.535A for red (1963),Nixon and Hornung(1968), Meyer (1968; pyrope and ll.558A for "green" pyrope. Brookins 1970),Sobolev, Lavrent'yev, Pospelova, and Sobolev (1967) concluded that, becauseof the high value of (1969),Reid and Hanor (1970),Meyer and Boyd the unit cell and refractive index, the "green pyrope" (19'72),and Gurneyand Switzer(1973) show colors may be even more chromium rich than the red ranging from pale orangeto red and dark violet, pyrope. Other garnetsfrom eclogitexenoliths in the causedby an increasingchromium content from I to Stockdale kimberlite have pyrope-almandinecom- 3 wt percentup to l0 wt percentof CrrO, and more. position and "appear unrelatedto the pyrope garnets These garnets have been defined as "chrome which occur as xenocrysts within the kimberlite" pyropes."Similar variations of pyropegarnets from (Meyer and Brookins, l97l). many Europeangarnet peridotiteshave been in- vestigatedquantitatively (Rost and Grigel, 1969; Methods Amthauer,Brenneis, Grigel, and Rost, l97l; Rost, Preparation Wannemacher,and Grigel, 1974). In Riley County,Kansas, 6 kimberlitediatremes About 20 kg of garnet-bearingkimberlite rock was are known (Brookins, 1970),among which the crushedand sievedto grain sizesbetween 0.3 and 0.8 kimberliteof Stockdalehas the remarkablecontent mm, and treatedwith dilufe hydrochloric acid to dis- of4 percentpyrope garnets (Eastwood and Brookins, solve carbonates and decompose serpentine and 1965;Rosa and Brookins,1966). Bagrowski (1941) other minerals.Heavy mineralswere separated by us- : had beenthe first to identify aspyrope a red mineral ing Clerici solution (D 3.5) and the garnetswere in the Stockdalekimberlite, previously believed to be concentrated by using a Franlz isodynamic "ruby spinel."Discrepancies in the analysisand in separator. This portion was then heated with hydrochloric acid (HCl: HrO : I :2) on a magnetic producing grainswith cleanand polished t Presentaddress: Professor Dr. F. Rost, FachrichtungMiner- agitor, thus alogie der Universit2itdes Saarlandes,D-6600 Saarbriicken. surfaces.Those garnet grains without impurities and F. R. G. inclusions were finally picked out by hand (enlarge- 675 676 ROST, BEERMANN, AND AMTHAUER mentof binocularmicroscope 16-25) to obtainabout centrationin Tables| , 2, and3. Garnetswhich could 2l g of pure garnet. only be separatedin amountsof a few mg wereex- The discriminationof differentlycolored garnets is aminedby determinationof theircell edges (Table 4). necessarilysubjective. The authors,however, suc- By comparisonof the resultswith the cell edge ceededin selecting4l garnetsamples of like color- values, the pyrope garnets from the Stockdale termedfractions-in differentlots (SeeFig. l, Table kimberlitefall into two main series. s). Each garnetfraction groupedinto SeriesI (Table l) possessesa definite value for co;with minorexcep- Analytical Methods tionsao increases steadily from I1.521to 11.581A as garnet fractions totaling more than 400 mg Those weight percentCrrO3 increases from 0.46 ro 7.20. with Philips X-ray vacuum spec- were analyzed a Thosegarnet fractions grouped into SeriesII (Table For garnet fractions less than 400 mg, trometer. 2) atsohave a definitevalue for aobut, unlike SeriesI was to determineFeO, emissionspectrography used garnets,their TiO, contentsgenerally exceed 0.50 wt (Rost and Hausner, CaO, MnO, TiO2 and CrzOs percent.For them ao showsa lesserrange than for 1954; Rost and Grigel, 1964; 1969). The standards SeriesI, all but onebeing between 11.540 and 11.548 used were predominantly common international A. SeriesII garnetshave largervalues of ao when standards and mixtures of these. The accuracy of comparedto SeriesI garnetsof approximatelyequal was between2 and 6 percent, both methods estimated CrrO, content.The CrzOecontent of SeriesII garnet and its concentration. dependingon the element fractionsdo not exceed2.5 wt percent. were determined by using the Guinier Cell edges Garnetfractions 6, 18,and 20 in Table3 yieldtwo 10.10.4reflection method, measuringthe distanceof distinctcell edge values. Since one value corresponds garnet against 4.4.4. refleclion of pure silicon; the of to SeriesI and the otherto SeriesII, fractions6, 18, was A. Single grains were ex- accuracy 0.001-0.002 and20 areprobably mixtures of SeriesI andSeries II camera (Gandolfi, amined by using the Gandolfi garnets.Fraction 37 in Table 3 consistsof violet- 1967);the accuracywas 0.005 A. colored garnetsthat give a broad reflectionin the Results Guiniercamera and undoubtedlybelong to SeriesI. Chemical Composition of the Stockdale Garnets Both pyropesstudied by Rosaand Brookins(1966) garnet The analytical results for the various garnet frac- and Brookins(1967) belong to SeriesI; thered garnet tions are arrangedaccording to their chromium con- correspondsin CrzOscontent and cell edgeto fraction3l in Tablel. Wt On the basisof color and cell edge,the authors L_l Seiles u favor inclusionof the garnetsin Table4 into SeriesI. 39 to 4lc compareto fractions38 I serles ]+ll mixed Thus fractions (Table l) and 37 (Table 3); they show the "alex- f_-] series t Tlst-r L Analyses of Stockdale Garnets, Series I Nr. SiO2 A12 Ot FeO MAo Cao Mno T1o2 ur2 ot 7 9.50 - b.oo o. J5 o.25 O.46 11.523 I 7 '90 - 5.Oo o.?) o.14 o.50 11.521 9 9.90 - 5.60 o. 44 o.22 O.52 11,52) rt 42.5 22.t+ 8.50 21.1 4.40 o.49 o.28 1.35 11.525 19 7.60 - 6.00 o.38 o.24 1.90 11.5)O 7,60 - 4.70 o.43 o.23 2.1O r1.531 7.50 - 6.00 o,42 o.25 2.1O r1,532 24 42.o 21.5 ?.45 21.4 4.45 o.45 o.2'l 2.18 11.533 6.7o - 5.80 o.43 o.65 2 .2o 11.5)4 29 42,O 21.4 7.53 20.6 I1.60 o. 54 o.14 2.)5 1r,532 3O 7.60 - 6.80 o,5J o.2J 2.35 11.533 41.6 22.2 7.65 21.4 4.45 o. 34 o.18 2.80 11.535 32 41.O 22.2 7.45 20.6 4.85 o.3) o.2o 2.85 11.539 33 4r.0 20,8 7.2O 20.4 5.40 o.52 o,12 4. oo 11 .546 - L1 Frdclion Nr 39 34 !+1.5 21.3 6.92 2o.o 5.40 o.51 o.17 4.t+5 11.549 (023 ||t%.1 J) 40.6 20.o 6.95 2o.o 6. oo 0. 70 o.l1 5.5O 11.56J 7.60 - 6.30 o.15 6.o5 t1.567 38 40.5 I 8.0 6.4o 18.2 7, OO o.65 o. 09 ?.2o r 1.581 012315678910111213 f, 41,3 ?1.) 7.55 o.47 o.26 3,18 WIok Cr20, Notes for Tables 2,3 and 4: Frc. l. Simplified histogram of the distribution of chromium con- Nr = number of garnet fraction tent in the Stockdale garnets. Series III : SeriesI plus Series II, - = oxides by spectrographical nethod not deternin€d fi = average composition of series mixed. PYROPE GARNETS IN KIMBERLITE INTRUSION Teglr 2. Analyses of Stockdale Garnets, Series II* T,c.sI-r4. Cell Edge,Color, and EstimatedChro- mium Contentof SomePeculiar Garnets* 2 42-.O 22_.5 4.95 2O.2 4,80 o. 39 o.84 o.32 11.54o Garnet Cell^Edge Wt percent Lolor 9, oo 5.90 o.41 o.70 o,35 11.5J5 No. (A) Cr 4 1O.50 6,40 o.1+2 o. 54 o.42 11.544 ror* 5 8. 70 6.50 a.j9 o. 62 o,45 11.542 10 8. 70 6.OO o. l5 o. 64 o.75 11.545 1 Pale orange ll.51f 0.2 ll 8. lo 6. oo o.44 o. 84 1.OO 11.544 39 Dark violet 1l .595 d.5 OO 4.7o 9. o.3t r.20 11.54J 40 il il 1l.611 10.0 8. 70 6.50 o.39 o. 66 1.2O 11.544 r4 9.20 5.60 O.4l o .52 1..1o 11,544 +ra I I .631 il.9 16 7.?o 4.7o o.3a r.40 1i.5L3 4lb 72.4 17 1+1.8 21.5 8.1o 2o.z 5 ,oo o.35 o.86 1.46 11.545 1.r.637 1Z Z 7.60 5.50 o.4l o.90 2 . oo lt .54'l 4Ic tr.647 26 6 .80 6,oo o.42 o.6o 2.2o I 1.544 ?.60 6,20 0.43 o.50 2.20 11.547 x Cr'rO, uaLuee estimated bg ertnapoLation 28 41.6 20,8 7 .35 21.4 5.15 o.43 o.
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