THE AMERICAN MINERALOGIST, VOL. 53, NOVEMBER DECEMBER, 1968 BEUSITE, A NEW MINERAL FROM ARGENTINA, AND THE GRAFTONITE-BEUSITE SERIES1 C. S. Hunr.nur, JR., exo L. F. AnrsrenerN, Departmentof Geolo gi c al Sci en c e s, H araar d, U nh ersity, Combrid ge, M ass achu s etts 0 2 1 3 I ABSTRACT Beusite, (Mn, Fe, Ca, Mg)3(POa),is the manganeseanalogue of graftonite and with it forms a continuous series. The new mineral was found in granite pegmatites in three localities in San Luis Province, Argentina: Los Aleros, Amanda and San Salvador. The mineral from Los Aleros has the following properties: It occurs in rough prismatic crystals up to 30 cm long, interlaminated with lithiophilite. Beusite is monoclinic, 2f m; space grotpP21/c,a:8.78,b:11.52andc:6.15 A,B:99"25',aibic:0.7621i1:0.5338;volume 613.7 N; Z:4. Strongest lines in X-ray powder photographarein A:3.50 (100), 2.863 (100), 2. 708 (60), 3. 13 (40), r.e24(40), 3.0 1 (3s), 2.Se (30), 2.4o2 (30). The new mineral is optically biaxial, positive,a:l702, p:1.703, t:1.722,2V:25", r)z strong, X:b ZAc:37o. The color is reddish-brown,cleavage: {010} good, {100} fair. The hardness is 5; density 3.702 (meas), 3.715 (calc). A wet chemical analysis recalculated to 100 percent is: CaO, 4 78; MgO, 2.64; FeO, 14.62; MnO, 36.56: PzOs, 41.40. DTA shows one endothermic peak at 960'C resulting from fusion. Optical properties, specific gravity and analyses by X-ray fluorescence and atomic absorption show beusite from San Salvador and Amanda to be similar to that from Los Aleros. The difierencesin the properties that result from changesin FeO:MnO ratio are offset by MgO and CaO making a chemical analysis necessary to distinguish between graftonite and beusite. The interlaminated lithiophilite is interpreted as exsolved from a high temperature disordered phase A wet chemical analysis of this mineral from Los Aleros recalculated to 100 percent is: LLO, 9.58; NarO, 0.03; FeO, 13.04; MnO, 23.02; Mgp,6.45; PzOr, 47.88. Optical properties of triphylite-lithiophilite from five localities and of graftonite from Ranquel and Cacique Canchuleta, all from San Luis, are presented. Beusite is named in honor of Professor Alexey Beus, formerly Professor of Mineralogy and Geochemistry, Moscow Polytechnical Institute. fnrnooucrroN During a field study of granite pegmatites in Argentina (1964 and 1965) specimensof what appeared to be graftonite triphylite inter- growths were collectedfrom four localitiesin the Province of San Luis (Fig. 1). The most remarkablespecimens, because of sizeand freshness, came from the small pegmatitelocally known as Los Aleros.The others camefrom the mines: San Salvador,Amanda, and CaciqueCanchuleta. An additional specimenshowing a similar intergrowth from the Ranquel mine in the sameprovince was given to us by Dr. Maria A. R. Benyacar. The physical propertiesand X-ray powder photographsof the "graf- l Mineralogical Contribution No. 457, Harvard University 1799 1800 C. S. HURLBUT, JR. AND L F. ARISTARAIN qEUSlf E_:O LOS ALERoS @AMANoA @sAN srLvADoR qn4rIeNrrE: El RANOUEL c cANcHULEI^ (r) 6 o o ) )i"""-;-:'i"'1Y"t-!o) SanLuis = .!". c) i l5o.*{ (I) I a , Concaran t9, S r,an.'sco dA lo ,C) tQ) (u q Frc. 1.Beusite and graftonite localities, San Luis, Argentina. tonite" pointed to the presenceof this mineral in all specimens.Ilowever, chemical analyses of the graftonite-like mineral from the first three aforementionedlocalities showed MnO to be in excessof FeO rather than the reversewhich is characteristicof graftonite. This indicated the necessitl, of discriminating between graftonite and the manganese analogue. A review of the literature shows uncertainty regarding the names applied to the minerals whose general formula can be expressedas (Fe, Mn, Ca, Mg)3(POr)r. De Tesris(1933) describedas mangualdite a mineral whose chemical anall'sis ,vieldeda formula close to 2MnO' CaO'PzOi, but whose ph-vsicalproperties were somewhat different from those of graftonite. Mason (1941) mentions the "graftonite- mangualdite group" (p. 119) thus suggestinga seriesexisted between the two minerals.Ilowever, in an addendum to the samepaper (p. 175) he found by X-ra.v examination of t1'pe material that mangualditeis a member of the apatite group. In 1950 Beus determined the weight percentagesof oxides in a graftonite-likemineral as: FeO, 16.95:MnO, 30.77;CaO, 0.54; XIgO, 9.50. The principal lines in a X-ray powder photograph of this mineral check with those of graftonite, but the physical properties are incom- plete. Beus stressedthe high content of MgO by naming the mineral magniophilite, although his was the first analysis of a graftonite-like mineral in which MnO was greater than FeO. Brooks and Shipwav (1960) also presenteda chemicalanall''sis (p. 522) of a "member of the graftonite isomorphottsseries" in which MnO exceedsFeO. Sincethe mineral of Brooks and Shipwal-(Australia) and the minerals BL.USITT. 1801 from Los Aleros, San Salvador and Amanda (Argentina) are ver,vlow in MgO (1.2 to 2.56 wt. /o) the name magniophilite is inapplicableto them. Recent chemical analysessuggest (see Fig. 5) that a complete solid solution exists, as in many other phosphates,between iron and manganesebut only partial solid solution with regard to magnesium and calcium. To avoid further confusionin the nomenclature,we pro- pose the name beusite for those members of the seriesin which the weight percent of MnO is greater than FeO. Beusiter (pronouncedbay' oosite)is named in honor of Alexey Beus, the first to report a mineral in the graftonite seriesin which Mn ex- ceeded Fe. Dr. Beus, formerly Professor of Mineralogy and Geo- chemistry, Moscow Polytechnical Institute, is at present a Technical Adviser to the United Nations Secretariat. GBor,ocrcer SBrrnqc The beusiteand graftonite localitiesin San Luis Province,Argentina are located on the easternslope of the Sierrade San Luis. Thesemoun- tains, part of the SierrasPampeanas, begin at the city of San Luis and extend to the Sierra de los Comechingones,approximately 150 kilo- meters to the northeast. They are fault-block mountains with sharp escarpmentsto the northwest but tilted toward the southeastand rise abovesea level to an elevationof 2000meters, or 1500meters above the surroundingplains. The basementcomplex of the Sierrade San Luis is formed essentially of schists,gneisses, and quartzites,that have been intruded by granite with the formation of migmatites. The metamorphic rocks strike N-S with, in general,an easterly dip ranging from 40o to 90o. Genetically associatedwith the granite are abundant pegmatites and aplites. The ageof the basementrocks is given as pre-Cambrian(GonzLlez Bonorino, 1950;Linares, 1959)but other authors considerit Lower to Middle (?) Paleozoic.(Pastore and Ruiz Huidobro, 1952; Pastore and Gonz6les, 1954).The erosionfollowing the uplift of the sierra during the Tertiary removed most of the pre-Tertiary and Tertiary sedimentary rocks Ieaving only minor patches of Permian rocks. Finally, rninor effusions of Tertiary andesiticrocks and Quarternary basalts complete the gen- eral geologicpicture of the sierra. For details of the geologyof the region one is referred to the papers by Pastore and Ruiz Huidobro (1952). Pastore and Gonz6lez(1954), Gonz|lez (1957) and Methol.2 I'fhe mineral and name have been approved by the Commission on New Minerals and Mineral Names, IMA. 2 Methol, E. (1964) Descripci6n geol6gica de la Hoja 22 h, Santa Rosa, Provincia de San Luis, Argent Reprh Dir. Nac. Mineria. Unpublished. 1802 C. S. IIURLBUT, JR. AND L F. ARISTARAIN OccunnBNcB The pegmatites of the Sierra de San Luis are tabular or lenticular, concordant bodies, generally zoned and manl' contain lithium and beryllium minerals.(Angelelli and Rinaldi, 1963;Herrera, 1963).All of them at which beusiteor graftonite werefound have commercialamounts of spodumene,beryl or muscovite, a characteristicof two-thirds of the reported graftonite Iocalities. The obscureLos Aleros pegmatite located in the Departamento de Pringles, San Luis Province, is 800 meters to the west of the large pegmatiteand beryl mine known as SantaAna. This mine can be reached by following a secondaryroad 35 kilometersfrom La Toma toward Paso del Rey and from this point following the mine road for 8 kilometersto the north. The Los Aleros pegmatite is tabular with N.S. strike and 80" dip to the east conformingto the enclosingmica schist.At the southernend of a small prospectpit (14 m long, 1.5 m wide,5 m deep) rough prismatic crystalsof beusiteup to 30 centimetersin length were collected.Triplite and tourmaline were found in addition to qtartz, microcline,plagioclase and muscovite. In the northern part of the Sierra de San Luis, approximately 100 kilometers from Los Aleros in the Departamento Junin, there are two beryl mines known as Amanda and San Salvador. On the dumps at both of thesepeglnatites, specimens of beusite were collected.Amanda is approximately 36 kilometers northwest of Santa Rosa south of the road between this town and Balde de Escudero. San Salvador is ap- proximately 20 kilometers NNW of Santa Rosa, west of the road from Santa Rosa to Punta del Agua. A few kilometerssouth of Los Aleros and south of the Pasodel Rey- La Toma road are the pegmatitesof Ranquel and Cacique Canchuleta operating for ber,'-land muscovite. Graftonite was collectedfrom the dump at Cacique Canchuletabut that from Ranquel was given to us. AII the beusiteand graftonite from theseArgentine localitiesare fresh with the exception of graftonite from Cacique Canchuleta which has partially altered to heterosite.This changeis similar to the first sl2oe nf alterationreported by Stanek(1955). Pnvsrcar ANDOPTrcAr, PnopnnrrBs Beusite. Fresh beusite from Los Aleros is reddish-brown with the cleavages:{010} good and {100} fair. Its hardnessis 5 and specific gravity, measuredon the Berman Balance is 3.702+0.005.