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Santaclaraite, a New Calcium-Manganese Silicate

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Santaclaraite, a New Calcium-Manganese Silicate American Mineralogist, Volume 69, pages 200J06, 1984 Santaclaraite,a new calcium-manganesesilicate hydrate from California RrcHeno C. Eno U.S. Geological Survey, Menlo Park, California 94025 eNo Yosnrrezu Onasnr Department of Geology, University of Pennsylvania Philadelphia, Pennsylvania19104 Abstract Santaclaraite,ideally CaMna[Si5O14(OH)](OH)' H2O, occursas pink and tan veins and massesin Franciscan chert in the Diablo Range, Santa Clara and Stanislaus Counties, Calif. It is associatedwith four unidentified Mn silicates, Mn-howieite, qtrartz, braunite, calcite, rhodochrosite, kutnohorite, barite, harmotome, chalcopyrite, and native cgpper. Santaclaraiteistriclinic,spacegroup Bl,a:15.633(l), b:7.603(l), c = 12.003(l)A,a= 109.71(lf,F = 88.61(1)',y:99.95(l)",Y = 13223Q)41,2= 4. The strongestlines of the X-raypowder pattern are (d,1, hkt):7.044, 100, 010; 3.003, 84, 501;3.152,80,410; 7 .69,63, 200; 3.847, 57, ( I 13,400); 3.524,39, 020. Crystalsare lamellarto prismatic (flattenedon {100}),with good cleavageon {100}and {010};H: 6%; D(calc.) : 3.398g/cm3, D(meas.) : -10.002), 3.31(t0.01);optically biaxial negative, with a : 1.681,F: 1.696,y = 1.708(all 2Vx:83 (tl)". Although chemicallya hydratedrhodonite, santaclaraitedehydrates to Mn-bustamite at about 550'C (in air). Santaclaraiteis a five-tetrahedral-repeatsingle-chain silicate and has structural affinities with rhodonite, nambulite, marsturite, babingtonite, and inesite. Introduction about fifty abandonedmanganese mines located in Santa The new mineral santaclaraitewas discovered in 1975 Clara, Alameda, San Joaquin, and Stanislaus Counties in an abandonedmanganese mine in the Diablo Range in near their common junction. Nearly all these mines lie northeasternSanta Clara County, Calif., by Messrs.John within a circle of 16-kmradius centered on Mount Board- L. Parnau and Albert L. McGuinness, who brought it to man at this junction. The mines and the geology of the us for investigation. They suspectedthat the mineral was areawere describedby Jenkins(1943) and Trask (1950), inesite from its color and prismatic habit, but our studies and a review with later production figures was presented proved the mineral to be new and to have a crystal by Davis (1957).The manganeseore bodies in the four- structure that helps to clarify the role of hydrogen in county area are ofthe Coast Rangesedimentary type that pyroxenoids(Ohashi and Finger, 1981).The mineral is occur in chert of the Franciscan Complex (Jurassic to named for the County of Santa Clara, the locality of its Lower Tertiary). first occurrence. Specimens of santaclaraite (holotype Our present information from the discoverers is that and cotypes) will be depositedat the SmithsonianInstitu- the abandonedmine in which santaclaraitewas discov- tion (National Museum of Natural History), Washington, ered is on property that is now private, posted against D.C. The name and descriptionhave been approvedby trespassing,and patrolled by a securitysystem. The mine the Commission on New Minerals and Mineral Names, workings are no longer accessible,and all of the manga- I.M.A. A descriptiongiven by Ohashiand Erd (1978)of nese mineral specimenswere found on old dumps at this an unnamed new mineral was a preliminary report for site. santaclaraite. The paragenesisof the specimensthat we have seen from this locality is noteworthy in that the more common Occurrence and paragenesis Mn minerals are scarceor absent and the Mn silicates so far found are all unusual in some respects. It seems Although the discoverers of santaclaraitedid not dis- worthwhile, therefore, to give a detailed account of the close the exact location of its occurrence, they have associated Mn minerals. Santaclaraite occurs both as provided us with many specimensfrom the locality and cross-fiber veins (the largest measures I cm in width by partial information on the occurrence. The mine is one of more than 9 cm in length) and irregular masses(10 cm in 0003-004x/84/0I 02-0200$02.00 200 ERD AND OHASHI: SANTACLARAITE 201 maximum dimension) in Mn-oxide-stained chert and A secondoccurrence of santaclaraitewas discoveredat quartz. Though uncommon in its overall occurrence, the Buckeyemine (locatedin sections2 and3, T. 5 S., R. santaclaraite is the most abundant Mn silicate at this 5 E., in Stanislaus County, Calif.). The mineral was Iocality. The next most abundantMn silicate is an uniden- identified by Erd in specimens collected rn 1942-1944 tified reddish-brown fine-grainedmineral that appearsto from the ore body by Dr. Max D. Crittenden,Jr., during be a memberof the friedelite series.Three other unidenti- his study of the geology of the deposit (Trask, 1950,p. fied Mn silicatesat this locality are similar to, but differ in 287-289).Santaclaraite occurs sparselyas pink prismatic some respectsfrom, the minerals parsettensite,welinite, crystals up to 3 mm long in quartz veins in tan chert and gageite (all currently under study). Mn-howieite is associatedwith rhodochrosite, the friedelitelike mineral associated with santaclaraite as yellow-brown fibrous (identical with that of the Santa Clara Co. occurrence), veinlets,masses, and small spherules,with a : 1.697,B braunite, and very minor chalcopyrite. It is probable that : 1.716,y = 1.727(all 10.002).Fine-grained rhodochro- at least some of the "inesite" identified in the mine by site is subordinate to the Mn silicates but is widely Crittenden is actually santaclaraite(Crittenden, oral com- disseminated throughout them and the quartz matrix. munication, 1980). The underground workings at the Calcian kutnohorite and calcian rhodochrosite were abandonedBuckeye mine are no longer accessibleand an found in a single occurrenceas small scalenohedra(to 0.6 attempt to find santaclaraite in the present-day (1981) mm). The central cores of the crystals consist of massive dumps was unsuccessful. white Ca-kutnohorite(a : 4.919,c : 16.525A; a = 1.702, : -10.002) e 1.518,both encrustedwith tiny euhedral CrystallograPhY rhombsof Ca-rhodochrosite(a : 4.824,c = 16.0lA; c.r: 1.78510.003,a : 1.518t0.002).A strongpositive micro- Morphology chemical test was obtained for Mn, but Fe could not be Santaclaraite occurs principally as radiated lamellar detected.The data indicate about 20 mole percent CaCO3 aggregates(Fig. 1) composed of thin prismatic to tabular in the rhodochrosite, which is near the limit for naturally subhedral crystals, flattened on {100}. The rough spher- occuring material (Deer er al., 1962,p.265-267). All the ules averageabout a millimeter in diameter. Where space specimensthat we have seenare stainedblack with a thin permits, thick prismatic euhedra, up to a centimeter in coating of X-ray amorphous Mn oxide, although very length on [fi)l], are developed;several ofthese are visible little of this material is actually present. The Mn mineral in the vug in Figure l. The mineral also occurs in cross- that was mined at this locality appears to have been fiber veins composed of prismatic to nearly fibrous crys- braunite,which occurs as massesand veins, up to 6 cm tals having a length/width ratio up to 40. Forms identified acrossin the specimensthat we have seen.Other associ- with a two-circle optical goniometer are b {010}, a {100} ated minerals are calcite (some manganoan),barite, and the most prominentform, m {110},/{l0l}, g {301},and ft rare harmotome, chalcopyrite, and native copper. Some Simpletwinning on {100}is common' ofthe quartz is colored dark grayish blue by inclusions of {401}. asbestiformriebeckite. X-ray data The preliminary unit-cell dimensions of santaclaraite were determined from single-crystal X-ray precession photographsusing Zr-filtered Mo radiation. Table I lists these data, refined by least-squaresanalysis ofthe X-ray powder data. The crystal structure of santaclaraite has beendetermined by Ohashiand Finger(1981); the mineral has centric triclinic symmetry (Pl for the primitive cell). An /-centered cell was employed by Ohashi and Finger for structural comparison of santaclaraite with other pyroxenoids;however, a B-centeredcell is selectedhere on the basis of morphology for the mineralogic descrip- tion. Table I comparesthe data for the various settings. The X-ray powder data are shown in Table 2. There is a moderately strong preferred orientation of hlcOreflections in the X-ray diffractometer pattern due to the good {010} and {100} cleavages.The intensities observed in powder photographs agree closely with the calculated intensities preferred Fig. l. Vuggy radiated pink santaclaraite, with prismatic and so are not listed in Table 2. The effect ofthe : crystals of tan santaclaraite(larger is 7.5 mm long) projecting orientation is most noticeable for the lines at d 2.692 into vug at center. Photograph by Lowell Kohnitz, U.S. and 2.9394, which are the strongest in powder photo- Geological Survey. graphs. 202 ERD AND OHASHI: SANTACLARAITE Table 1 Unit-cell data for santaclaraite Physical and optical properties Santaclaraiteis pale pink (Munsell color 5RP 8/2) or Sy6ten Tric 1inic Triclinic Triclinic moderate reddish orange (l0R 6/6). Although these two Sp€ce group !1 B1* r1* color varieties are quite distinctive, we observed no a (l) 9.738(2)** r5.633(r) 10.291(r) significant differences in their chemical composition or 9.970(r) 7.603(1) rr,934(2) 7.503(r ) 12,003(3) r2.oo3(4) optical properties. The color of the pale-pink variety 109.77(r)" r09.70(1)" 105.78(r). B 93.95(r). 88.6r(1)" 1r0.55(2)" darkensto orangewhen exposedto tungstenX-radiation. r04.97( 2 ). 99.95(1)' 89,09(r)" The mineral is transparent v (AJ) 651.0(r) r322,0(t 1322,0(4) and has a vitreous luster and a very pale pink streak. It is nonfluorescent. Cleavage is z 244 good on both {100}and {010}.Its hardnessis 6% (Mohs). D (s/c#) 3.398 (idealized compoaition) : 3.379 (chmical analysis) Santaclaraiteis biaxial negative:a 1.681,B: 1.696, : -t-0.002; = D 3.31(!0.01)r "y 1.708(all Na light); zvx 83(rl)"; r > v, moderate.
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