THE AMERICAN MINERALOGIST, VOL 46, MAY_JUNE, 1961 TEPHROITE FROM FRANKLIN, NEW JERSEY* ConNBrrus S. Hunrsur, Jn., Departmentof Mineralogy, Harvard, Uniaersity. Assrnlcr A study of tephroite specimens from Franklin and Sterling Hill, New Jersey showed in all of them the presence of thin sheets of willemite believed to be a product of exsolution. .fhese sheets are oriented parallel to the {100} and [010] planes of tephroite with the o and r axes of tephroite and willemite parallel. It is believed that Iittle zinc remains in the tephroite structure and that much of it reported in chemical analyses has been con- tributed by intergrown u'illemite. This conclusion is supported by experiments syn- thesizing tephroite. The indices of refraction and d spacing of {130} vary as would be expected with changes in amounts of MgO, FeO and CaO. INrnooucrroN 'fephroite, Mn2SiO4,a member of the olivine group, was describedas a new mineral from SterlingHill by Breithaupt in 1823.A chemicalanal- ysis of the original material was published by Brush (1864) together with severaladditional chemical analysesof tephroite made by others. These analysesreport ZnO in varying amounts which Brush attributed to invariably associatedzincite. Palache (1937) did not agree with Brush and stated " . that the molecularratios in someanalyses more nearly satisfy the orthosilicateformula when zinc is regardedas essen- tially a part of the mineral rather than as a constituent of mechanical inclusions." The present study was undertaken for the purpose of in- vestigatingthe variations in the propertiesof tephroite with changesin chemical composition, particularly the effect of zinc. Relationships were not expectedto be simplefor analysesshow, in addition to ZnO, variable amounts of MgO, FeO, and CaO. Dosctuptrox oF TrrE SpBcnrBNs For this study 27 specimensin the Harvard collectionlabelled tephro- ite and roepperite were assembled. Several additional specimens were kindly made available by Mr. John L. Baum of the New Jersey Zinc Company and Mr. John Albaneseof Union, New Jersey. The name tephroite was given to the original material becauseof its gray color (from the Greek-ash-colored.)but only six of the specimens studied were gray; thirteen were reddish brown; four rose-red.Roep- perite is black. It was also consideredfor it has beendescribed as a vari- ety of tephroite containing notably high amounts of FeO and ZnO. Of * Contribution No. 395 from the Department of Mineralogy and Petrology, Harvard University. )+t 550 C. S. IIURLBUT. JR. the four roepperitespecimens available, three proved to be black wille- mrte. When examined microscopically,most tephroite is seen to contain minute inclusions.These inclusions have beenstudied by Metsger, Ten- nant and Rodda (1958) who concludethey are franklinite and are re- sponsiblefor the color variation. Black franklinite is present in roep- perite and red franklinite (spinel) is present in the red tephroite. " . paler shadesof gray and fleshpink . are due to the sparsescat- tering of inclusions." It has previously been thought that zincite inclu- sions were responsiblefor the red coloring. However, in some tephroile specimensflakes of both red zincite and black franklinite are present associatedwith wiliemite sheetsand elongatedparallel to the c axis of the tephroite. Portions of all the specimensgave under ultra violet light the char- acteristic greenishluminescence of willemile. Some of this fluorescencc resultedfrom massivewillemite intergrown with the tephroite but much of it seemedto be coming from the tephroite itself. Closerexamination showed thin fluorescingsheets of colorless willemite lying along the {100} and {010} planesof the tephroite.Sections normal to thesetwo directions,{001}, show the tracesof the sheetsintersecting at right angles (Fig. 1). Under magnification more and thinner sheetsbecome visible forming a minute crosshatchedpattern. This is best observedin thin section where it is seen that the willemite sheetsalthough discon- tinuous are all crystallographicallyoriented with the c axis of willemite parallel to the c axis of the tephroite host, Fig. 2. The major and mosl continuouswillemite sheetsare irregularly spacedand may have a thick- nessas great as 0.10 mm. The lessersheets are much lesscontinuous with thicknessranging from 0.05 mm. to lessthan 0.005 mm. A similar inti- mate intergrowth of willemite with glaucochroitewas reported by O'Dan- iel and Tscheischwili(1944). The cleavageof tephroiteis given as {100} and {010}, the directions of the interlayeredsheets of willemite. On most of the Franklin tephroite when examinedin detail thesedirections of breaking do not yield plane surfacesbut are striated in a direction parallel to the c axis; an observa- tion early made by Brush (1864).They appearto be more a parting than a cleavage,separating easily along the planes of the major willemite sheets.On most specimensit is impossibleto developa cleavagebetween these parting planes and in thin section no cleavagecracks are visible between the willemite layers. Small transparent crystals of Franklin tephroiteshow no cleavageparallel to {100} or {010} but a good {001} cleavage. TEPHROITE FROM FRANKLIN, N. T. Frc. 1. Polished surface of tephroite crystal cut nearly normal to the c axis. Photo- graphed under ultraviolet light with fluorescing willemite recorded as light lines and areas. Full scale. In addition to the parallelismof the c axesof the tephroite and the in- cluded willemite there is a parallelism of other axial directions. X-ray rotation photographs were taken of the willemite with the rotation axes perpendicular to the sheets. One set of sheets, parallel to {100} of tephroite, yields 13.97 A as the identity period (o6 of willemite); the other set of sheets at right angles and parallel to {010} of tephroite Frc. 2. Photomicrograph of tephroite thin section cut nearly normal to the c axis. Crossed nicols. Light lines are willemite. Area:2X1.7 millimeters" 552 C. S. HURLBAT,JR. yields 24.20A as the identity period. For the willemite co:9.35 A. 1ltte axial dimensionsof the willemite and tephroite thus compareas follows: Tephroite oo: 4.87 A bo:10.64 co:6.23 Willemite oo:13.97 it t(1010):24.2O co:9.35 For willemite Bragg and.Zachariasen(1930) give as:13.96 A, co:9.34. The closest structural correspondenceof the two sets of dimensions is found in the valuesfor c6,for 3 Xcoof tephroite is 18.69A, nearly equal to ZXco of willemite,18.70 A. The constant orientation of the willemite with respect to the tephroite host as well as the intimate intergrowth stronglysuggest that the willem- ite is a product of exsolution.Moreover, it is almost certain that much of the zinc reported in chemical analyses was contributed by willemite. CnBurcar, CouposnroN Palache (1937) lists 13 chemical analysesof tephroite from Franklin and Sterling Hill. All of theseshow the presenceof FeO (3.3370max.), and all but the earliesttwo MgO (l5.gVo max.) and ZnO (18.9O/6max.). CaO (2.70/6 max.) is present in six analyses. With the possibility of so many elements substituting for manganese in the tephroite struc- ture, it at first appeared a hopelesstask to correlate in any significant manner the physical constants with chemical composition. Of the 13 specimensrepresented by the analyses given by Palache (1937, p.77) only one (number 11) was availablefor study. This is a red- dish-brown tephroite with abundant interlaminated sheetsof willemite. The analysismade byL. H. Bauer (No. 1, Table 1) givesZnO: l2.IsTa. With the thought that willemite may have contributed much of the zinc to the analysis, a magnetic separation was made on minus 325 mesh ma- terial. Tephroite is slightly magnetic, willemite non-magnetic. After many passesthrough the Frantz isodynamic separator' even the most magnetic material was far from being free of willemite as shown by in- spection under the microscopeand ultra-violet light, and the presenceof willemite lines in aL rc-raypowder photograph. An analysis of this admit- tedly contaminated sample by Mr. Jun Ito gave 6'I/s ZnO, approxi- mately one half of that reported in the earlier analysis. With zinc present in the unseparated willemite as well as in the franklinite of the coloring inclusions,the amount in the tephroite structure must be considerably less.Could this mean that zinc is a negligiblevariant in the composition of tephroite? Of the 27 specimensstudied, one was selectedfor a completeanalysis. This was of a large reddish brown tephroite crystal from which only the coarse associatedwillemite had been separated.The analysis, No. 2, TEPHROITEFROM FRANKLIN, N, I. 553 'rable 1, compares very favorably with several earlier analysesreported by Palache (1937). Partial analyses were made of seven other tephroite specimensfrom Franklin and Sterling Hill and one of roepperite. rn these zinc is not reported since willemite and probably franklinite was present in all and its determination would not have been significant.For com- parison a partial analysis of tephroite from Liingban containing no zinc was alsomade. rn most of theseanalyses the chief oxide other than Mno is Mgo and thus most of the specimensbelong essentiallyto the series Mg2SiOa-MnzSiOa.Glasser and Osborn (1960)point out that a continu- ous solid solution seriesexists between Mg2SiOa and MnzSiOa. Tenr,r 1. Arlr,vsns ol Tnpnnorrn* 10 Sioz 29.53 30.17 ZnO 12.15 10.22 none MnO 43.64 49.57 36.0932.82 58.42 53.18 46.19 s1.31 28.68 55.65 MgO 9.64 4.92 15.28r5.76 2.40 1.07 8.72 2.66 5.65 10.75 FeO 4.41 3.94 1.26 1.N 0.79 0.78 1.90 0.29 24.28 0.09 CaO 0.05 0.63 0.10 0.05 4.40 tr 2.81 rr 1.10 Ig. loss 1.00 99.37 99.87 Molecularratio MnO:MgO MnO 72.02 85.13 57.29 55.92 93.26 96.58 75.29 gt.64 74.27 74.64 MeO 27.98 14.87 42.7t 44.08 6.74 3.42 24.71 8.36 26.j3 25.36 * 1.