THE OPTICAL PROPERTIES of the HUMITE GROUP Espbn S. Lensbn

354 TEE AMERICAN MINERALOGIST

THE OPTICAL PROPERTIES OF THE HUMITE GROUP EspBn S. LensBN

A considerable number of specimensof the humite group were examined optically in an attempt to find some consistent differences that would enable one to quickly and with assurance,dis- tinguish between the members of the group. Assuming that the specimens were correctly labelled, the resulting data and in par- ticular the indices of refraction for each member of the group showed a surprisingly wide variation and within about the same range for each of the species,so that no satisfactory data for deter- minative purposeswas had and the suspicion was raised that many of the specimensmight be incorrectly labelled. It was accordingly decided that any reliable data on the group must be based on the examination of material that had been analyzed or otherwise positively identified. I was fortunate enough to secure the material studied by Pen- field and Howel from Professor Ford of Yale University, and that studied by Sjcigren2from Professor Aminoff of Stockholm. I ex- press my sincere thanks to Professors Ford and Aminoff for the use of these materials. The study was made by the immersion method, and the indices of refraction were determined in sodium light and should be in error * 0.001. Some of the plates of Sjnorbergite, are optically f , and all have large to very large axial angles. The pleochroism is stronger for the darker colored specimenand in all the pleochroic formula is: X: yellowish brownl y:paler yellowishbrown; Z : colorless. Taer,u I gives the indices of refraction for five analyzed specimens of chondrodite from as many localities, for a specimen that was similar to the analyzed sample from a sixth locality and for the mineral from four other localities. The indices of refraction for all of the five analyzed specimensare very much alike, those I Penfield, S. L. and Howe, T. H. On the Chemical Composition of Chondro- dite, Humite, and Clinohumite. Am. J. 5c.,47,188-200, 1894. 2 Sjdgren, IIj. Chondrodite von Kafveltorp, Zei.t. Kryst. Min.,7, ll3-152,1882, Beil,ritge zur Minerdogie Schweilens, Bul,l. Geol. Inst. IJpsuI,a, I, t-54, 1892; 2,39-54, 1894. JOURNAL MINERALOGICAL SOCIETV OF AMEruCA 355

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;o ! gf,9ssE?$"sB$*.ii ,.i . >-x o6 daa- NFt 356 TE E AM ERICAN M I N ERALOGIST for the Franklin mineral are considerably lower, while those for the Weston, Nordmark, Custer County, and Tilly Foster minerals are much higher. Analyses are available for the mineral from Nord- mark and Tilly Foster but the exact materials that furnished the analyses were not available for optical study.

Tesrr II. ANlr,vses nNn Oprrcer,PRoPERTTEs or Huurte Monte Vesuvius Nordmark Nordmark Somma Yale Sjiigren Sj6gren 4102 Brownish Yellowish Light dark yellow Sray SiOz 36.63 36.74 35.M 35.2r Tio: 0.07 0.07 Mgo 56.45 56.31 47.22 48.33 CaO o.t7 0.11 FeO 2.35 2.22 10.96 7.93 MnO 1.47 1.69 ALOa 0.19 0.07 FerOa 0.68 1.06 F 3.68 3.96 3.79 4.59 KrO 0.17 0.20 NazO o.29 0.34 HrO* 2.45 t.28 f Hro- 11.8s

100.96 101.36 101.73 101.4s OforF 1.26 1.66 1.60 r.93

99.70 99.70 100.13 99.52 Sp. Gr. 3.194 3.183 3.32 3.24 3.201 3.225

d, l.62ls 1.625 1.632t0 1.627 1.635 r.643 t.649 1.656

2V 70"

RnrrnrNcrs ron Teslos I lNo II s Barth, Tom. On Contact Minerals from the Pre-Cambrian in Southern Norway, Norsk, Geol.Tid.,Vnl,94-96, 1924. 'von Eckermann, Harry. The Rocks and Contact Minerals of the Mansjti Mountain, Geol. F or., Stockholm, 27 9-383, 1922. 5 Eskola, Penti. The Contact PhenomenaBetween Gneissand Limestone in Western Massachusetts,Jour. Geol., XXX, 270, 1922. TOURNAL MINERALOGICAL SOCIETY OF AMEMCA JJ/

6 Optical data on brown crystals similar to thoseanalyzed but not on identical material. 7 Shannon,E, V. The Minerals of Idaho. U. S. Nar. Mus., Bull. 131,33F334, 1926. t Specimensfrom Tilly Fostergave: . d 1.627 1.632 1.635 1.643 p 1.638 r.640 1.645 1.655 'y 1.657 1.661 1.663 1.670 The last figuresare by GillsonnJ, L., Atn. Mineral.,ll, 284, 1926, e Optical data not closelytied to the analysesbut on specimen4806 of the Brush Collection which is very pale brown and answersthe description of the analyzed specimen. r0 No.'62 Plate 2 of Sjdgren-Optical data not closelytied to analysis.

Tegr.p III. Ane,rvsrs eno Oprrcll Pnopnnrrns ol Clnqosulrrrn. Vesuvius Vesuvius Nordmark Chafiev'su Piedmontra YaIe Brush Sjdgren Lake, Ontario 4t43 2046 No.63 si02 37.78 38.03 35.86 37.42 36.83 TiOz .06 |.14 r.92 Meo 53.05 54.00 M.66 56.32 51.53 CaO trace FeO 5.64 4.83 t4.25 |.27 -4.W MnO t.t9 0.10 .28 AbOa 0.07 FerOs o.22 0.46 0.42 F 3.58 2.06 4.t6 5.04 0.03 KrO 0.15 NazO o.29 o.t2 HrO+ 1.33 1.94 1.58 o.s6 lr.u HrO- t0.12

100.38 100.86 102.42 L02.3t 99.2613 OforF 1.50 .86 r.75 2.r2

99.88 100.00 100.67 100.19

Sp.Gr. 3.219 3.184 3.35 3.17 3.258 3.222 a 1.632 1 .62514 l.652rt 1.628 1.66411 p t.6M 1 .638 1.663 t.&2 r.67s .| L.664 1 .653 1.698

2V 74" near 90o 76027', 62" Ext. 120-150 70"29', 358 TH E AM ERICA N M I NERALOGIST

RelpruNcrs ron Tnsrn III.

11Optical data not closely tied to analyses. 12Walker, T. L., and Parsons, A. L.-Clinohumite {rom Chafiey's Lake, Ontario. Conh. Can. Mi.n., p. 17-18, 1927. 13Zambonini, F., -Sur la veritable nature du Titanolivine de la valler d'Ala (Piedmont), Bull. Soc.Fr. Minr., XLII, 1-30, 1919. Contains PbO.0.14,'BaO 1.30, CrrOB0.03, NiO 0.07, Optical data by Larsen. la Not uniform-Indices vary*0.005 or more. Data are for about averages.

fn Tasrc II optical data are given for three analyzed specimens of humite from two localities. In Tesln III optical data are given for five analyzed specimens of clinohumite from four localities. The corresponding data for two specimens of analyzed norbergite are given in the preceding paper. Assembling the data for the four members of the humite group, Tnnr,B IV shows that on the whole, as the MgzSiOr increasesin amount the indices of refraction increase,but there is much overlapping. The norbergite occupies a low range and can be dis- tinguished from the others by its indices of refraction. Most of the analyzed chondroditeshave p between 1.616,and 1.622, and are higher than any of the norbergites and lower than any of the humites but some of the chondrodites have B as high as any clinohumite except that from Piedmont. The humites mostly have B lower than does clinohumite but there is overlapping.

Teer,n IV. Opucer, Darn ron rnr Huurro Gnoup

Norbergite Chrondrodite Humite Clinohumite Compo-Mg(OH, F):. Mg(OH, F): Mg(OH, F), Mg(OH, F)z' sition MgrSiOo 2MgrSion 3Mgrsioo 4Mg2SiOr q 1.561-1 .563 t.60t-t.643 t.62r-1.632 r.623-1.664 a 1 .566-1. 570 1.616-1 .655 r.627-1.643 t.636-1.673 7 1. 587-t .590 t.636-1.670 r.649- 1.651-1 .698

Ext. Orth. 22'-29" Orth. 7"-15" 2V 45'-50' 73"-90" 700- 620-90"

The variation in the indices of refraction is brought out by com- paring the data for the three minerals (chondrodite, humite, and clinohumite) from the same localities. Such data are available from Vesuvius and from Nordmark and are shown in Tnsr,B V. For both localities the indices of refraction of the chondrodite JOURNAL MINERALOGICAL SOCIETY OF AMERICA are lowest, that for humite intermediate.and that for clinohumite highest though for the Vesuvius mineral the differences are not great. For each speciesthe Nordmark mineral has considerably higher indices of refraction and iron content than the Vesuvius mineral.

T,tsrn V. A Colperrsox or MeMsnnsol rnn Huurrr Gnor,'prnou Vnsuvrus AND 'RoM Nonolt'Ltx Fromvesuvius Chondrodite Humite Clinohumite a 1.609 |.621 1.625 r.632 1.625+ p 1.619 1.627 1.635 r.6M 1.638+ "y |.641 1.649 1.656 1.664 1.653+ FeO*MnO 3 .66 2.35 2.22 5.64 4.83 '58 F 5.15 3.68 3.96 3 2.06 HrO 2.82 2.45 2.t3 1.33 1.94

From Nordmark Chondrodite Humite Clinohumite a 1.619 1.632 1.652 p r.632 r.643 1.663 r 1.653 .

FeOfMnO 11.78? 9.62 15.M F 5.40? 4.59 4.16 HrO 2.30? 1.85 1.58

CorvcrusroNs.An attempt to correlatethe chemicalcomposition with the optical constants is disappointing. One would expect a simple and systematic relation in which the indices of refraction increase with the iron and titanium content and decreasewith the increase in fluorine. There appears to be some tendency in this direction as indicated by the Nordmark minerals which are high in iron and have high indices of refraction but the relations are far from simple. While for the analyzed chondrodites, the optical data are rather uniform (F:1.616 to 1.622) although there is considerable variation in chemical composition (FeO*MnO: 2.64 to 6.62); the Tilly Foster mineral shows very nearly the chemical composition of the Kaveltorp mineral described by Penfield and Howe, yet its indices of refraction are much higher (for Kaveltorp F: 1.620 for Tilly Foster P:1.645). These data are so inconsistent as to lead one to the suspicion that some con- stituent, such as titanium, has been.overlookedin the Tilly Foster mineral.