New biopyriboles from Chester, Vermont I. Descriptive mineralogy

American Mineralogist, Volume63, pages 100A-1009, 1978 New biopyriboles from Chester, Vermont: I. Descriptive mineralogy Dnvn R. VnsLeNl lNo CHlnr-rs W. BunNHenr Department of Geological Sciences, Haruard Uniuersity Cambridge, M assachusetts 02 I 38 Abstract Four new magnesium-iron chain-silicate minerals have been identified from a metamor- phosed ultramafic body near Chester, Vermont. They occur with anthophyllite, cummingto- nite, and talc between the chlorite and actinolite zones at the boundary ofthe body. The cell parameters of the minerals are diagnostic: (l) j imthompsonite is orthorhombic, Pbca, a : 1 8 . 6 , b : 2 7 . 2 , c : 5 . 3 0 A ; ( 2 ) c l i n o j i m t h o m p s o n i t e i s m o n o c l i n i c , C 2 / c , a : 9 . 8 7 , b - - 2 7 . 2 , c : 5 .32A ,0 : 109 .5 " ; ( 3 ) ches te r i t e i s o r tho rhomb ic ,A2 ,ma ,a :18 .6 ,b :45 .3 , c :5 .30A ; (4 ) an unnamed mineral is monoclinic, A2/m, Am, or A2, a : 9.87, b : 45.3, c : 5.294, B : 109.7o. The physical and optical properties are close to those of low-Ca amphiboles. The cleavage angles (37.8' and44.7") are lower than those of amphiboles, and intergrowths of the minerals with anthophyll ite and cummingtonite are petrographically distinctive. The minerals are biopyriboles and are chemically intermediate between anthophyll ite and talc. The ideal chemical composition for j imthompsonite and clinojimthompsonite is (Mg,Fe),oSi,roor(OH)n, and that of chesterite is (Mg,Fe),rSiroOEl(OH)6. The new minerals might easily be confused with amphiboles if their electron microprobe analyses were considered alone. Introduction Physical similarities among pyroxenes, amphi- boles, and micas led Johannsen (1911) to call these mineral groups collectively the "biopyriboles." Solu- tion of the major biopyribole structure types later showed that the similarities were not fortuitous, but rather were a direct result of structural similarities. Thompson (1970) has pointed out that most amphi- boles can be thought of as alternating slabs of mica and pyroxene structure cut parallel to (010) along the ideal C2/m mica a-glide planes and pyroxene c- glides. These M (mica) and P (pyroxene) slabs, when assembled with the proper operations, form the MP double-chain amphibole structure. Thompson (1978) has also suggested that slab mixtures with different mica-pyroxene ratios might be found. Our work con- firms this prediction and reveals the biopyriboles as a coherent mineral family, comprising several distinct but closely related structure types. This paper describes four new minerals and their occurrence in a talc quarry at Chester, Vermont (Veblen and Burnham, 1975, 1976; Veblen, 1976l' I Present address: Departments of Geology and Chemistry, Ari- zona State University, Tempe, Arizona 85281. Veblen et al., 1977). Physical and optical data, unit- cell parameters, electron microprobe chemical analy- ses, and X-ray powder diffraction patterns calculated from the refined crystal structures are reported for three of the new minerals. Nomenclature for these three well-characterized minerals is also presented. Only preliminary unit-cell dimensions and possible space groups are reported for the fourth mineral, which remains unnamed. All four minerals, which are structurally and chem- ically intermediate between anthophyllite and talc, were found during examination of single crystals by X-ray precession photography, and the crystal struc- tures of three of them have been solved and refined using diffractometer-measured X-ray diffraction data. Two of the minerals consist of triple silicate chains connected by octahedral cation strips, while the other two are the first known mixed-chain silicate structures, containing both double and triple silicate chains. The structural crystallography is described in a subsequent paper (Veblen and Burnham, 1978). Identification of the new minerals is not simple, because they occur together as fine intergrowths. Nevertheless, the information provided in this paper should enable mineralogists and petrologists to dis- 0003-0Mx / 7 8 /09 l 0- I 000$02.00 VEBLEN AND BURNHAM: NEII BIOPYRIBOLES t inguish the new minerals from other biopyr iboles, and also from each other. Nomenclature Names for three of the new minerals have been approved by the International Mineralogical Associ- ation Commission on New Minerals and Mineral Names. The orthorhombic and monoclinic minerals with b axes of -27 A that contain only triple silicate chains are named jimthompsonite and clinojim- thompsonite, after Professor James B. Thompson of Harvard University. The orthorhombic mineral con- taining mixed double and triple silicate chains with b = 45A is named chesterite, for the locality near Ches- ter, Vermont. Although what is presumed to be the monoclinic analog of chesterite has been identified, its occurrence as very fine intergrowths in chesterite has so far precluded measurement of its physical properties and confirmation of either its chemical composition or crystal structure. Fol lowing the usage of Johannsen ( l9l l ) and Thompson (1970), biopyriboles are minerals that can be represented as mixtures of (010) pyroxene and mica slabs; pyriboles are the subset of biopyriboles excluding the micas. In this structural classification, talc is considered as a member of the mica group. Cell parameters and space groups The new minerals were first distinguished from amphiboles by their b cell dimensions. Figure I com- pares 0-level a-axis precession photographs of ensta- tite, anthophyllite, jimthompsonite, and chesterite. The photographs show the effects of differing b-axis lengths and exhibit the following extinction criteria: k : 2n 1 l missing for enstatite and jimthompsonite, and k * I : 2n * I missing for anthophyllite and chesterite. Table I compares the cell parameters and space groups with those of anthophyllite from Chester. The cell dimensions of anthophyllite, chesterite, and jim- thompsonite were determined by least-squares refine- ment with 84,95, and 98 measurements respectively from precision back-reflection Weissenberg films, us- ing the program LcI-sQ (Burnham, 1962) with sys- tematic correction terms for absorption, film shrink- age, and camera eccentricity errors. All data for chesterite and jimthompsonite were obtained from the same crystals used for X-ray intensity measure- ments by remounting the crystals to obtain a second orientation. The clinojimthompsonite cell parameters were refined by a least-squares method using twelve diffractions manually centered on a four-circle dif- 1001 Fig. l. 0-level a-axis precession photographs of enstatite (En), anthophyl l i te (An), j imthompsoni te (Jt ) , and chester i te (Ch), showing differences in D* length. fractometer, while those of the unnamed mineral were measured from precession photographs. Space groups of the minerals listed in Table I were determined by examination of extinction criteria on long-exposure precession films. The space group of jimthompsonite (Pbca) is uniquely determined, but the others are not; reasons for selecting one ofseveral diffraction-equivalent space groups for each mineral are discussed in conjunction with the structural crys- tallography in a subsequent paper (Veblen and Burn- ham. 1978) . Occurrence and associations The new minerals all occur in the blackwall zonez of a metamorphosed ultramafic body that is exposed in the Carleton talc quarry near Chester, Vermont. The geology of the quarry has been described by Gif lson (1927), Phillips and Hess (1936), and Chides- ter et al. (1951). The generalized zoning sequence from country rock to ultramafic body is: (l) mus- covite-quartz-garnet gneiss; (2) altered gneiss; (3) biotite and chlorite blackwall; (4) actinolite; (5) talc; (6) talc, magnesite, and serpentinite. The material used in this study is from a single block of blackwall found on the quarry dump, al- though similar material has been found in place on the northern wall of the flooded quarry (Richard 2 The term "blackwall zone" is commonly used to describe the chlorite- or biotite-bearing metasomatic reaction zones that are frequently found at the boundaries of metamorphosed ultramafic bodies. VEBLEN AND BURNHAM: NEW BIOPYRIBOLES J inthonpsonlte a = 1 8 . 6 2 6 3 ( 3 ) A b = 27 .2303(6)A c = 5 . 2 9 7 0 ( 3 ) A v = 2686.6(2)A3 Pbca z = 4 [ (Mg,Fe) r ' s112032(oH)4 ] Ches terlte a = 1 8 . 6 1 4 0 ( 3 ) A b = 4 5 , 3 0 5 ( I ) A c = 5 . 2 9 6 6 ( 3 ) A v = 4 4 6 5 . 8 ( 3 ) A 3 A 2 , u z = 4 [ (Mg,Fe) r rs120054(o t t )6 ] Anthophyll lte a = 1 8 . 5 8 6 3 ( 2 ) A b = 1 8 . 0 6 4 9 ( 2 ) A c = 5 . 2 8 9 5 ( 4 ) A v = 1 7 7 6 . 0 ( 1 ) A 3 PM z = 4 [ (Ms,Fe) 7S18O22(OH)2] C11noj iEthoDpsonlte a = 9 . 8 7 4 ( 4 ) A b = 2 7 . 2 4 ( 3 ) L c = 5 . 3 1 6 ( 3 ) A B = 1 0 9 . 4 7 ( 3 ) ' v = 1 3 4 7 . ( 3 . ) A 3 c 2 l c z = 2 [ (Mg,Fe)10s112032(oH)41 Unnaned mlneral a = 9 . 8 6 7 A b = 4 5 . 3 1 A c = 5 .292A, B = L09.7" v = 2227.A3 A2ln, An, or 42 Table l. Cell parameters and space groups of low-calcium chain silicates. Chester. Vermont Orthorhomblc Monocllnlc tremolite to anthophyllite. At Chester the anthophyl- lite is replaced by chesterite, the unnamed mineral, jimthompsonite, clinojimthompsonite, and fibroirs talc. The new minerals are thus part of a retrograde reaction sequence from anthophyllite to talc. Habit, color, cleayages, and partings The new minerals occur as intergrowths parallel to (010) in anthophyllite and cummingtonite, and form radiating sprays of prismatic crystals up to 5 cm long. The anthophyllite and the new minerals are trans- parent and have the same color, ranging from color- less to very light pinkish brown. All are colorless in thin section. Cleavage angles were measured with a reflection goniometer on the single crystals used for X-ray in- tensity measurements. Chesterite possesses perfect { l l0} c leavage intersect inBat44.To and 135.3o, whi lejimthompsonite has perfect {210} cleavage at 37.8o and 142.2". In addition, both minerals break along {100} and {010}, but these direct ions may be part ings, {100} resulting from breakage along fine monoclinic lamellae and {010} from separation along lamellae of another orthorhombic pyribole. The cleavages should be valuable diagnostic properties in well-crys- tallized specimens, but some material from Chester consists of several minerals so finely intergrown that the cleavage is not apparent; these specimens are often fibrous. Structurally, the chesterite and jimthompsonite cleavages are analogous to the {210} cleavage of the orthopyroxenes and orthoamphiboles. Because the monoclinic polymorphs from Chester occur only as lamellae, their cleavages could not be observed, but on structural grounds one would predict a {120} cleavage for the unnamed mineral and a {110} cleav- age for clinojimthompsonite. Optical properties Optical data for jimthompsonite, chesterite, and anthophyllite from Chester were obtained by using a spindle stage (Wilcox, 1959). The jimthompsonite and chesterite crystals were the same ones used for X- ray intensity measurement. The crystals were first mounted with their b axes coincident with the spindle axis, precise orientation being achieved by directly remounting the crystals from their X-ray goniometer- head mounts. Optic axial angles (2V,) for sodium light were measured directly by spindle rotation, and dispersion was observed in blue-filtered light from an incandescent bulb. Principal indices of refraction were then measured for sodium light by mixing index Sanford, personal communication). The block may be a section of a continuous blackwall zone, or it may be part of an isolated pod. The simplified zoning sequence observed in this block is: (l) chlorite; (2) fibrous talc; (3) fibrous talc, jimthompsonite, clino- jimthompsonite, chesterite, the monoclinic analog of chesterite, anthophyllite, and cummingtonite; (4) anthophyllite, cummingtonite, chesterite, the mono- clinic analog of chesterite, and actinolite; (5) acti- nolite and massive talc. Magnetite is found through- out as an accessory mineral, and cummingtonite has been observed as lamellae in actinolite and an- thophyllite. Actinolite and anthophyllite commonly occur as oriented intergrowths on planes near (100), attaining lengths of 5 cm. Three similar occurrences of tremo- lite and anthophyllite from the Gouverneur mining district in New York have been interpreted by Ross et al. (1968) as representing retrograde alteration of VEBLEN AND BURNHAM: oils until a match was obtained (Becke line method); the index of the matched oil was measured with an Abb6 refractometer calibrated with oils matched to n(fluorite) and <.r(quartz). The crystals were then re- mounted with their c axes coincident with the spindle axis, and the indices were checked. Optic axial angles calculated from the indices of refraction agree with the observed values of 2V, within the estimated preci- sion of the measurements, The consistency of these measurements was further checked by measuring re- tardation ratios of the components of multiphase grains in thin section with a Berek compensator. Optical properties (Table 2) can be used to differ- entiate between anthophyllite, chesterite, and jim- thompsonite from the Chester wall zone, but com- par ison o f these da ta w i th the numerous determinations for anthophyllite given by Rabbitt (1948) indicates that optical methods are probably not the best determinative means for distinguishing these minerals at other localities. Anthophyllite and gedrite show a wide range of optical properties, and the new minerals will probably show similar varia- tions with compositional changes. Clinojimthompsonite occurs only as thin lamellae and was not suitable for optical measurements. How- ever, the extinction angle NAc = 10o was measured. The new minerals closely resemble amphiboles in thin section. In (001) sections the lower cleavage an- gles are revealed in some instances, but the cleavage is often indistinct or fibrous. The minerals usually occur together in (010) intergrowths, however, leading to a striking appearance under crossed polars for crystals with b near the plane of the section (Fig. 2a-c)s. Because pyribole exsolution features are usually re- stricted to planes near (100) and (001), there should be little difficulty in recognizing these intergrowths; anthophyllite-gedrite exsolution on (010) could, however, cause confusion. When the intergrowths have b inclined more steeply, but not normal, to the section plane, they can take on a softly striped ap- pearance (like a multi-colored candy cane) in crossed polars (Fig. 2d). Portions of some grains possess interference colors intermediate between those shown by anthophyllite, chesterite, and jimthompsonite, resulting in streaks parallel to (010) (Fig. 2a). These intermediate optical properties suggest that anthophyllite, chesterite, and NEW BIOPYRIBOLES 1003 Table 2. Optical properties of anthophyllite, chesterite, and jimthompsonite from the Chester wall zone Anthop[yll1te Chesterite Jirthonpsonite c = a - I - 2\I L.620 1 . 530 1. 5l+r & c d = r . o f I B = r .632 Y = r.5l+o Z = c x NegatiYe r>v, veak d = f . o u ) B = r .626y = r .533 X = a Z = e 2''I = 62o x Negative r>v, veah e A color photograph of such an Science, October 28, 197'l (Veblen also shows the cleavage angles of j imthompsonite. intergrowth is on the cover of et al., 1977). This photograph anthophyllite, chesterite, and Est i ra ted er ro rs : re f r . ind . : lO .OOO5, 2V: 12o j imthompsonite are intergrown on a scale too fine to be optically resolved, or that there are stil l more structural ordering schemes or areas of disordered chain sequence. Most grains, however, are optically uniform, and intergrowths usually exhibit optically- sharp boundaries between anthophyllite, chesterite, and jimthompsonite. Chemical analysis The results of electron microprobe analyses in- dicate that the major-element compositions of the new minerals are intermediate between those of an- thophyllite and talc. Owing to the intergrown nature of the new minerals, pure samples could not be sepa- rated for water analysis. Analyses (Table 3) were performed on an auto- mated MAC Model 5 electron microprobe. All speci- mens and standards were carbon-coated. Operating voltage was l5 kV, with a reference current of 300 mA. A diopside-jadeite glass was used as the stan- dard for Si, Al, Mg, Ca, and Na; natural aenigmatite, ilmenite, forsterite, chromite, rutile, and orthoclase were used for Fe, Mn, Ni, Cr, Ti, and K, respectively. Counting time was 20 seconds for both background and peaks; counting was stopped when the number of counts exceeded 30,000. Compositions were cor- rected by the alpha-matrix method of Bence and Al- bee (1968), using alpha factors calculated as de- scribed by Albee and Ray (1970). The microprobe analyses show that the Chester pyriboles contain Mg, Fe, Mn, Ca, Al, Si, and Na; the elements Ni, Cr, Ti, and K are not present in measurable amounts. Energy scans performed with an Eonx 707A energy-dispersive analyzer on a Cam- eca MS46 microprobe confirmed the absence of sig- nificant amounts of unexpected elements. The jim- thompsonite analysis is the average of 1l analyzed points on the same crystal used for X-ray intensity measurement, plus three points from optically-identi- 1004 YEBLEN AND BIJRNHAM. NEW BIOPYRIBOLES Fig. 2. Chester b iopyr iboles in th in sect ion. Scale bars represent 0.1 mm. (a) Sect ion paral le l to (001) o lgrain consist ing most ly of anthophyl l i te. The anthophyl l i te is beginning to t ransform to chester i te, producing streaks paral le l to (010). The stra ight crack paral le l to (100) (arrowed) is probably a remanent "herr ingbone" twin p lane, indicat ing that th is grain was or ig inal ly a monocl in ic amphibole crystal . Crossed polars. (b) A (010) intergrowth of anthophyl l i te and chester i te, wi th the b axis near the plane of the sect ion. Crossed polars. (c) A sharp (010) intergrowth of anthophyl l i te, chester i te, and j imthompsoni te. The b axis is in the plane of the sect ion, but no cleavage is v is ib le. Crossed polars. (d) A (010) intergrowth of the pyr iboles wi th the b axis near the sect ion normal . Crossed polars. f ied grains in polished thin section. The clinojim- thompsonite analysis is the average of four points on one of the lamellae in the crystal used for intensity- data collection; because the lamella width (2p) is narrower than the volume activated by the electron beam, this analysis is undoubtedly "diluted" by j im- thompsonite, which is the host mineral. The chester- ite analysis is the average of l4 points in thin section. In addition, anthophyll ite, foliated talc, and acti- nolite grains from the blackwall zone were analyzed, and the results given in Table 3 are averages of 17 points from anthophyll ite, f ive from talc, and three from actinolite. The average analyses of chesterite, j imthompson- ite, and clinojimthompsonite are consistent with the refined structures (Veblen and Burnham, 1978). The ideal chemical formula for chesterite is M,rSi2o O,n(OH)6, and that for j imthompsonite and clinojim- thompsonite is MroSirrOrr(OH)n, where M refers to Mg and Fe'+. The chesterite formula is thus the sum of the anthophyll ite formula, M?SisOrr(OH)r, and the jimthompsonite formula. Figure 3, a ternary plot of octahedral cations us. tetrahedral cations us. HOo.r, shows that the new pyriboles ideally are chemically intermediate between anthophyll ite and talc and are coll inear with enstatite, anthophyll ite, and talc. The analyses show that small amounts of Mn and Ca substitute for Mg and Fe; the Mn may be respon- sible for the l ight pinkish color, observable in hand specimen, of the low-Ca pyriboles. Only clinojim