6 p. : ill. ; 24 cm.Includes bibliographical references (p. 5-6).Pyrite, French Creek, Chester County, Pennsylvania -- Smoky quartz, El Paso County, Colorado

Frondel, Clifford, 1907-

American Museum of Natural History Scientific Publications

AMERICAN MUSEUM NOVITATESPublished byNumber 829 THE AMERICAN MUSEUM OF NATURAL HISTORY March 19, 1936New York City54.8, 5TWISTED CRYSTALS OF PYRITE AND SMOKY QUARTZBy CLIFFORD FRONDELTwisted crystals are so rare in nature that any information regardingnew occurrences is worthy of record. General accounts of the phenome-non of twisting in crystals have been given by Spencer (1921),Bernauer (1929), and Rossmann (1934).Fig. 1. Pyrite crystal twisted on the trigo- Fig. 2. Diagram illustrating thenal axes of symmetry. Natural size. sense of the twist in the trigonal axes.PYRITE, FRENCH CREEK, CHESTER COUNTY, PENNSYLVANIAAxially twisted pyrite crystals have not heretofore been described,although bent crystals and crystals with concave or convex faces areknown. The present instance is afforded by a suite of 16 specimensfrom French Creek, Chester County, Pennsylvania, which exhibittwisted and twisted skeletonized cubes of pyrite implanted on drusycrystals of magnetite and specularite: The pyrite crystals on a few ofthe specimens are partly covered by deeply etched crystals of calcitethat are colored green by included fibers of byssolite.AMERICAN MUSEUM NOVITATESA typical twisted cube is shown in Fig. 1. The axes of torsion arethe trigonal (octahedral) axes of symmetry. The twist in these axes issuch that the opposite corners of each trigonal axis are twisted in thesame sense, with the two trigonal axes in each vertical diagonal plane ofsymmetry each twisted in an opposite sense. This relation is sketched inFig. 2.An extreme instance of such twisting is shown in Figs. 3a-3b. Thiscrystal is a skeletonized and markedly composite cube with the cornerstwisted, in the manner described, through approximately 60°. Thelarger individuals forming the twisted, composite crystal are composed ofa bFigs. 3a-3b. Two views of a skeletonized and twisted pvrite cube. The upperright corner of the cube face represented in Fig. 3b has been broken off. Naturalsize.much smaller crystals -closely aggregated in nearly parallel position.The largest of the sub-individuals are those occupying the corner posi-tions.Several small pyrite crystals in the suite are not twisted; but exhibita peculiar composite structure in which eight separate cubes are groupedtogether to form a single large cube. The several parts are not quiteparallel, but diverge at small angles and may be separated, in part, byopen cracks. One of these crystals, in which the four crystals formingthe base of the larger, composite cube are subordinate in size and do notappear in the photograph, is shown in Fig. 4. The composite structure2 [No. 8291936] TWISTED CRYSTALS OF PYRITE AND QUARTZin these crystals, and in the preceding, twisted crystals, appears to bethe result of lineage growth, as described by Buerger (1934).Cobalt and arsenic were found by Genth (1875) as impurities inthe pyrite from French Creek, and the twisting and composite growth ofFig. 4. Composite cube formed of eightseparate cubes in approximately parallel posi-tion. Only the upper four of the small cubescan be seen in the photograph. Natural size.Fig. 5. A pale smoky quartz crystaltwisted on a lateral, digonal axis. Thecrystal is distorted by elongation andflattening parallel to this axis. The horn-zontal face at the top of the crystal is aprism face-the crystal is oriented withthe twisted a-axis vertical-and the smalltriangular face immediately below it be-longs to the trapezohedron (5161). Theunusual habit gives the crystal an ortho-rhombic aspect. St. Gotthard, Switzerland.Natural size.the crystals is possibly caused by the presence of these elements in solidsolution. The dissimilarity of properties-size, polarizability, deform-ability, etc.-of the foreign atoms and the Fe and S atoms would distortthe crystal lattice, and the lattice strain set up in this way would very3AMERICAN MUSEUM NOVITATESlikely cause abnormalities in the growth of the crystal. If this is thecase, the directional nature of the distortion suggests that the foreignatoms are arranged along certain planes only.The locality at French Creek has also yielded pyrite crystals thatare remarkable in being tetragonal or orthorhombic in outward sym-metry, the crystals appearing as curved vicinal pyramids elongated inthe direction of one crystallographic axis. These crystals have beendescribed by Penfield (1889), in part from specimens originally con-tained in the Bement collection and now in the collection of the AmericanMuseum.Fig. 6. A dark smoky quartz crystal twisted on the vertical trigonal axis.El Paso County, Colorado. One-half natural size.4 [No. 8291936] TWISTED CRYSTALS OF PYRITE AND QUARTZSMOKY QUARTZ, EL PASO COUNTY, COLORADOTwisted smoky quartz crystals from several European localitieshave been described. Two types of twisting have been found: theaxis of torsion being either a lateral, digonal axis, or the vertical, tri-gonal axis. A crystal of the former type, from St. Gotthard, Switzer-land, is shown in Fig. 5. Similar instances from other localities inSwitzerland have been described by Tschermak (1894), RosickS (1933),and others.A smoky quartz crystal twisted on the vertical, trigonal axis,from El Paso County, Colorado, is shown in Fig. 6. The crystal is dis-torted by flattening parallel to a pair of prism faces and measures 21 X2.9 X 1.7 cm. It is a right-handed individual, twinned on (1010), withcorroded faces of the positive and negative unit rhombohedrons, anunidentified steep rhombohedron, and the right positive trapezohedron(5161). The twist is right-handed and is approximately 450 in 21 cm. oflength.The occurrence of twisting in the smoky variety of quartz and itsgeneral absence in the colorless and amethystine varieties suggests aconnection with the pigmentation of the crystals, as was found experi-mentally by Bernauer and others for twisted crystals of various sub-stances grown from solutions containing organic impurities. Thiswould oppose Holden's contention that the pigment of smoky quartzis of a secondary origin, produced in an originally colorless crystal bythe action of radiation. The writer (1934) has described further evi-dence that indicates a primary origin of the color of smoky quartz.REFERENCESBERNAUER, F. 1929. ' "Gedrilite" Krystalle': Forschungen zur Krystallkunde,Heft 2, 102 pp., 18 text figs., 12 plates, 49 figs.BUIERGER, M. J. 1934. 'The Lineage Structure of Crystals': Zeits. Krist.,LXXXIX (A), pp. 195-220.FRONDEL, C. 1934. 'Origin of the Segmental Coloration of Amethyst and SmokyQuartz': Amer. Mus. Nat. Hist. Novitates, No. 758, pp. 1-15.GE1rmi, F. A. 1875. 'Mineralogy of Pennsylvania': 2nd Geol. Surv. Penn.,Prelim. Rept., p. 20; Amer. Jour. Sci., (3) XL, p. 114.PENFIELD, S. L. 1889. 'On some curiously developed pyrite crystals from FrenchCreek, Delaware County [Chester County], Pa.': Amer. Jour.SCi., (3) XXXVII, pp. 209-212.RosICKY, V. 1933. Spisy Vyd6,van6 Pfirodov6deckou Fakultou MasarykovyUniversity, No. 187, pp. 1-24, 24-31 (French r6sum6).RossmANx, E. 1934. 'Cornuted Crystals': Jour. Soc. Chem. Ind., LIII (T), pp.135-137.56 AMERICAN MUSEUM NOVITATES [No. 829SPENCER, L. J. 1921. 'Curvature in Crystals': Mineralog. Mag., XIX, pp. 269-271.TSCHERMAK, G. 1894. 'Ueber gewundene Bergkrystalle': Denkschr. Akad. derWiss. Wien, LXI, pp. 365-400, P1. ii, figs. 8a-b-c, P1. iII, figs.- 9a-b-c.

Twisted crystals of pyrite and quartz

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