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Quartz with Rhombohedral Cleavage from Madagascar

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Quartz with Rhombohedral Cleavage from Madagascar American Mineralogist, Volume 66, pages 596-600, I98l Quartz with rhombohedralcleavage from Madagascar Orro W. FLORKE,HnrNz G. Mmrre, JtlncnN Wnrcnr,nt Institutfiir Mineralogie,Ruhr-Universitrit Bochum, Postfach, D-4630 Bochum, Germany AND HOLGER KULKE DEMINEX, DorotheenstraBe1, D-4300Essen, Germany Abstrsct Anhedral quartz slabsfrom a vein depositin Madagascarshowing smooth cleavageplanes parallel to positive rhombohedra r {l0ll) are described.Only two of the three equivalent pairs ofthe form {l0Tl} show cleavage,although ofdiflerent quality (smoothness).The best developedplanes lie parallel to Brazil-twin lamellae;the lesswell developedplanes do not. We presumesmall tectonically induced thermal shocksto be the releasingmechanism for rhombohedral cleavagefractures. In combination with hydrolytic weakening and directed stress,large smoothcleavage planes were formed. Sincethe r-facesof quartz are F-faces,they have a minimum of free surfaceenergy. We assumethis to be the reasonfor cleavageparallel to positive rhombohedralfaces. Introduction surfaceswith pearly luster over areasof l5 to 20 cm'z. Faint, unevenlyspaced, parallel striationsindicated a Many authors describe qtrartz that, beyond its secondand a third direction of parting which were well-known conchoidalfracturing, under specialcon- not as well developedas the first. Thin sectionscut ditions shows cleavage of decreasing quality normal to the principal parting plane showedparallel (smoothnessor perfection)on r {l0Tl}, z {0lTl}, lamellae of varying width. The alternating arrange- and m {10T0}, respectively.In most caseshowever ment of these lamellae resembled albite twinning. no distinction has been made betweenr and z. The Therefore the authors concluded that this peculiar cleavageparallel to ru usually is not well developed. parting might be explained as a pseudomorph of Rose (1844) mentioned that cleavage of quartz single-crystalquartz after triclinic feldspar,though it may occur to an equal extent parallel to the positive was admitted that pseudomorphic replacement by and negativerhombohedra. The l9th century litera- single crystalsinstead of fine-grainedpolycrystalline ture on quartz cleavage was reviewed by Hintze material is very unusual. Jahns (1953) stated that (1915),and the literature up to 1960 by Frondel there existsno evidencethat large crystalsin pegma- (1962). All authors reported that pronounced platy tites grow by replacementof other minerals. cleavage or parting occasionally occurs in vein The production of cleavagein quartz through ther- qtarlz, mostly in the inner parts of vein fillings. mal shockby Kenngott (18M49) and under directed Rogers (1934) mentioned huge anhedral blocks of stressby Judd (1892) was mentioned by Hintze vein quartz from California pegmatites.Since parting (1915).Borg and Maxwell (1956)subjected qluartz wasnot observedparallel to all three equivalentpairs sand to a directed stressof about 2 kbar at 300'C. of rhombohedral faces, Rogers rejected the term under a confining pressureof about I kbar trans- cleavageand concludedthat parting might be due to mitted by alkaline hydrous solutions.They observed polysyntheticDauphin6 1ryinning, mechanicallypro- a remarkableincrease of fracturing parallel to r and duced during the high-low transformation of the z, but none parallel to lrt. The percentageof grains qvafiz. with deformation lamellaedecreased simultaneously. Murdoch and Webb (1938) describedquartz with The fractures occurred rarely in parallel sets and oc- nearly perfect platy parting in pegmatitic veins from casionallyconformed to more than one crystallogra- another California localitv. The slabs showed flat phic plane. Bloss (1957)crushed quartz crystalsboth 0003-004x / 8| /0506-0s96$02.00 596 FLORKE ET AL.: QUARTZ WITH RHOMBOHEDRAL CLEAVAGE at room temperature and between 650 and 700"C. tions perpendicularto the two recognizablecleavage He obtained a pronounced cleavageparallel to the planeswere measured to be 87!2". rhombohedra r and z and to a much lesser extent A systemofshort and lesslinsa1, healed fissures M parallel to the prism m. also exists (Fig. 2). Angles of intersection between theseflssures and R, or R,, are about 130to l40o in Geologicalsetting and mineralogy the same sectionsas above. The M-fissures.which One of us (HK) collected anhedral fragments of have healed after fracturing, representcharacteristic large single crystalswith almost perfect platy cleav- accumulations of aligned fluid inclusions. The R- age from qvartz veins in NE Madagascar.The hills, fracturesare unhealed. bordering D'Antongil Baie (Fig. l) are part of an old Universal-stagemeasurements proved the R-frac- cratonic block of Gondwana. Granites, migmatites tures to be the tracesof rhombohedralfaces and the and gneisseswere cut by a great number of quartz M-fissures to be atgned almost parallel to prism veins and basalticdikes during the Late Cretaceous. faces. At that time the separation India of from Madagas- Experimentalresults car causedintense shearing and mylonitization of the basementrocks (Hottin, 1969). X-ray measurements,according to the orientation Deep lateritic weatheringobscures the attitude and method of Keppler (1975),proved that the observed thicknessof the quartz veins. Furthermore, frequent cleavageplanes are rhombohedralfaces. It is difficult landslides have produced accumulations of quartz to distinguish between r- and z-rhombohedrausing blocks at the baseof the slopescontaining the origi- X-ray methods.Although the intensitiesof the reflec- nal outcrop. tions l0Tl and 0lTl are different (International Ta- Our samplescome from quartz veins of about 2 m blesfor X-ray Crystalography,1969, Vol. I, p. 465) thickness, presumably crystallized from hydro- their exact determination is nearly impossible, be- thermal solutions at the expenseof the quartz con- cause both reflections are strongly affected by ex- tent ofthe shearedhost rocks.The quartz variesfrom tinction effects.Literature data (International Tables, milky or slightly smoky to absolutelytransparent in 1962,Yol. III, p. 79; Le Page et al., 1976)are con- various parts of the vein. Cleavageoccurs especially tradictory on the question of which reflection is the in the inner parts of the vein fi[ing. The remainder more intense. exhibits conchoidal fracturing only. The best devel- An unequivocal experimental indication of oped cleavageswere observedin blocks of approxi- whether the rhombohedralcleavage planes are posi- mately I m' volume composedof alnost clear quartz tive r- or negative z-planes (Bravais-setof crystal- with only minor trains of inclusions and cloudy lographic axes,see Dana Frondel, 1962)can be ob- schlieren(Fig. 2). tained by hydrothermal growth. Prisms measuring 5 x 5 x 15rrm were cut from platy fragmentswith two natural cleavageplanes forming two faces of these Sampledescription prisms. These served as seeds.Growth conditions Thin sections of various thicknessesproved the were: seedtemperature 380oC, nutrient temperature samplesto be single crystals; however, the micro- 400"C, pressure1.5 kbar, duration 14 days;nutrient: structure is characterized by twin lamellae following broken quartz of the same sample, solvent: I m the Brazil law. Neighboring lamellae show differ- NarCO, in H, O. After the runs the seedcrystals were encesin extinction position of 2o. The lamellaethem- terminatedonly by real facesm, r and z, with r prom- selves extinguish homogeneously.Specimens with inently developed parallel to the original cleavage macroscopicallyplanar cleavagesshow systemsof planes of the seed.The facesm and r were morpho- long linear fractures parallel to the best developed logically dominant, whereasz-faces were subordinate cleavageplanes. We refer to this fracture system as due to their higher growth rate. The product crystals R,. The twin lamellae boundaries are frequently exhibited an unusual pseudoorthorhombic-prismatic identical with the R,-fractures.On the universalstage habit becauseof the prominent developmentof the a secondsystem of linear fractures,here termed R,r, rhombohedral faces. In agreement with literature running parallel to a secondpair of rhombohedral values,the measuredvelocities of growth VG were: faces r, could be measured.These fractures are sig- VG- < VG, << YG" (cf. Jost, 1955).Our growth ex- nificantly shorter than the R, type. The anglesof in- perimentsat different temperatures(pH ll, pressure tersectionbetween fracture systemsR, and R,, in sec- 1.5to 3 kbar) show that in the temperaturerange up FLORKE ET AL.: QUARTZ WITH RHOMBOHEDRAL CLEAVAGE D E gronrtes 1O81m vetn t wtthc , @ o o O lO 2okm t-t-t Fig. 1. Sketch of deposit, to 300oCthe free surfaceenergy difference between r that rhombohedral face r which representsthe best and z is so large that z showsmuch higher growth developedcleavage plane. rates than r. With increasingtemperature, however, Untwinned, single crystal slabs(3-5 cm diameter) the growth rates of r and z approacheach other and of transparent quartz fron diferent localities (Ar- at 500"C there is almostno differencebetween them. kansas,Goiaz, Madagascar)were slowly heated(1" / Twinning was analyzed by etching cuts per- min) in air (l atm) to l00oC and subsequently pendicularto the c-axis(45Vo }JF,20"C, 30 min), re- quenchedin water to room temperature.After this vealing polysyntheticBrazil twinrningparallel to only procedure a few linear fractures parallel to the '.1'K' r{1: 1, \ ''-"# Rrt if )*:.-o-', *, .tI *, Fig. 2. Photomicrograph of linear rhombohedral cleavage fracture systems R1 and R1y and of healed
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