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Some Unusual Minerals from the "Mottled Zone'' Complex

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Some Unusual Minerals from the MINERALOGICAL NOTL.S SralvmrN, L. I. (1955) Ascharite and other borates in magnetite ores of contact-meta- somatic deposits. Dokla.dyAhad. Nauh SSS,R, l0l, 937-940 (in I{ussian). (1961) The so called magnioborite. Zopiski Vses. Minerolog. Obshch.90, 754-755 (in Russian). T.trfucur, Y. (1952) The crystal structure of magnesium pyroborate. Acta Cryst. 5, 574-581. 'l'onorov, 'I'he N. A. aNo P. F. KoNovarov (1940) binary system magnesium oxide-boric anhydride. Zltur. ltiz. Kkim.14,1103-1109 (in Russian). War,waln, Taroo (1953) Suanite, a new magnesium borate from Hol Kol, Suan, North Korca. Minerol. Jour. (Japon), L, 54-62; (abs.) Am. M'ineral.40, 941 (1955) AN MINERALOGIST, VOL. 48, JULY-AUGUST, 1963 SOMEUNUSUAL MINERALS FROM THE "MOTTLED ZONE'' COMPLEX,ISRAEL Y. K. Boxror., GeologicalSuraey of Israel and.Hebrew (/n'iversity,Jerwsalem AND S. Gnoss ANDL. I{ellrn, GeologicalSwraey oJ Israel. In most parts of Israel a chalky-shalyrock sequence,the Ghareb and Takiye formations is found overlying flint beds of Campanian age. In subsurfacesections this sedimentary rock sequenceis generally highly bituminous and may contain 25 per cent or more organic matter. In a number of widely separateareas, however, such as the Hatrurim region and the BeershebaValley in the northern Negev, near Ramleh in the coastalplain and at l{aaleh Adumim in the Jordanianpart of the Judean Desert, a peculiar rock complex,the so-called"NIottled Zone," appears instead of this normal sequence.The "NIottled Zone," which reachesa maximum thicknessof 220m, comprisesa largenumber of rock typessuch as limestone, chalk, marl, shale as well as gypseous,ferruginous and phosphaticrocks, all distinguishedby a pronouncedbut highly irregular coloration.These various rock types occur without any apparent regu- larity and passinto eachother without distinct boundaries.No bedding can be observed,but a schist-likestructure is fairly common.This rock sequencecontains an abundanceof trace elements,among which Ag, V, Ni and Cr are prominent. Chromium, in the form of green chromium silicatesin particular is found in many veins up to 1 m thick and may make up 6 per cent of the vein material. One of the most widespreadrock types of the "IIottled Zone" is a very hard, dense,splintery, brown, violet or black rock. I,Iicroscopicexamina- tion showsit to be composedmainly of calcite and spurrite,l approxi- I Spurrite has also been found in these rocks by E. Sassand tobermorite by C Milton (private communication). M I N L,RALOGI CA L NOT ]':5 925 mately in the ratio 2: 1. Calcite appears as xenomorphic grains, some twinned and up to 0.6 mm in size.The spurrite crystals,which reach up to 0.3 mm, are idiomorphic to hypidiomorphic with pseudohexagonalor prismatic outlines.They show high relief, medium birefringenceand are biaxial negative,with small 2V, weak dispersion(r)v), x:b, y,\c: -33", z\a-0o. The crystalsexhibit two setsof cleavages,one distinct, the other poor, at an angleol 79o.Two types of twinning, one simple,the other polysynthetic,are characteristic;the angle betweenthe two planes is 57o.The crystals appear granulateddue to intensecracking and gen- erally contain round or elliptical, drop-like inclusions of calcite up to 0.02 mm across.Nl[inor constituents of the rock are ch]orite and an as yet unidentified iron sulfide. This composition,derived by microscopicex- amination, is compatiblewith the resultsof chemicalanalyses, which in addition indicatethe presenceof sometenths per cent of organicmaterial. The rock is traversedby a criss-crosssystem of small veinlets,rarely exceeding1 mm in width. In theseveinlets a number of uncommon min- eralshave beenidentified, which include vaterite, portlandite, bayerite, minerals of the tobermorite group,l an as yet unknown calcium silicate hydrate related to tobermorite, thaumasiteand, very much more abun- dantly, ettringite. With the exception of portlandite, thaumasite and ettringite these minerals occur in a very fine state of subdivision and frequentiy show a fibroushabit. Bayerite,A1(OH)8, occurs as white fibers with parallel extinction, asso- ciated with calcite in one specimenand with calcite and gypsum in an- other. The mineral wasidentified by its distinctive r-ray powder pattern. Of the three polymorphic forms of Al(OH) 3 gibbsite is very common, nordstranditehas recently beenreported from two localities,(Wall et al., 1962;Hathaway and Schlanger,1962), but this appearsto be the first re- corded natural occurrenceof bayerite, although the material is readily prepared artificially. Vaterite,the third polymorph of CaCO3,has previously been reported from Bailycraighy (nIcConnell, 1960). In the present samplesit was found intimately mixed with calcite and aragoniteand in one specimen also with barite. The crystals are too small to be identified under the microscope;they were recognizedby their r-ray powder pattern, which resemblesthat of the mineral from Baliycraighy. Portlandile,Ca(OH)2, forms tabular crystals which, together with fluor- apatite and ettringite, line the walls of the very thin veins in the black spurritic-calciticrocks. They are colorless,transparent, with pearly Iuster and high birefringence,uniaxial negative,c,::1.576I.001. The x-ray pat- 926 MINERAI,AGICAL NOTES tern is identical with that of synthetic Ca(OH)2,after correctionfor pre- ferred orientation. Portlandite in nature is very rare; it has previously been found in Scawt Hill (Tilley, 1933), Vesuvius (I{inguzzi, 1937; Carobbi, 1940)and Ettringen (Hentschel,1961). Minerals oI the lobermoritegroup, predominantly 14 A calcium silicate hydrate but lessfrequently also the 11 A modificationwere identified in severalsamples scraped from veins in the reddish-brownrocks. The min- erals occur as white fibers with silky luster and form mostly radial aggregatesbut locaily alsolenses up to 2 mm in size.Optical data: Nega- tive elongation,biaxial positive, 2Y very small, very Iow birefrigence,B approximately 1.552. X-ray data for the present samplesof the 14 A mineral very closelyresemble those of the type mineral from Crestmore (Heller and Taylor, 1956).The 11A modificationprobably correspondsto a lower hydration state of the samematerial, sincethe hkO spacingsare identicalfor all the 14 and 11A mineralsexamined. In addition to 11 and 14 A tobermoriteanother white fibrous material wasfound which, althoughit givesrise to a distinctive r-ray pattern, may be a memberof the tobermoritegroup. The fibersshow parallel extinction and mostly posi^tiveelongation. The strongestr-ray spacingsare at 10.40, 2.92 and 1.806A; the repeat distancein the fiber direction is 7.2 A, with pronouncedpseudohalving. This value is slightly lower than the 7.3 A spacingusually observedfor fibroushydrated calcium silicates,but pseu- dohalving of the fiber axisis a characteristicof the group. The mineral usually occursintergrown with someettringite and tober- morite. Both the unknown material and ettringite exhibit strongly pre- ferred orientation along the fiber axis, but completerandomness in the perpendiculardirections. Ettringite is oriented with its hexagonalc as fiber axis. The associatedtobermorite showsno preferredorientation. Grinding the unknown mineral with water doesnot afiect its r-ray pat- tern. On heating up to 850oC. the morphology of the fibersremains un- changed.Well oriented wollastonite and partially oriented Iarnite are obtained, with their b axesin the fiber direction. At lower temperatures various oriented but as yet unidentified products were formed;at about 400' C. the material is apparently amorphousto r-rays. The thermal decompositionproducts of this mineral and the similarity of its r-ray pattern to thoseof fibrous hydrated calcium silicatessuggest that the chemicalcomposition is xCaO, SiO2,yH2O, where 1 1x 12. Unfortunately it has so far proved impracticable to isolate sufficient quantities of the pure material even for microchemical quantitative analysis. The question whether this mineral representsa distinct speciesor merely a further modification of the tobermorite group must at present remaln oDen, MINERALOGICALNOTES 927 Thaumasite,CaSiOs.CaCOs.CaSO+.14.5H2O, has been identified in two specimens,apparently pure in one and mixed with aragonite, calcite and a modifiedform of ettringite in the other. It forms very thin prisms with parallelextinction, negative elongation and fairly high birefringence. The *-ray powder pattern is identical with that of thaumasite from County Down, Ireland (Knill, 1960), of which a specimenwas kindly suppliedby Dr. P. Sabine. Etlringite,ideal composition6CaO.Al2O3.3SOr.31 H2O, is relatively widespreadin veins and cavities in "Mottled Zone" rocks both in the trrc 1. Well-developed prisms of ettringite. Hatrurim and Ramleh areas. rt is transparent and occurs in three morphologically difierent modifications: as lemon-yellow, hexagonal prisms, as colorlessprisms and as very thin white fibers. The yellow crystals,which showvitreous luster, are only up to ] mm acrossand have no terminal faces,but perfect (1010)cleavage. eualitative microchemical analysisindicates the presenceof tracesof Fe in addition to the major Ca, Al, SO+ and H2O, which may account for the coloration. The colorlessprisms are exceptionally well developed,up to 3 mm across and terminated by rhombohedralfaces (Fig. 1). They are coated with an opaque white powder, which may be a dehydration product. The fibers form radial aggregatesor grow perpendicular to the vein walrs. The optical data for the three modificationsare as follows: Ertinction Elongalion e @ yellowprisms parallel
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