615

Thz Camdinn Mineralo gist Vol. 34, pp. 61,5-62r(1996)

TWINNEDHAMBERGITE FROMTHE GILGITDISTRICT, NORTHERN PAKISTAN

R. PETERRICHARDS WaterQuality Inboranry, HeidelbergCollege, 310 East Market Street, Tffin, Ohio44883, U.SA.

ASSTRACT

Hambergitecrystals from a locality in northern Pakistanoccur atached to elbaite, and show two different habits on the samespecimen. Neither habit has been previously described.Crystals of the first habit are twinned by reflection on {110} and consistof platesflattened parallel to the twin plane,and boundedby {001}, {100}, {010}, {210}, {110}, and {341}. Crystals of the second habit, including tle main crystal on the specimen,are also twinned and have a conspicuously hemimorphichabit They are doubletwins by reflection on {110}, composedof a large central crystal in twinned relationship to two platy crystals, one on each side. Twin boundariesare marked by re-entrantgrooves, and by optical discontinuitix observableeven in unpolarizedlight at low magnificationunder a binocularmicroscope. The forms presentinclude thoseon the platy twins, ptus {241}. The differencebetween the two habits results from differencesin the effect of twinning on crystal growth in the presenceof one or two twin planes.The observedhemimorphy is a property of the twinned agglegate,and does not call into questionthe holohedralsymmety ofuntwinned hambergite.

Keyvords: hambergite,twinning, crystal morphology,Gilgrt, Pakistan.

Sotr[Naens

Des cristaux de hambergiteprovenant dbn gisementdu nord du Pakistan,prds de Gilgit, se trouve attachdb I'elbai'te,et montre deux morphologiesdistinctes sur le m6me 6chantillon. Ni I'une, ni I'aufe de ces morphologiesn'avait 6tE d&nte auparavant.Les cristauxdans le premiercas sontmacl6s par r6flexion sur { 1I 0 } et sontfaits de plaquettesparallbles au plan de macle,delimit6es par les formes{001}, { 100}, {010}, {210}, { 110},et {341}. lrs cristauxayant le secondhabitus, y compris le cristal le plus imposantde l'&hantillon, sont aus$imacl6s, et possbdentune morphologieh6mimorphique 6vidente. Ils sont doublementmaclds par r6flexion sur {110}, et comportentun gros cristal central en relation de macle avec deux cristaux en plaquetie,un de chaquec6t6. ks interfacesentre les maclesse manifestentpar desrayures rentrantes, et par desdiscontinuit6s optiques observablesmOme en lumibre non polaris6eh faible grossissement,avec un microscopebinoculaire. l,es formes pr6sentessont celles qui figurent sur les maclesen plaquetteset, en plus, {24Ll.Ia dttr5renceentre les deuxhabitus rdsulte des diff6rencesdans I'influence du maclagesur la croissancecristalline, soit qu'il y ait un ou deux plans de macle.Lh6mimorphie observdeserait une propri6t6des agr6gats mac16s, et ne remetaucutrement en questionla sym6trieholoddrique de la hambergite non mac16e. (Traduit par la R6daction)

Mots-cl6s:hambergite, maclage, morphologie, cristaux, Gilgit Pakistar

Inrnoouctrox faces are typically sfriated parallel to the c axis. Hambergitehas perfect cleavageon {010} and good Hambergite is a rare beryllium borate, Bq@O3) cleavageon { 100}. Crystalsmay be twinnedon { 110} (OH,D, found in syenitic and granitic .It is @rugman& Goldschmidt1912, Switzer et al. L965). orthorhombic, with space-groupsymmefry Pbca Recently, twinned crystals of hambergitewith two (T,achariasen193L, Tachariasenet al. 196!) and unit- unusual habits were found in northern Pakistan, cell para:netersa9.776,b 12.194,c 4.430A for nearly reportedly at Stak Nala along the SkarduRoad in the pure OH end-membercrystals Qlimalaya mine, San Gilgit District. Thesetwins of hambergiteare attached Diego County,California: Burns et al. 1995).Although to prisms of tricolored elbaite. Both are it may show hemimorphic developmento several partially coated with white fine-grained borian independent studies indicate holohedral symmetry muscovitein vermiform crystalsand anhedralmasses. (pointgroup 2lm2lmTm\ AccordtngroPalacheet al. This assemblageis consistent with formation in a (1951),hambergite crystals are usually prismatic,with pocket environmentin a granitic . longestdimensions parallel to the c axis,and its { 100} The morphology of ttresehambergite twins is not 616 similar to any shown in Goldschmidt(1918) or Msrnoos describedin the more recentliterature. nor is it similar to other Pakistani specimensthat dealershave seen. The hambergitecrystals were examinedunder low The purposeof this note is to describethese new habits magnification using a binocular microscope.Selected of twinned hambergite. small crystals were removed from the specimenand examined witl a petrographic microscope under Marmran crossedpolarizers to identify the presenceoftwinning. The identity of crystals of both habits was confirmed One specimenwas availablefor study; on it, a large by X-ray examination.A small quantity of the mica hambergite crystal (25 mm long) is attachedto an also was removedand submittedfor X-ray idenffica- elbaitecrystal @igs. 1,2). This crystal is hemimorphic tion. X-ray analysis was done with a Guinier-Hdgg and prismatic with pyramidal moffications. Much of focusing cameraand C\rKcq radiation with synthetic the surface is coated with an aggegate of white spinel as an intemal standard.Cell parameterswere vermiform mica crystals,which partly encrustboth the refined using the program of Appleman & Evans tourmaline and the hambergite.Some of the mica has (1973),as modified by Garvey(1986). beenscraped away to showthe major mineralsto better Twinning was initially assumedto be on {110}, advantage,but differences in surface luster indicate as reported in the literature. This assumption was that some portions of the major crystals wgre never validatedby comparisonsof calculatedand measured covered. About a dozen additional small hambergite interfacial anglesacross twin boundaries. crystals,not exceeding2 mm in maximum dimension, The small Hl crystal was mountedwith its end face were discoveredduring examinationof the specimen parallel to the stage of the pefrographicmicroscope, under low magnification (20x). These are mostly or and the presenceof two twin planeswas revealedby completely embeddedin mica. Severalof thesehave examinationunder crossedpolarizers. The angles the samemorphology as the large hambergitecrystal between extinction directions acrossthe twin planes (hereaftercalled Hl for'Habit 1"), but the resthave a were measured, as were angles between twin different, platy morphology (H2, see Fig. 3). A1l planes, crystal faces perpendicularto the microscope crystalsof both habits appearto be fwinned. stage,and intemal cleavagecracks. The samecrystal

Ftc. 1. Hambergiteon elbaite, from northern Pakistan.The hambergitecrystal is 25 mm wide. TWINNED IIAMBERGITE FROM PAKISTAN 617

FIc. 2. Doubly twinned00 hambergite (H1). A. Clinographic projection in staadard orientation. B. Orthographic projection with {010} of the large central individual parallel to page and [001] horizontal, indicating the orientation of striations. C. Orthographicprojection, rotated 90o aboutthe vertical axis of the pagerelative to B, showingthe morphologyofthe end ofthe crystal andthe shapeofthe cross-section. D. Cross section in same orientation as C, showing location of twin planes and orientation of extinction directions and cleavages.E. Forms shown are c {001}, m {110},and u {241}. was measuredwith a two-circle optical goniometer, study were available, so an initial morphological and the faces presentwere determinedby comparing interpretation was based on comparisons with the the stereographicprojections of the measuredcrystal measuredHl crystal. An incomplete Hl crystal was with thoseof morphologicalinterpretations constructed mounted with one of the large faces parallel to the usingSHAPE (Dowty 1980). stage of the petrographic microscope, and angles No II2 (platy) crystals appropriatefor goniometric defined by edgesof the face were measuredto help

f,l:] r1l XIX

\iiIt D

Ftc. 3. Singly twinned hambergite (H2). A. Clinographic projection in staadard orientation. B. Orthographicprojection with twin plane parallel to page ald [001] horizontal,indicating the orientationof striations.C. Orthographicprojection, rotated 90o aboutthe vertical axis ofthe pagerelative to Fig. 28, showingthe morphologyof the end of the crystal and the shapeof the cross-section.D. Cross-sectionin same orientation as C, showing location of twin plane and orientation of extinction directionsand cleavages.E. Formsshown are a {100}, c {001}, m {110}, n {210}, andX {341}. 618 estab[sh the identity of the faces involved in the Crystalsof habit HI angle.The samecrystal was mountedon its end face, perpendicularto the twin plane,and the anglebetween The small Hl twin, mountedon a {001} face and extinction directions in each sector of the twin was examined under a petrographic microscope with measuredto confirm the twiming relationship. The crossedpolarization, is cleady a doubletwin. The twin angle betweeninternal cleavagecracks and the large consistsofthree individuals, a relatively blocky cental face also was measured.These angles were compared individual flanked by two platy individuals, with with corresponding angles calculated from the different directions of extinction (Fig. 2). The angles morphologicalinterpretation using SHAPE,to confirm between extinction directions in the central and side the validity of the interpretation. crystalsmeasure 76' and 80o. The angle betweenthe twin planes measures76.5'. In addition to internal REsuLTs tracesof the {010} ,a few tracesof the poor {100} cleavageare present.Both twin planesand all X-ray examinationshows that crystalsof both habits internal tracesofboth cleavagesin all threeindividuals are hambergite. Refined cell parametersbased on are perpendicularto the stage.Thus both twin planes 16 indexedpowder-diffraction lines area 9.7343(53), must be in the [001] zone. The angles between b 12.1,837Q4),c 4.4350(13)A (Robert F. Martin, extinction directions acrossthe twin planes comFare pers. comm., 1995). The cell paxameters4 and c are well with the calculatedangle across (ll0),77 .2' . T\e consisient with the replacement of about 20Vo of angle betweenthe twin planescompares well with the the OH with F, using the relationships determined calculatedangle between (110) and (Ll}),77.2",but by Bums et al. (L995). For reasonsthat are not clear, not with that between(110) and (1T0) (102.8), and the E parameteris indicative of no F substitutionfor indicates that the twin planes are symmefiical about oH. (100) rather than about (010). This observationis A successfirlunit-cell refinementof the mica based consistentwith the orientationof the {010} cleavage on 18 indexedlines showsthat it is a borian muscovite, relative to the twin planesGig. 2D). 2M1 polytype" with cell parametersa 5.I218(1,I), Hl twins show pronounced hemimorphic sym- b 8.8652(22), c 20.0125(24)A, and B 94.460(47)'. metry. The left half (as drawn in Fig. 2A) is composed A small amount of he IM polytype also may be of a pair of {110} faces that belong to the platy present. individuals and are parallel to the respective fwin All hambergite crystals of both habits examined planes. These faces convergeat the left edge of the under crossedpolarizen are reflection twins, and have fwin, making a1 angle less than 90' with each other, one or more cleady visible contactplanes. The stong and are lightly striatedparallel to [001]. The right half similarity in habits between these and the crystals of the crystal consists of four symmetrically rcmaining on the specimen, together with feafures equivalent,somewhat curved faces of a dipyramid,and observableunder the binocular microscope,such as two smaller fiiangular faces that belong to an lhl{} re-entrantangles, intemal cleavagecracks, and internal prism. All of these faces belong to the central optical discontinuities across twin boundaries, individual. The top and bottom ends of the twin are demonstratethat all crystals on this specimen are truncatedby small, flat facesof {001} sharedby all twinned. three individuals. AI twins havea pair of well-formed large facesthat Narrow re-entrant grooves occur along the edges are oriented parallel to a twin plane and are sffiated wherethe platy individuals meetthe centralindividual parallel to the longest dimension of the twin. All (Frg. 2C). The bottom ofeach gtooveis the intersection crystals of both habits have small faces (the 'end of the twin plane with the surface(Fig. 2D). The twin faces" referredto above)that are perpendicularto the planes, conspicuousunder crossedpolarizers, are striatedfaces, the twin plane(s)and the perfect {010} faintly visible in unpolarizedlight under the binocular cleavage.Assuming that the twin plane(s) belong to microscopeas optical discontinuities,if one looks into {110}, they intersectthe perfectcleavage along [001]. the crystalthrough the {001} endfaces. Sincethe small end facesare perpendicular to all these Along the left edgeof the twin (Ftg. 2A) are a few planes,they are alsoperpendicular to their intersection; small flat-bottomedchips that reflect the perfect (010) therefore,the end facesbelong to {001}. The sftiated cleavageofhambergite. Planar cracks in the interior of facesbelong to {110}, andthe striationsare parallel to the aggregateare parallel to the cleavage surfaces [001], as reportedin the literature. presentin the chips, and showthat the chippedportion All twins are hemimorphic,but tle hemimorphyis belongsto the centralindividual. much more conspicuousin Hl twins than in H2 twins. The sametwin was measuredon a Stoe two-circle Hemimorphyis sfictly a consequenceof twinning, and optical goniometer.Because of its small size, many does not call into doubt published studies indicating faces on this twin do not give a reflected light figure; that single crystals of hambergite have holohedral thesefaces were brought into positionfor measurement orthorhombicsymmetry. by direct observationof the light reflected from the TWINNED T-IAMBERGITEFROM PAKISTAN 619

the literature, and experimentation using SHAPE proposed indices were adjusted lY rho Oualltyot En€ed lghffgure in which Miller iteratively to bring the stereographicprojections more 0"- C€nter€d at rhef t E; rery taint Effsated [gh flguc 900 23f Falil r6tl6d llght ffguE et@rcd out along phl nearly into coincidence. The results are listed in t30' No refl# llgtrl figuc Tahle2 and shownin Figure 2E. mo 1450 No Gi€c1d nght fguB In Figure 4, the stereographicprojection of the e0" 2't3" No Gfrsted brlt ngu@ measuredfwin is shown using filled circles to mark D 90. 1660 No crlgcied lght ffgurei not in skstch 900 3580 No r€'tleoied llgtrl figre; not ln slGtch the polesofthe faces.The polesofthe facesofthe final s" str Refle4led [ght ffguE rery hlnt bd ot good quafiy model for the fwin aremarked with larger opencircles. s0" s{tr Good Ei@t8d [dnffguF The fwo projections are very similar, and agree qr 92" No Gfloriod llgfttflgur€. Rdleodom lpm @ntrun gmvo by F sufficiently closely that the model can be acceptedas 90" No ciact€d rcrtt figuB. valid. RotlodoF lrom r€ntmnt g@9 by F H 60" 310 Voryfrff cff€ded llgttffgure H 60" 3260 No €fl616d llgtrt flguF Crystalsof platy habit H2 I ano' 4a20' ModsEtEly blund Ff,@ted ligil ffguE trcm|@toD otthlswdl@ petrographic I 63p20' 314040', Mod€mtely blur€d r€'flsted llght ffgure Jpinning is easily detected by trom M bD ol thb cuiled la€ examinationof thesecrystals under crossedpolarizers. A single fwin-plane is parallel to the large sniated s.*h*ot'€cry'hrro,ffi$' { 110} faces.No evidenceof polysynthetictwinning or ldsntflcdonotlq@ ^4I qg of complex composition-surfaceswas seen. On a ----\ w" _ fragmentof a platy twin orientedwith { 1l0 } parallel to the stage, extinction directions are parallel and 'Phl @n€!€d lo allgn @nterlndMdual e thal fa@ ot f, l, and h aE appDdntsldy symmstdcabout phlEo. LettsE mlgned io fa@ are for book&osplnodurlng perpendicular to the striations, and the angle .r goniometryonly, sd m rct hbnd€d to lmplyMiler lndl@ (Frg. 38) measures155". With the same fragment orientedwith {001} parallel to the stage,extinction in each member of the twin is inclined 38' to the twin planeGig. 3D). fyeinning is revealedon the surfaceof the crystalby face into the viewing telescope.This methodleads to the presenceof a re-enfrant groove (along the right measurementsthat are reproducibleonly to about +1o, edge of the crystal in Fig. 3A). The interfacial angle as opposed to the t4' accuracy provided by the between the faces in the re-entrant groove was reflected light figure. Many of the more important estimated at 30' by examination using a binocular faces yielded reflected light figures, and could be microscope.Facets of the {010} cleavageoccur in measuredmore accurately(Table 1). chips alongthe sharpedges between the largeflat faces The forms present on the twin were established and the re-entrant gxoove;they have the orientation by comparing the stereographicprojection of the shownin Figure 3D. measluementswith stereographicprojections of forms Assuming that the twin plane is {110}, these chosenusing the observationsabove, forms reportedin observationsare sufficient to establishthe forms shown in Figure 3E. The secondaryfaces belonging to { 110}, l2l0\, {010}, and {34L} were identified by comparisonwith correspondingfaces on the indexed double twin, and checkedfor reasonablenessagainst IABLE 2. MII I FR INDICESFOR FORMS the observationsabove. The angle betweenthe ON DOUBLY.TWINNEDHAMBERGITE' {010} cleavageand the {110} twin planeis calculatedto be Form Miller F@dslgnalon 38.6o,the re-entrant angle is calculatedto be 25.6o.The lettgr indl6 forgonlomsty fTablo 1l anglex, measuredon a SHAPE drawing with the twin planeparallel to theplane ofthe paper,is 153.3'.These 100 D resultsare all in good agreementwith the observations. 010 E c 001 A DIscussloN m 110 F,E,g n 210 Both of the habits of hambergitedescribed in this u 24'l H paperare hemimorphic,which is inconsistentwith the x 341 I reported symmety of hambergite. The presenceof ' Urlderl|n€dbrns w6€ moa8urodon ths twinniag on {110} is sufficient to explain the platygklg orlrbb; ther6rt w8rom@urgd hemimorphyof the twinned aggregates.A contacttwin 0n thg largeaenral crFtal. FormlstbrE on has aggregatesymmetry 2mrn, ard only the followhs taago In Palached a/. (1951) {110} andS!$Esr atal. (1965). mirror planeperpendicular to the c axis coincideswith 620 TTIE CANADIAN MINERALOGIST

Hc. 4. Goniometricprojections of a measuredcrystal and a morphologicalinterpretation generatedusing SHAPE. The crystal at left is in the same orientation as the stereographicprojection: both the viewing axis and the projection axis are normal to the pinacoidal face {001}, Plotting positions ofmeasured faces are shown using small filled circles,and those of faceson the model crystalare shownusing largeropen circles.

a synmetry elementof the untwimed crystal.Thusthe fwin planes on each side, and the geome0y of the hemimorphicnature of thesetwins doesnot contradict configuration allows this cental individual to capture the holohedralsymmetry of untwinnedhambergite. most of the massof depositedmaterial. The fwo habits of twinned hambergiteare srikingly different from eachother. This might be explainedas a Comparisonswith hambergitecrystals consequenceof two generationsof growth, but the from other localities paragenesissuggests that twins of both habits grew simultaneously.Furthermore, twins of eachhabit share Kazmi et al. (1985) describedhambergite from Stak a common elementof identical morphology:the platy Nala, Gilgit District, Pakistan as occurring in two crystals,which flalk the prismatic fwin and comprise habits:tabular with the pinacoids{100}, {010}, and fte platy twin. {001} dominant, and dipyramidal with {111} The different habit of single and double twins on demin4af and with courmon6arinning. A photograph this specimenattests to the power of twinning to affect showsthe tabularcrystals to be quite platy but provides crystalmorphology. It is well known that new layersof no details; no photographicdocumentation was given growth arenucleated in the re-entant anglescreated by for the dipyramidal habit. No measurements or twinning Qlenderson 1983, Harnnan 1955), causing drawingswere given to documentthe morphology,and the faces in the notch to grow faster than those the twin law was not described.It is possiblethat the elsewhere,even faster than other faces of the same two habits referredto are the sameas thosedescribed form. This growth advantageconferred by twinning above, but only if their morphological analysesare often results in twins being larger than untwinned enttely wrong. In the absenceof more sqmplete crystalson the samespecimen. It also accountsfor the information, it must be assumed that the habits distortionin habit that accompaniestwinning, of which described in Kazmi et al. we differenl from those typical Japanlaw twins of quartzand butterfly twins of describedin this paper. calcite (contact twins on {01T2}) are well-known Goldschmidt ( I 9 18) ilustrated hambergitecrystals slamples. from Norway and from several localities in In the case of the platy twins of hambergite,the Madagascar.The crystals are all prismatic, and most effect of this differential nucleationof growth layersis areelongate along [001]. No habit illustratedresembles to accelerategrowth on faces that intersect the twin thosedescribed here. One twin on { 110} is illustrated, plane, leading to the platy habit. In the prismatic but no pronounced distortion due to twinning is double twins, the sametrnocess accounts for the platy apparent. flanlcing individuals. However, the individual in the Switzer et al. (1965) illustrated crystals from the middle gains a symmetrical growth-advantagefrom Little Three mins, f,amsn4 and from the Himalaya TWINNED HAMBERGIIts FROM PAKISTAN 62L mine, Mesa Grande, botl in San Diego County, Rnmnsvcrs Califomia. All illustrated crystals are prismatic and show no similarity to the habits described here, Apeuuall, D.E. & EvANs,H.T., Jn. (1973):Job 9214: ' of powder although the form i:ai) is presenton some of their in{9xlng and least-squaresrefinement crystali from both iocalities.-Contacttwins on Geol'san" com'pur'contrib' 2'0 {110} {ktpo Doc'*P:P811618E)' Yf' wire reportedbut not described. NTIS Marcusson(1985) i[ustrated a hambergitecrystal F.C. (1995): from the Himalaya mine as a drawing traced from a BuRNs,P.C., NovAK, M. & Hawruonir, Fluorine-hydroxyl variation in hambergite: a crystal photograph.No morphological inforrnation is given, structurestndy. Can. Miwral,33, 1205-1213. but the drawingshows portions of prism anddipyramid possible faces and traces of {010} cleavageon the Dosrry, E. (1980): Computing and drawing crystal shapes' surfaceof the dipyramid faces.If the long direction of Am. . 65. 46547 l. the crystal is taken to be parallel to the c axis, it bears somesimilarity to the prismatichabit describedabove, Dnucuar, I. & Got oscuunr, V. (191'2): Ein but has a somewhatsteeper dipyramid (approximately Hambergitzwilling von Madagascar,Z. Kristallogr. 50, {561}) and lacks the 6pinningthat characterizesthe 596-597. material describedin the currentpaper. Hambergite is not a common mineralopublished Genvn, R. (1986): LSUCRIPC, least squaresunit-cell morphologicaldescriptions are uncommon,and some refinement with indexing on the personal comput€r. are quite incomplete or qualitative. This makes Powd.Dffi 1(l), 114. morphological comparisons difficult. Given the descriptions available in the literatureo however, GorDscHr,IDr, V. (1918): Atlns cler Krystallfurmen, Catl the habits of twinned hambergitedescribed here from Winters Universitlitsbuchhandlung,Heidelberg, Germany. norttrem Pakistan appear to be different from any Reprintedin 1986by the RochesterAcademy of Sciences, previously described. Rochester,New York (1955): growth twins. Z. CottclustoNs HARTMAN,P. On the morphologyof Kris tallog r. lU7, 225-237 . Hambergite from the Gilgit District, Pakistan, Hq.IDERsoN,W.A., Jn. (1983):Microrninerals: flat twins, fans occurs as twinned crystals with two different andballs. MineraL Rec. 14.363-368. hemimorphic habits that have not been previously described.Both habits involve contact twinning on KAz\4r,A.H., PersRs,J.J. & OBoDDA,H,P. (1985): Gem {110}. The hemimorphyis a consequenceof the pegmatitesof the Shingus-Dussoarea, Gilgit, Pakistan. contact lyTinning, and does not contradict the Minera.l.Rec. 16. 393411. holohedral symmetry previously determined for hambergite. Platy I12 trvins are composed of two MARcussoN,C.R. (1985):Recent work at the Himalayamine. individuals, whereasfwins of the more prismatic Hl M ineral. Rec. 16, 419424. habit are composedofthree individuals: a large central individual twinnedto platy individuals on eachside, by PAr.ACHE,C., BBRMAN,H. & Fhonos., C. (1951): Dann's reflectionon (110) and (T10) or, equivalently,(lT0) Systemof Mineralngy (seventhed.). tr. John Wiley and and (T-10).The habitsdiffer from eachother because of Sons,New York. the different influence of the twin plane on crystal growth, dependingon whetherone or two twin planes Swnm, G., Ct-ARre, R.S., SNKANKAS,J. & WoRIIm.rc, arepresent. H.W. (1965): Fluorine in hambergite.Am. Mfueral, 50, 85-95. ACIA\TOWLEDGH\mNTS ZAcIARIAsET.I,W.H. (1931): The crystalline structure of Z Krisnllngr. 76, 289-302. My thanks to Dr. Robert F. Martin, who provided hambergite,Be2BO3(Otf . X-ray analysesthat identified the mica" confirmed H.A. & MAREzo, M. (1963): The that the small platy trvins were hambergite, and Pt-prrnlcm' structure and of hambergite,Be2BQ'OH. provided parameters the unit-cell and inferred extent Acn Crysnllo gr. 16, 11441146. of fluorine substitutionfor thesecrystals. Dr. Richard Gaines provided the specimen and allowed the judicious removal of materials for study. Dn. Abe Rosenzweig and Van King reviewed an earher version of the manuscript and offered suggestions that improved the content and clarity of the current Received.May 26, 1995, revised. manuscript accepted version. February15, 1996.