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Liddicoatite, a New Calcium End-Member Ofthe Tourmaline Group

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Liddicoatite, a New Calcium End-Member Ofthe Tourmaline Group American Mineralogist, Volume 62, pages I I2l-l 124, 1977 Liddicoatite,a newcalcium end-member of thetourmaline group Psrn J. DUNN,DeNrrr- E. ApplrnaeNAND Josrpn E. Nsr-rN Department of Mineral Sciences,Smithsonian Institution Washington,D.C., 20560 Abstract Liddicoatite,a new calcium end-memberof the tourmalinegroup, ideally Ca(Li,Al),AluBsSi6Or?(O,OH)B(OH,F),is rhombohedral, R3m, with a : 15.867(4),c : 7.135(4)A,and cell volume Z : 1555.6(9)A3.Liddicoatite is the calcium analog of elbaite.It is a commonmineral in the detritalsoils of Antsirabe.Madasascar. Introduction (1922,p.4ll-432). Of specialinterest is the analysis During a systematicsearch for compositionalvari- of a pale pink crystal from Maroando with 2.58 per- ation in natural tourmalines, we noted that many cent CaO, and a crystal from Antaboko with 4.10 large, multi-colored tourmalines from Madagascar percentCaO (Duparcet al.,l9l0). Theseare the only have an unusually high calcium content. A compre- extant prior analysesof this new mineral. Their sig- hensiveinvestigation of 75 Madagascartourmalines nificance was not apparent at the time they were resultedin the discoveryof a new end-memberin the made, becausethe extent of solid solution in the tourmaline group. This new mineral is the calcium tourmaline group was not well understood. analog of elbaite.We have named the mineral liddi- Locality coatite in honor of Mr. Richard T. Liddicoat, Presi- dent of the Gemological Institute of America, in The locality of liddicoatite can be stated only in recognition of his contributions to gemological general terms. The crystalsoccur as detritus in the knowledgeand education.The mineral and the name soils near Antsirabe in Madagascar,and a specific wereapproved in advanceof publicationby the Com- site for the occurrenceof the type material is not missionon New Mineralsand New Mineral Names, known. IMA. Holotype material is depositedin the National Museum of Natural History, Smithsonian Institu- Morphology tion, Washington,D.C., under catalog1135815. Ad- Liddicoatite is found as large well-formed crystals ditional holotype material has been depositedin the of obvious trigonal symmetry, displaying the com- British Museum (Natural History), London, Eng- mon tourmaline morphology. The crystalsare stout, land, the Royal Ontario Museum, Toronto, Ontario, prismatic, and range in size up to 25 X l0 cm. The Canada, and the Geological Survey of Canada, Ot- crystals are deeply striated parallel to c. There are no tawa, Ontario, Canada. The pronounciation is obvious or distinguishingdifferences in the morpho- lidd'-i-coat-ite. logy of liddicoatite when compared with elbaite;the forms presentare typical of tourmalinesin general. Previous work The tourmalines from the pegmatitesof Madagas- Color zoning car were first described by Lacroix (1893, p. The most striking feature of some liddicoatite,but 102-105),and the beautiful zoning of thesecrystals not all, is the extremelybeautiful symmetricalcolor was initially noted by him. Duparc et al. (1910),in zoning (Fig. I ). The most beautiful specimensare their comprehensiveinvestigation of the pegmatites zoned green, pink, and red, but browns, blues, and of Madagascar,described these tourmalines in great many hues are frequently observed.The orientation detail and presentedoptical, morphological,and ana- of the zoning is of particular interest, inasmuch as lytical data. A further description was by Lacroix most gemmy zoned elbaiteis either zoned parallel to n2l tI22 DUNN ET AL LIDDICOATITE Fig L Symmetricalcolor zoning in Madagascarliddicoatite {0001}or {1010},{1120}, or both of the latter forms. almost constantfrom zone to zone.Color is of no use In most liddicoatite, however,the predominant zon- in the determinationof the species.Both elbaiteand ing is parallel to a pyramid. This causesa slight liddicoatite come in all the previously noted colors. blurring of zone margins when cross-sectionsof the Although the specimenchosen for type material and crystal are viewed parallel to c, but the effectis none- describedherein is brown in color, this is not a char- thelessbeautiful. The calcium/sodium ratio remains acteristicor diagnosticfeature of liddicoatite. Physical and optical properties Table L Chemical analysis of liddicoatite, holotype specimen NMNH #135815 The type liddicoatite is brown, transparent, and has a vitreousluster on fracture surfaces.Cleavage is wt. z weak parallel to {0001}or absent.The streak is very sio2 37.7 light brown. The mineral is not fluorescentin either A1203 37.9 short or long wave ultraviolet radiation. Liddicoatite is uniaxial negativewith e : 1.621,o: Tioz 0.38 : 1.637in sodium light. Birefringenceis moderate Feo 0.83 (0.016). The mineral is strongly pleochroic (r,r) e) 1,1n0 O.27 with e : light brown, c,l: dark brown. The density, MCo 0,11 determinedby Bermanbalance with temperaturecor- rection, is 3.02 g/cm3, which is in reasonableagree- CaO 4.27 ment with the calculatedvalue of 3.05g/cm'. Appli- Naro 0.88 cation of the Gladstone-Dalerelationship, based on Li2o 2.48 the constantsof Mandarino (1976)and the chemical analysisof Table l, yields a calculated index F I.72 average of refractionn : 1.636when the observeddensity is r2o3 10. 89 used.This givesbetter agreementwith the observedu 2,69 : 1.632than doesthe calculateddensity. "ro* The hardnessof liddicoatiteis about 772(Mohs). Total 100.06 Liddicoatite has physical and optical characteristics o= F 0.72 too similar to elbaite to permit characlerizationwith- ' Total 99.34 out analysis.The aboverefractive indices and density apply to the holotype specimenonly. The refractive analyst: Joseph Nelen indicesof liddicoatitevary with the iron and manga- nesecontent, as do the indices of most tourmalines SLructural formula based on 31 anions (02-,O"-,r-) : (Deerer al,1962, p.300-319). (c"0. (Lit. 7zNto. z r) sso1t. t3""0. ttTio. osho. o+*to. o gtr o. or) A16.ooBg.ooSi6 (F0.87(oH)o.ra), for ttre .02(o2j.27(CH)2.73) Chemicalanalysis general fornutu X I3 Z6 83 Si6(o,oH)30(oH,F), Initial chemical study of liddicoatite was done by electron microprobe analysis,using an ARL SEMQ DUNN ET AL, LIDDICOATITE electron microprobe and NMNH microprobe stan- Table 2 Structural comoarison betweenliddicoatite and elbaite dards. Becausemicroprobe resultsare inadequateto characterizein detail the complex solid-solutionrela- Site and Ideal Elbaite Ideal Liddicoatite Multipliclty Donnay (Present Study) tionships in tourmalines, it was decided to perform completewet-chemical analyses on some representa- 18c Si si tive Madagascarspecimens. The analysisof the ho- 9b B B lotype material is given in Table 1 In consideration 18c (z) A1 AT of the rarity good wet-chemical of analyses of 9b (Y) (A1, Li) [Li,A1) tourmalinesin the literature,the procedureswe used 3a (x) Na Ca are outlined in somedetail. 3a 01 (F 'oH) ot (F, oH) Analytical procedures 9b o2 o2 Reduction of samplesize was done flrst in an iron 9b 03(o'0H) 03(0 ' oH) percussionmortar to -40 mesh; removal of metal 9b oq oa contaminationwith a magnetwas followed by further 9b os os -100 grinding in a tungstencarbide mill to mesh. 18c o6 o6 Moisture, SiO2,AI2O3, FeO, CaO, and MgO were l8c ol o7 determinedon 0.5 g portions by classicalchemical 18c oa os methods (Peck, 1964).Prior to filtration of silica, boron was volatilized by repeateddehydration with an HCl-methanol mixture (Hillebrand et al., 1953). mensions were obtained from a refinement of the Iron in the RrOs portion was determined using 0- powder diffraction data using the program of Apple- phenanthroline; titanium as the tiron complex; man and Evans(1973). They are a : 15.867(4),c : aluminum was precipitatedwith 8-hydroxyquinoline 7.135(4)A,cell volume Z : 1555.6(9)A3,Z : 3. (Jarosewich, 1966). Using a separate sample the Silicon was usedas an internal standard.l Liddicoat- MnO was dissolvedby fusion with NarCOr, acidified, ite is indistinguishablefrom elbaiteby X-ray powder- oxidized to permanganate and determined color- diffraction techniques,and the cell dimensions fall imetrically. For total water the Penfield method within the range of reported values for lith- (Peck, 1964)was used,the HrO+ being the difference ium-aluminum tourmalines(Epprecht, 1953). we do betweentotal water and moisture. For fluorine 0.5 g not think that cell dimensionsalone are reliable cri- sampleswere fused with ZnO-NarCO, flux and fil- teria for determining this species.Although c/a of tered (Grimaldi et al., 1955). The fluorine in the liddicoatite is closeto the c/a of some dravite,/schorl filtratewas distilledusing a constant-temperaturedis- solid solutions,if one has true c and true a from X- tilling apparatus(Huckabay et a\.,1947),precipitated ray studies, it is clear from the charts of Epprecht as the lead chlorofluoride complex, filtered off, and (1953) that liddicoatite is a lithium-aluminum dissolvedin 5 percentnitric acid. The fluorine in the tourmaline.Although cell dimensionscan be usedto lead chlorofluoride complex was determined in- differentiate between elbaite, dravite, and schorl, directly by precipitatingthe chloride-equivalentwith there is insufficientevidence to indicate that the cell excesssilver and titrating the excesssilver by the dimensionscan be used,with reliability,to differenti- Volhard method (Hillebrand et al., 1953). Alkali ate between sodium/calcium analogs such as el- content was determinedflame-photometrically after baitelliddicoatite and dravite/uvite (Dunn et al., sample decompositionaccording to the J. Lawrence 19',77). Smith method. Boron was titrated with NaOH and mannitol after removal of interfering substancesand Crystal chemistry neutrafization. Precisionof method was t 0.2 per- Liddicoatite is a calcium lithium aluminum tour- (a cent BrO, on NBS 93 l2.76Voborosilicate glass). maline end-member. It is the direct structural ana- Crystallography log of elbaite, the sodium lithium aluminum end- member. This analogyis illustratedin Table 2, which Liddicoatite from the holotype specimenwas stud- ied by single-crystaland powder X-ray diffraction techniques.Like all other tourmalines,liddicoatite is ' Powder-diffraction data for liddicoatite mav be obtained from rhombohedral, with spacegroup R3m. The cell di- D.
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