American Mineralogist, Volume 68, pages 420425, 1983 Hydrobiotite, a regular 1:1 interstratification of biotite and vermiculite layers G. W. BnrNorpv Materials Research Laboratory and Department of Geosciences The PennsylvaniaState University University Park, Pennsylvania 16802 Pernrcre E. ZN-sxr Materials ResearchLaboratory The Pennsylvania State Universiry University Park, Pennsylvania 16802 eNo Cnerc M. BBrure2 Department of Geosciences The Pennsylvania State University University Park, Pennsylvania 16802 Abstract Well-ordered hydrobiotites consist of a regular alternation of biotite and vermiculite layers. Current requirementsfor use of a specialname for an interstratified mineral specify that the coefficient of variation, CV, of the basal spacings obtained from ten or more reflections should be less than 0.75. Data for three hydrobiotites from different sourcesgive mean basal spacingsof 24.514 measuredto 0.0.31for individual flakes and 0.0.22for orientedpowder layers. The CV valuesfor these samplesare 1.25,0.35,and 0.98 when equal weight is given to all reflections. The large values for the first and third samplesarise. primarily from the first-order difractions which are least accurately measurable;when these are omitted, the CV values are 0.67,0.29, and 0.20. Observed and calculated structure factors for fi)/ diffractions up to / = 3l vary similarly with the index /. Values of d(00/), calculated by Reynolds' method and by a modified Mering method for ordered mixed-layer sequencesfrom 40-&Vo biotite layers, show that the three samplesstudied have 45Vo,53Vo,and 49Vobiotitelayers. The calculationsalso show that ifthe percentageof biotite layers falls outside the range45-55% biotite, variable basal spacingswith CV > 0.75 will be obtained. Introduction with a 24A basal spacing and also more complex se- quences. in which "the sequence The name hydrobiotite has been in the mineralogical He found two samples . adefinite ratioof l:l biotite: literature for at least a century. It was used by Schrauf of layers . .may approach vermiculite Boettcher (1966,p. 284), discussing (1882,p. 3E1-383)for a mineralfrom Kremze resembling units." the origin of vermiculites and hydrobiotites, stated that biotite in its chemical composition but containing consid- "the hydrobiotite has been used in a variety of erably more water. It was mentioned by Dana (1892, p. name ways and should be defined wherever used"; he specifi- 632). Gruner (1934)recognized that the X-ray diffraction cally defined hydrobiotite as "a regular l: I heteropolyty- data of hydrobiotites "show clearly that [they] are made pic (mixedJayer) biotite:vermiculite" and (p. 286) stated up of interstratified single or double layers of mica and that "the sharp X-ray peak at 25.5A and the high order of vermiculite." He consideredan alternating1:1 sequence rational basal peaks obtainable in the X-ray patterns of I On leave of absence from: Comisi6n de Investigaciones hydrobiotite attest to the regularity of spacing." Cientificasde la Provincia de Buenos Aires - CETMIC, Calle 47 Foster (1963) made a critical review of the available y l15. Dept. Tecnologia, 1900LaPlata, Argentina. chemical data for vermiculites and hydrobiotites and, 2 Current address: Department of Geology, University of despite inherent difficulties in arriving at structural for- Illinois, Urbana, Illinois 61801. mulae in the absence of cation exchange data, showed m03-004)v8310304-0420$02.00 420 BRINDLEY ET AL.: HYDROBIOTITE. A REGULAR I :1 INTERSTRATIFICATION 421 that the formulae of hydrobiotites are intermediate be- Optical properties of the north-western Transvaal ma- tween those of biotite (or femrginous phlogopite) and terial were as follows: F = 1.560-1.562,y : 1.565-1.567, vermiculite, and therefore, are consistent with an inter- a biaxial negative interference figure, 2Y" - 19". stratification of layers. Hey (1962)recognized hydrobio- X-ray difraction data were obtained using a Philips tite as "a mixed-layer mineral, an interstratification of diffractometer with Ni-filtered CuKa radiation and were vermiculite and biotite," but did not mention regularity of recorded at l" 29lmin and with chart recording on a scale the mixed layering. of 2" Z?linch. When samplesin the form of single flakes Despite these studies, the name hydrobiotite seemsnot were used, the incident X-ray beam divergence was to have been fully accepted as a valid mineral species. choosenso that the radiation was confined to the surface The name is not included by Fleischer (1980) in his of the flake but this condition was not wholly satisfiedat "Glossary of Mineral Species" among a long series of 2e < l0'. To record diffracted intensities. it was neces- hydro-minerals.In a critical review of interstratified layer sary to make changesin the silt system, the X-ray tube silicates, Buley et al. (1982)made no mention of hydro- operating conditions, and the recording conditions. Sev- biotite. They specifiedthe following requirementsfor the eral diffractions were recorded before and after each use of special names. An interstratification of two layer change of experimental conditions so that all recorded types, A and B, should have sufficient regularity to give a intensities could be placed on a common basis. The well-defined series of at least ten essentially integral diffractometer was calibrated with respect to a silicon orders of reflection from the summation spacing, d4s = standard and a secondary tetradecanol standard (Brind- d^ + dB, and the coefficient of variation of das should be ley, l98l). less than 0.75 to demonstrateadequate regularity oflayer Attention has been given particularly to the evaluation alternation. The diffraction breadths of odd and even of basal X-ray difractions. Imperfectly interstratified order reflections should be closely similar. If odd orders layer sequences give diffraction peaks which do not are absent,their absenceshould be shown to be consis- follow precisely the Bragg reflection equation. The appar- tent with calculated intensities. ent spacingsd(00I) multiplied by the apparentorder / give The presentstudy was undertakento establishwhether a variable product /d(00D = d1and the degreeof order in interstratified biotite/vermiculites exist that satisfv these the layer sequenceis evaluatedby a statistical treatment requirementsfor use of the name hydrobiotite. of values of dr. If d is the arithmetical mean value, the mean deviation, AZ, is (2"ldrt1l)ln, and the coefficient of Materials and methods variation, CV, is [I"(dr- A2l@ - l)]t/2 x lfi)/Z, where n is the number of terms in the summations. Vermiculite ores from the Enoree area of S. Carolina. made available many years ago to one of us (G.W.B.) by Results the Zonolite division of W. R. Grace & Company, measurements showed a varied mineralogy that included vermiculite, Basal spacing biotite, and variously ordered mica./vermiculites.From Table I summarizesthe results for two samplesexam- material describedas a Cooper concentrate, small black- ined in flake form and one sample as a thin oriented ish flakes were selected under a binocular microscope. powder layer. The two samplesexamined as flakes yield- They were lightly ground in water with a small pestle and ed diffractions up to 0.0.31 and the powder data were mortar. The dispersedmaterial was allowed to dry slowly measuredas far as 0.0.22. The mean basal spacingsare in air to form thin layers on glass slides. Only basal respectively 24.46t0.17, 24.5I -+0.05 and 24.45* 0. 13A difractions appearedin the recorded patterns. X-ray data where the -r values are mean deviations. The CV values for this material are presentedin section 3 of Table l. are respectively1.25,0.35 and 0.95when equalweight is A number of sampleswere made availableby Dr. S. W. given to all reflections. The secondsample easily satisfies Bailey and from among these, a well-developed flaky the condition CV < 0.75 to justify the acceptanceof a material from north-western Transvaal provided thin specific mineral name. flakes up to about 8 x 8 mm size, which could be The large CV values for the first and third samplesarise examineddirectly without reduction to powder. Many of largely from the first order diffractions which are the least the flakes were contaminatedwith apatite and, to a lesser accurately measurable because of the small 20 angles, extent, with zircon. Uncontaminatedflakes were selected 3.6-3.8', at which they occur. When the first order under a microscope and were mounted directly on glass diffraction data are omitted, the mean spacings(as given slides with a trace of adhesive. A sample supplied by in Table l) are respectively24.5110.10, 24.5110.04, and Wards Natural ScienceEstablishment. labelled vermicu- 24.51'+0.044and the CV values are 0.67,0.29,and 0.20, lite from north-easternTransvaal, proved to be a regular- all of which satisfy the condition of CV < 0.75. Although ly interstratified biotite/vermiculite. Golden yellow and we cannot give an unequivocal explanation for the low pinkish flakes were selected for study but were smaller values of the first order spacings for the first and third than those from north-western Transvaal and were less samples, the balance of evidence appears in favor of suitable for X-ray intensity measurements. retaining hydrobiotite as a specific mineral name. 422 BRINDLEY ET AL,: HYDROBIOTITE, A REGULAR I : I INTERSTRATIFICATION Table l. X-ray difraction data for basal reflections from hydrobiotites (1) (2) (3) Flake, North-western Transvaal. Flake, North-eastern Transvaal. Oriented powder, South Carolina d(002) d(001) I(peak) d(002) d(001) I(peak) d(002) d(001) I(peak) I 23.3 23.3 IO 24.3 24.3 7 23.6 23.6 20 t 12.30 24.60 80 LZ. Z5 24.46 60 L2.26 24.52 >100 J 1 7.99 23.97 8.27 24,8L ) 8. 18 Lq. )c ) 4 6.146 24,58 I 5 4.880 24.40 30 4. 909 L+. )) 30 4.