Canadian MineralogistVol. 17, W. 5aI-547 (1979)

STRUCTURE OF HYALITE FROM THESPRUCE PINE PEGMATITE DISTRICT, NORTH GAROLINA

WILLIAM J. FURBISHDepartment of Geology, Duke (Jniversity, Durhatn, North Carolina 27708, US,A.

ED L. SCHRADER, JN.Department of Geology and Geography, University of Alabama,

University, Alabama 35486, US-{,

AssTRAcr

Hyalite opal from the Spruce Pine, North Caroli-na, pegmatite district gives an X-ray pattern similarto tle pattern of vitreous silica, on which peakrof quartz, tridymite or cristobalite may be superim-posed. Scanning electron micrographs show parti-culate, relatively even-sized spherical structures thatoccur in semi-close-packed arrays as well as random-ly dispersed spheres of various sizes. The matrixconsists of layers of apparently structurelees andloosely consolidated particles; particulate sphericalstructures are either enclosed within single layersor lying across the layer interfaces.

SoMMAIRE

Une opale hyaline Fgmatitique de Spruce Pine(Caroline du nord) donne aux rayons X le dia-gramme de poudre de la silice vitreuse, auquel sesuperposent, dans certains cas, les pics du quartz,de la tridymite ou de la cristobalite. Au microscope6lectronique i balayage, on voit des particulessphdriques de mOme diamdtre en assemblagps quasi-compacts, ainsi que des sphdres de diff6rentestailles dispersees au hasard, dans une pite de parti-cules mal-consolid6es, i l'aspect amorphe, dispo-s6es en couches; des amas r6guliers de particulessph6riques se trouvent soit au sein d'une couchesoit sur finterface entre deux couches.

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After Greig (1932) indicated the presenceof crystallinity in certain .opals, Taliaferro(1935) pointed out that apparently amorphoushyalite gave definite B-cristobalite X-ray pat-terns. Jones et al. (1963) concluded that mosthyalite is near-amorphous. In 1.964 Jotes a al,reiterated that hyalite gtves an X-ray diffrac'tion pattern with only a diffuse band at 4.1A.Jones & Sednit (1971) used X-ray diffractionpatterns as their principat criterion for the classi-

fication of opals into three structural groups.Hyalite was placed in category "Opal-A," -char-acterized by a diffuse X-ray band at 4.I4. .-

Controveisy has continued over the specificnature of the crystalline components of opalsince Greig's (1932) work. Quartz, d--cristo-balite, disordered a-cristobalite, cristobalite, a-tridymite, disordered a-tridymite, p-tridymite,disordered B-tridymite, tridymite and higlfydisordered to near-amorphous phases have allbeen reported by various authors. Flbtke et al.(1973) felt that, even though all natwal hyalitethey examined seemed near-amorphous, thereremained a possibility that hyalite with disor-dered cristobalite and tridymite componentsmight form under suitable natural conditions.This conclusion followed from their experi-mental work, in which they obtained suchmaterial in their high-temperature or high-depositional-rate hydrothermal runs (or in both).

The discovery by Pense (L963) of the exist-ence of discrete particulate structures in opalled to re-evaluation of the physical and opticalproperties of opal. Jones el al. (1964) firstietateA the X-ray pattern to internal particulatestructures, Sanders (L964) considered the

"color" of precious opal to be caused by diffrac-tion by the lattice of voids between regularlypacked and sized particulate spheres within theopal host.-

Little thought was given specifically to theorigin of hyafte until the paper of Fl6rke et ol'appeared in 7glZ. They conducted a series of

eiieriments in which silica glass with the-physicai properties and appearance of natural hyalitewaslormed by vapor transport in a hydrotherrnalsystem. They suggested that this might also be

the mode of formation for hyalite in at leasttwo natural ocqurence$ and, if so, the originof hyalite is then genetically distinct from mostopal deposits that originate through depositionfrom solution or colloids.

541

542 THE cANADTAN

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Hyalite quite commonly occurs in the SprucePine, North Carolina, pegmatite district in frac-tures in alaskite, in pegmatites enclosed by thealaskite, or in pegmatites peripheral to the alas-kite. The Spruce Pine pegmatite district islocated in the extreme west-central portion ofNorth Carolina and is geographically limited toAvery, Mitshell and Yancey counties.

Generallyo hyalite from this area occurs inthin, sheet-like masses on fracture surfaces. Thehyalite-coated surface consists of botryoidal tostalactitic projections of transparent, clear tolight yellow, green or blue glass-like masses thatmay cover all or only part of the fracfure sur-face.

Samples for t}lis study were collected fromvarious geological environments within the area.They were from: (1) a large pegmatite enclosedconformably by a hornblende gneiss and located

MINERALOGIST

about 450 m from any known alaskite body, (2)a pegmatite enclosed within the host alaskite,(3) various fractures within the alaskite bodyeither in close proximity to the enclosing biotiteand hornblende gneisses and schists or wellwithin the body.

Refractive indices from the hyalites of thesesample locations vary from sample to sample:1.443 to 1.445 (one value 1.458); in someinstances, they even vary slightly within indivi-dual samples. A random difference in intensityand reaction to various wavelengths of ultra-violet radiation was noted among the specimens.This was taken to indicate the presence ofdifferent chromophors among samples.

ExpsRrlvrsNrer, Dere

Dilferential thermal analysis

Differential thermal traces of four hyalite

Frc. 1. Differential thermograms of four hyalite samples showing re-active variations. Sample I from large pegmatite, 450 m from knownalaskite aody; samples 2 and 3 from separate fractures (15 m apart)in-alaskite-body; sample 4 from fracture in alaskite body 25 rn'iroma large included pegmatite. Heating rate l0oC/min, maximum T 1140oC.

STRUCTURE OF IIYALTTE 543

samples chosen from different local depositionalenvironments are shown in Figure 1. Al1 sam-ples were dry-mortar ground, sized to 200 mesh,desiccated over silica gel for ten days and anal-yzed in a uniform fashion. Variations in inten-sity and position of reaction peaks are shownto exist, and the contrast between samFles canreadily be seen. Numerous other sample anal-yses, not shown in this paper, gave a similarvariation in differential-thermal-analysis patternsto the four shown in Figure 1, even though manywere obtained within one metre of each otherin the same depositional environment.

X-ray diflraction

X-ray diffractograms of both heat-treated andnormal unheated samples were made. A widevariation in patterns exists from sample to sam-ple in both the heat-treated and unheated mate-rial. Unheated hyalite samples give X-ray dif-fractograms with peaks that range from a broadswell at 4.1A to a well-defined a--cristobalitepattern. Superimposed upon these patterns, insome instances, are subdued peaks of eitherquartz or tridymite (Fig. 2). The specirnens wereheat-treated in open crucibles. The successivetemperatures to which the samples were raised,before being X-rayed, were correlated with the

Frc. 2. X-ray diffraction patterns of samples 2and 3 (of Fig. l) from separate fractures (15 mapart) in alaskite body' Cu Ka radiation.

postreactive areas of D.T.A. patterns for a por-tion of the same sample that was used forD.T.A. Preliminary control-heating mns weredone on both unground and ground materialfrom single samples. X-ray diffraction patternsfrom the controlled runs showed that far lessintense peaks were obtained from the pregroundmaterial. Whole material was used for all sub'

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Frc. 3. Heat-treatment curves of samples 1 through 4 based on relativeX-ray poak heights with increased T. Curves were drawn on change of20oC T increments between 800 and 1140oC.

lO SAUPLE 3

544 TIIE CANADIAN I\4INEMLOGIST

sequent heat runs and ground for X-ray anal- ature increments of 20oC were used, and eachysis after heat treatment was complete. Beyond increment peak temperature wa$ held for twenty-800oC, to a maximum T of 114,0'C, temper- four hours. Conversion to e-cristobalite was

Flc. 4a. Scanning-electron micrograph of hyalite growti-surface showingstalactitic growth forms, smooth and rough surfaces. Bar scale =100 g,m.

Ftc. 4b. $6nniag electron micrograph of the etched fracture surface ona single stalactitic growth unit. Concentric growth layen can be seeu.Bar scale = 60 pm.

Frc. 4c, d. Scanning electron micrographs of normal hyalite sample sur-faces showing a sequence of surface morphologic expression from lens-shaped raises of uncertain origin through semi-spherical forms that aredefinitely a surface expression of an internal structure. Bar scale =30 pm.

STRUCTURE OF I TALITB

Flc. 4e. Scanning electron micrograph of a normal hyalite growth sur-face showing pafiiculate-sphere growth configuration. The lack ofstructureless silica material and the interconnecting necks are especiallynoticeable. Bar scale - 2 ym'

Frc. 4f. Scanning electron micrograph of a single growth layer in whicbthe volume of spheres and structureless material are similar. Bar scale- 2 p'n.

FIc. 49. Scanning electron micrograph of a slngle growth layer in whichthe deposition of structureless silica material predominates over de-position of particulate spheres. Bar scale = 1 pm.

Fta. 4h. Scanning electron micrograph of a cross-section of growth layers.This surface has been etched and shows particulate spheres dispersedat random in structureless silica. Some spheres cross over the boundarybetween layers. Bar scale - 1 U,m.

545

546 THE cANADIAN

initiated over a temperature range from -880to -1100oC with those samples that started toconvert at the lowest temperature lsasfoing themaximum total conversion (Fig. 3). Usuallythose samples with an initial X-ray patternsimilar to that of vitreous silica converted toa-cristobalite most readily (1 and 4, Fig. 1),whereas those initially with a more definitecrystalline pattern showed slight to only moderateamount of reaction (2 and 3; Figs. I and 2). Notridymite patterns were produced.

Scanning electron miwoscopy

Scanning electron micrographs were obtainedfrom hyalite samples that were on either normalgrowth surfaces, etched growth surfaces, fracturesurfaces or etched fracture surfaces. A recordof both external morphological and internalstructures was thus made available.

The external surfac€ of the hyalite is notplanar but rather uneven and consists of bluntstalactite-like projections up to 0.5 mm across(Fig. 4a); these coalesce at their base to forma planar growth zone on the initial hyalite-hostrock interface. These projections consist of suc-cessive layers of opaline material (Fig.4b: broken,etched surface). The unbroken, unetched sur-face of the stalactitic projections may have aperfectly flat surface or may show various sizesand shapes; some cases definitely show internalstructure but in others this is less certain. Thishabit seems to have totally random distribution.Figures 4c and 4d reflect what appears to be atransition from lens-shaped forms to sphericalexternal expressions of internal particulate struc-tural forms. The lens shapes are somewhat exag-gerated in Figure 4c because of the inclinedelectron beam; in Figure 4d the beam is notinclined, and non-spherical forms exist.

Figure 4e represents an unetched natural sur-face growth area involving mostly particulatespheres in a semi-close-packed array. In thisfigure the continuous necked interconnection be-tween particulate spheres can be seen. A smallamount of structureless, loosely consolidatedmaterial can also be seen filling the voids be-tween the spheres. Unlike Figure 4e, Figure 4frepresents a growth surface in which the parti-culate spheres are fewer and more dispersed inthe structureless, sponge-like silica material. Thelayering of spheres is present but no intercon-necting necks between spheres are visible.

Figure 49 shows still fewer particulate spheri-cal units than Figure 4f, and where the growthlayer has developed it consists predominantly of

MINERALOGIST

apparently non-structured siliceous material.Figure 4h, representing a pattern prevalent

throughout the study, shows an etched cross-section surface of hyatte in which the spongy,structureless siliceous material forms layers andis the predominant siliceous form. Particulatespheres are dispersed at random throughout themass and some are seen to lie across laver inter-faces.

DrscusssroN eNp CorqclusroNs

This study shows that hyalite opal may exhi-bit crystallinig (Fig. 2) and may contain parti-culate spherical strustures (Fig. af). Neitler ofthese features, however, necessarily occurs inmethodical patterns in the material studied.Both vary from sample to sample or within asingle sample.

Two varieties of solid material make up thehyalite body. Particulate units of spherical orovoid shape may be present (Figs. 4c and 4d).These units either lie in layered semi-close- toopen-packed arrays (Fig.4e) or occur at randomthroughout the,opal body (Fig.4h). Their sizemay be relatively constant within a single layeror sample (Fig. 4e), or variation may occur(Fig. h). A porous unit of loosely aggregatedparticles constitutes the remainder of tle opalbody (Fig. 4h). It is layered, but the thicknessof the layers varies.

The physical relatipnship of the two units iswell documented by the electron micrographstaken during this study. Single or multiple layersof particulate spheres may accrue on the growthsurface with no infill of aggregate particles(Fig. 4e). Such a situation is probably only atemporary surface-growth feature. Particulateunits together with interspherical infill of ag-gregate particles may constitute a single layer(Fig. 4f), or the spherical units may be dis-persed at random throughout the aggregate unitparticles. Where this occun the spherical unitsmay be contained within a single layer of ag-gregate material, or they may cut across layerboundaries and be contained within two or morelayers (Fig. 4h).

Such relationships indicate that two inde-pendent but interrelated modes of dqrositionalprocesses are contributing to the hyalite growthpatterns; predominance of (1) production ofone unit or (2) its deposition over the otherduring hyalite growth results in the final spatialrelationships and ratios of the rwo units involved.

In 1964 Sanders suggested that visible color

STRUCTURB OF ITTALITB

in precious opal is the result of light diffrastiontrom a three-dimensional array of close-packedsilica particles within the range of 15O0 to40004 in diameter. Sanders & Darragh (197I)later refined refined this concept, pointing outthat the minimum diameter of spheres able todiffract visible light in the violet part of thespectrum is O.16 pm and that. the maxirnumdiameter of spheres able to diffract in the redpart of the spectrum is 0.28 ;.cm. Requirementsfor the diffraction of light within the visiblerange are generally not met by the hyalite opalmaterial of this study. Although close packing ina three-dimensional pattern does occur, it is notextensive, and the size of the particulate silicaspheres usually varies so that a visible diffrac-tion effect cannot be obtained.

AcrNowr,sDcsMBNTs

The writers express their appreciation to theDuke University Research Council for partialsupport of the study through a grant. The scan-ning electron microscope used was supportedby National Science Foundation grant OCE74-125554OI.

RennnrNcEs

Friinrs, O.W., JoNes, J.B. & SncNrt, E.R. (1973):The genesis of hyalite. Neues lahrb. Mineral.Monatsh., 82-89.

Gnerc, J.W. (1932): The existence of the high-temperature form of cristobalite at room temper-ature and the crystallinity of opal. J. Amer. Chern.Soc. 64,2846-2849.

JoNes, J.8., Sanurns, J.V. & SrcNtr, E.R. (1964):Structure of opal. Nature 204, 990-991,

& SroNrr, E.R. (1971): The nature ofopal. I. Nomenclature and constituent phasesl. Geol. Soc, Aust, 18, 57-68.

--, - & Nrcrsox, N. M. (1953):Differential thermal and X-ray analysis of opal.Natare 198, 1191.

PsNsg J. (1963): Ein Electronenmikroskopisherbeitrag zud Optik der Edelopale. Fortschr. Min-eral. 41, 166 (Abstr.).

SeNorns, I.Y. (t964): Color of precious opal.Nature 204, Ll5l-t153.

& Dunrcn, P.J. (1971): The microstruc-ture of precious opal. Mineral. Record 2' 261-268.

TALTAFERRo, N. L. ( 1935) : Some properties of opal.Amer. l.,Sci. 230, 450-474,

Received October 1978; revtsed manuscript ac-cepted January 1979.