A Pleochroic Variety of GemLabradorite From the Rabbit HillsArea, Lake County, Oregon*By ERNEST H. CARLSONAssociate Professor of GeologyKent State University

IntroductionLabradorite rarely is considered to

be a gemstone. One important butpoorly known occurrence is foundabout 22 miles north of Plush ineast-central Lake County, Oregon(Figure 1). It lies in the northwest partof the Rabbit Hills NE quadrangle mapof the U.S. Geological Survey, Thematerial is called sunstone locally, andthe site of the occurrence is designatedas the "Sunstone Area" on. this map.

ORE G 0 NLake County

Figure 1. Gem labradorite locality in theRabbit Hills area,Lake County,Oregon.

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andMICHAEL A. KIRCHERGeologist and GemologistMarcus, Ohio

The Rabbit Hills area lies within thesouthern portion of an extensive pro-vince of volcanic rocks that passesnorth and east into Washington andIdaho. Stewart, and others (1966, p.178-180) note that labradorite occursas phenocrysts in porphyritic lavaflows in the vicinity of the RabbitHills. They described crystals as largeas 86 x 26 x 8 mm.

A parcel of cut stones and roughmaterial from the Rabbit Hills areawas investigated by the writers. Trans-parent gem quality stones as large as13 x 7 x 5 mm were examined. Thediscovery of some previously undes-cribed properties led to the work uponwhich this report is based. The assis-tance of James Pettit, AssistantManager of J.R. Rodgers, Ltd., Sher-man, Oregon, who supplied all thespecimens, is gratefully acknowledged.

*Department of Geology, Contribu-tion No. 134.

GEMS & GEMOLOGY

Figure 2. Debye-Scherrer x-ray diffraction photograph of labradorite from the Rabbit Hillsarea, Oregon.

Previous work on the properties ofgem labradorite from southern Oregonis scant. Aitkens (1931, p.8) noted theoccurrence of a labradorite that isremarkably similar to the materialdescribed here, from an unspecifiedlocality -in southern Oregon. Bank(1970, p. 134-136; 1973, r- 58-59)described similar feldspars (bytownite,labradorite) from an unspecified peg-matite locality near Plush. Gem labra-dorite from the Rabbit Hills localityhas been noted recently by Rodgers(1976, p. 120).

X-Ray DiffractionPositive identification was made

from a powder sample, using a Debye-Scherrer camera of 114.7 mm

diameter, and a filtered copper radia-tion source. The resulting pattern isshown in Figure 2. Twenty reflectionswere measured and the correspondingd-spacings are listed in Table 1. Inten-sities were estimated visually on a scaleof 10. The pattern is typical of plagio-clase and no reflections due to im-purities were recognized.

The structural state of theplagio-c1ase was determined by accuratelymeasuring the 20 r 3 1 - 201 3 1 spacingwith a Nore1co diffractometer. Thespacing was found to be 2.10 degrees.From the determinative curve given byBambauer, and others (1967, p. 342),a high structural state for the plagio-clase is indicated. This result is con-sistent with a volcanic origin.

Table 1. Intensity and measured d-spacing for labradorite from the Rabbit Hills,Oregon

d-spacing d-spacingIndices Intensity (Angstroms) Indices Intensity (Angstroms)

021 .". "' ........ 1/2 4.68 131 2 2.82201

•• 1 •• •••• •3 4.03 132 1 2.64

111 of ••••• •••• • 1/2 3.88 241 4 2.51130,.f11. ........ 3 3.75 241 2 2.13

130 .............. 2 3.62 151 1 2.09f1.2 ...•...... 1/2 3.47 422,422 ...... 1/2 1.92112 .......... 1 3.35 333,260 ...... 1 1.87

040. 202. 002 •...... 10 3.19(broad) 400 1 1.83131 .• I •.•• •••••

1 3.02 113 1/2 1.79041.022 ......... 3 2.93 204 1 1.77

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Refractive Indices andSpecific Gravity

Refractive indices were determinedin sodium light using a Duplex IIrefractometer. From eleven deter-minations, ne ranged from 1.560 to1.563, averaging 1.562, and nv rangedfrom 1.569 to 1.570, averaging 1.570.The average birefringence was 0.008.

Specific gravity determinationswere made using an Ainsworth, doublepan, analytical balance, and toluene asthe displacement fluid. For sevendeterminations, the specific gravityaveraged 2.713, ranging from 2.711 to2.717. The accuracy of the determina-tions is estimated to be ± 0.010.

The composition of the plagioclase,in terms of the percent albite (Ab) andanorthite (An); can be determinedfrom its structural state, refractiveindices and specific gravity. Using thedeterminative curves of Barth (1969,p. 159) and Smith (1958, p. 1189),the composition was found to becalcic labradorite (Ab32-An6s). Allof the above data is in close agreement

with the results of Stewart and others,(1966, p. 182-185), seeming to indi-cate that their material and ours cor-respond.

Color and PleochroismColors of varying tone and intensity

were observed in the labradoritefromthe Rabbit Hills. The variations incolor, as observed in diffuse southdaylight during January, are given inTable 2. The color descriptions usedfollow standard North American gem-ological nomenclature. In addition, thecolor was noted to vary slightly withthe source of illumination - a red-orange color being more predominantunder incandescent light, and a bluish-green predominating under fluorescentlight.

One of the most unusual propertiesof the labradorite is its pleochroiccharacter. This pleochroism, which isweak in the pale yellow material,increases in strength with depth ofcolor. In more deeply colored speci-mens the strong pleochroism imparts amulticolored effect that can be seen

2. Color and pleochroic character of labradorite from the Rabbit Hills,Oregon.

Color of Stone

Table

yellow

red-orange and blue-green(multicolored effect)

bluish-green

red-orange

yellowish green

orange

bluish-green and violet(parti-colored)

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Pleochroismcolorless; light yellow

bluish-green; light red-violet; reddish-orange or orange

bluish-green; light orange; colorless

orange; light reddish-purple

bluish-green; light orange

orange; reddish-orange

red-violet; reddish-orange; blu ish-green

GEMS & GEMOLOGY

Figure 3. Saucer-shapedclustered inclusions(x25).

without the aid of a dichroscope orpolariscope as the stone is rotated.'This effect is similar to that observedin transparent andalusite. The datagiven in Table 2 were obtained from agem dichroscope with the base of anllluminator Polariscope serving as thelight source. The pleochroism un-doubtedly is due to a unique combina-tion of the high transparency, deep

Figure 4. Zigzag patterns viewed on edge(x25).

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coloration and relatively large size ofthe crystals of the Rabbit Hillsmaterial.

The labradorite lacked fluorescencein both short- and long-wavelengthultraviolet radiation. No characteristicabsorption spectra were recognizedwith a Rayner Prism Spectroscope.

InclusionsInclusions of microscopic size are

common in the Rabbit Hills labrador-ite. With the unaided eye, the presenceof inclusions in some specimens isindicated by an aventurescent effectsimilar to that of sunstone. As viewedwith dark-field illumination undermagnification, the inclusions are seento reflect light strongly and to beoriented in planes. The distribution ofthe inclusions is irregular and theyoften occur in clusters (see Figure 3*):The inclusions sometimes appear as aseries of minute, parallel streaks thatform a zigzag pattern when viewedalong an edge (see Figure 4*). Whenrotated from this position, reflectivesurfaces of the inclusions appear, in-dicating a plate-like habit (see Figures5*, 6*, and 7*). The size of the

Figure 5. Reflections from plate-like in-clusions (x100).

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Figure 6. Reflections from plate-like in-clusions (x25J.

inclusions is approximately the samein a given plane, but may vary betweenadjacent planes and from specimen tospecimen. In one specimen of roughmaterial, sheets of inclusions were seento lie parallel to a prominent directionof cleavage. This cleavage was es-tablished to be the {DOI} directionsince it was cut by broadly spacedalbite twin lamellae. It was not pos-sible to identify the platy inclusionswith the available equipment. The

Figure 7. Strong reflections from inclusionsviewedperpendicular to plates (x25J.

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strong doubling effect under magnifi-cation, which is readily apparent in thephotomicrographs, increased the dif-ficulty of resolution.

In one stone, a solitary inclusionwas resolved at 100 power magnifica-tion (Figure 8*). This inclusion has theequant habit and color that are char-acteristic of pyrite, and is obviouslyprotogenetic in origin. Noting the sizeof other inclusions in this photograph,one clearly can see why this particularspecimen presented a cloudy appear-ance under low magnification.

Gem PotentialThe limitations to the use of this

gemstone in jewelry stem from its lowhardness relative to other gem ma-terials and its ability to cleave easily intwo directions. Similar factors· havebeen overcome in other gemstoneswith careful attention to the type ofmounting used, and by exercising carein setting the stone. Examples of

*References to magnification refer tothe original size of the negative (24 x36 mm)., All photographs were takenunder dark-field illumination.

Figure 8. Large solitary inclusion surround-ed bV minute inclusions [x 100).

GEMS & GEMOLOGY

gemstones of a similar nature arekunzite and opal, both of which areused widely in jewelry.

The quantity of better material thatis potentially available is unknown atthe present time. Since there may be alimited supply of quality stones,probably the appeal will be restricted.The high degree of transparency andthe unusual pleochroism displayed bythe Rabbit Hills material, however,combine to form an exceptional gem-stone for the collector or other dis-criminating individual.

ReferencesAitkens, I., 1931, Feldspar gems: U.S.

Bur. Mines, Inf. Circular 6533, lOp.Bambauer, H.U.,Corlett, M., Eber-

hard, E., and Viswanathan, K.,1967, Diagrams for the deter-mination of plagioc1ases using x-raypowder methods: Schweiz. Min.

Petro Mitt., V. 47, p. 333-349.Bank, H., 1970, Durchsichtiger blass-

gelblicher Plagioklas aus denU.S.A.: Zeits. deutschen Gemmolo-gischen Gesell., v. 19, p. 134-136.

Bank, H., 1973, Klar durchsichtigeLabradorite aus Oregon, U.S.A.:Zeits. deutschen GemmologischenGesell., V. 22, p. 58-59.

Barth, T.F.W., 1969, Feldspars: NewYork, Wiley-Interscience, 261 p.

Smith, J.R., 1958, Optical propertiesof heated plagioc1ases: Am. Min., v.43, p. 1179-1194.

Stewart, D.B., Walker, C.W., Wright,T.L., and Fahey, J.J., 1966,Physical properties of calcic labra-dorite from Lake County, Oregon:Am. Min., v. 51, p. 177-197.

Rodgers, J.R., 1976, Classic Oregonlocations: Lapidary Jour., V. 30, no.1, p. 116, 118,120,124,126,128.

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