Optical Properties of Common Rock-Forming Minerals
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AppendixA __________ Optical Properties of Common Rock-Forming Minerals 325 Optical Properties of Common Rock-Forming Minerals J. B. Lyons, S. A. Morse, and R. E. Stoiber Distinguishing Characteristics Chemical XI. System and Indices Birefringence "Characteristically parallel, but Mineral Composition Best Cleavage Sign,2V and Relief and Color see Fig. 13-3. A. High Positive Relief Zircon ZrSiO. Tet. (+) 111=1.940 High biref. Small euhedral grains show (.055) parallel" extinction; may cause pleochroic haloes if enclosed in other minerals Sphene CaTiSiOs Mon. (110) (+) 30-50 13=1.895 High biref. Wedge-shaped grains; may (Titanite) to 1.935 (0.108-.135) show (110) cleavage or (100) Often or (221) parting; ZI\c=51 0; brownish in very high relief; r>v extreme. color CtJI\) 0) Gamet AsB2(SiO.la where Iso. High Grandite often Very pale pink commonest A = R2+ and B = RS + 1.7-1.9 weakly color; inclusions common. birefracting. Indices vary widely with composition. Crystals often euhedraL Uvarovite green, very rare. Staurolite H2FeAI.Si2O'2 Orth. (010) (+) 2V = 87 13=1.750 Low biref. Pleochroic colorless to golden (approximately) (.012) yellow; one good cleavage; twins cruciform or oblique; metamorphic. Olivine Series Mg2SiO. Orth. (+) 2V=85 13=1.651 High biref. Colorless (Fo) to yellow or pale to to (.035) brown (Fa); high relief. Fe2SiO. Orth. (-) 2V=47 13=1.865 High biref. Shagreen (mottled) surface; (.051) often cracked and altered to %II - serpentine. Poor (010) and (100) cleavages. Extinction par- ~ ~ alleL" l~4~ Tourmaline Na(Mg,Fe,Mn,Li,Alk Hex. (-) 111=1.636 Mod. biref. Generally pleochroic; maxi- [SieO,al(B03k to 1.698 (.020) mum absorption across the (OH,F)4 length. Colors chiefly brown, green, blue. Triangular outline on basal section. Amphibole (110) Characteristic 124°-56°cleav- age. Generally pleochroic. Anthophyllite- (MgFeMSiAI)a0 22(OHh Orth. (+) and (-) 13=1.605- Biref. mod. to Z=c; Y=b; may be pleochroic Gedrite + Na in gedrite 2V=80± 1.689 low (.013 to in tans; found only in metamor- Cummingtonite Same but no Na Mon. same 1.65-71 .026) phic rocks; parallela extinction. Tremolite Ca2MgsSia0 22(OH)2 Mon. (-)2V=85 13=1.615 Biref. mod. to Zl\c=20o; colorless; a meta- low (.026) morphic mineral. Actinolite Ca2(MgFe)sSia0 22(OH)2 Mon. (-) 2V = 75 13=1.642 Mod. (.020) Zl\c = 15°; pleochroic yellow to green. Hornblende Complex AI amphibole Mon. (-) 2V near 13=1.63- Mod. (.020) Zl\c= 15°-25°;pleochroic yel- e.>I\) 80 1.7 low, green, brown. ...... Arfvedsonite Soda-rich amphibole Mon. 2V large, 13=1.69- Low (.005) )(/\c=200; strong pleochroism (-) 1.70 in blues and greens; found chiefly in sodic igneous rocks. Glaucophane Na.Mg3AI2Sia022(OH)2 Mon. (-)2V=45 13=1.638 Mod. (.018) Zl\c=5°;distinct lavender blue to blue-black pleochroism. A metamorphic mineral. Sillimanite AI2SiOs Orth. (010) (+) 2V = 13=1.658- Mod. (.020) Y .1 single perfect cleavage; 30 r>v 1.670 Z=c; may be in interlacing nee- dles (fibrolite). Metamorphic rocks. End section: t Calcite CaC03 Hex. (1011) (-) 111=1.658 Extreme (.172) Differential relief on stage rota- E=1.486 tion; one index far above 1.54, other below; rhombohedral cleavage; often twinned; told from dolomite etc. only by , index. Chemical XI. System and Indices Birefringence Mineral Composition Best Cleavage Sign,2V and Relief and Color Distinguishing Characteristics Chloritoid H2FeAI2Si07 Mon. (110) (+ ) 13=1.720- Low (.007- Pleochroic in greenish-black to 2V 36-68 1.770 .016) gray. Epidote has better cleav- age. X parallel to (010) parting. Zoisite HCa2A13Si30'3 Orth. (010) (+) 13=1.700 Low (.006) Abnormal blue interference 001- 2V = 30±, ors; one cleavage; parallel" ex- r<v tinction; colorless. Epidote H(CaFe)2AI3Si30'3 Mon. (001) (-) 2V = 75 13=1.753- High (.033-to Abnormal interference colors; 1.763 .051) Zl\cleavage= 14°-24°;weakly pleochroic in pistachio greens; usually in fine aggregates. Kyanite AI2Si05 Tri. (-) 2V = 83 13=1.722 Low (.012) High relief; 2 good cleavages, w best is (100); Zl\c=300; alters I\:) to sericite; nice colorless co blades. Pyroxene (110) 93-8r Cleavage characteristic; cf. am- phibole. Enstatite MgSi03 Orth. (210) (+)2V=55 13=1.652- Low biref. Parallel" extinction. Colorless. 1.670 Hypersthene (MgFe)Si03 Orth. (210) (-) 2V = 75 13=1.680- Low biref. Parallel" extinction; faint pink 1.702 to green pleochroism dis- tinctive. Pigeonite (MgFe)Si03 + CaSi03 Mon. (110) (+) 2V = 13=1.654- Low biref. Colorless; Zl\c=22°-45°; low 0-20 1.660 2V diagnostic. Diopside CaMgSi20 s Mon. (110) (+) 2V = 60 13=1.672 Mod. biref. Zl\c=38°; colorless; pure form common chiefly in contact metamorphic rocks. Continued (Pyroxene, cont.) Augite Ca(MgFe)Si2O. Mon. (110) (+) 2V = 60 13=1.676- Mod. biref. Zl\c=48°-54°;may twin on 1.745 (100) or (001); light colored in thin section, but reddish or vio- let when n is abundant. Aegirinaugite Ca(MgFe)Si20. - Mon. (110) (+) 2V = 65 13=1.680- Mod. biref. Zl\c=65°-75°;pleochroic in NaFeSi2O. 1.720 Green greens; may show inclined dis- persion; common in alkalic rocks. Apatite Cas(F,CI)(P04la Hex. (-) 11)=1.632- Low (.004) Small hexagonal prisms; sha- 1.648 green surface; colorless. Small inclusions common. Andalusite AI2SiOs Orth. (110) (-) 2V = 85 13=1.640 Low (.011) Faint pink pleochroism; X=c; carbonaceous inclusions; al- ters to sericite. Ul B. Intermediate Positive Relief N co Micas (001) Perfect basal cleavage Muscovite KAI2[AISi31O,o(OH)2 Mon. (-) 2V = 13=1.582- High biref. High index parallels cleavage; 45± 1.610 colorless. Sericite Fine grained muscovite. Biotite K(Mg, Fe),(AISi310,o(OH)2 Mon. (-) 2V = 13=1.564- High biref. Pleochroic; max. absorption o - 10 1.690 Green, brown and high index parallel cleav- age; pleochroic haloes around inclusions; green ferric, brown titanian. Paragonite NaAI2[AISi31O,o(OH)2 Mon. Na analog of muscovite, with which it may be intergrown in schists; X-ray or probe needed to identify. Chlorite (Mg,AI,FeMAI,SiI40,o(OH)e Mon. (001) (+) or (-) 13=1.570- Very low B; Perf. (001) cleavage; may small-zero 1.65 pleochroic in show abnormal interference greens colors; max. absorption paral- lel to cleavage; high index par- allel or normal to cleavage. Chemical XI. System and Indices Birefringence Mineral Composition Best Cleavage Sign,2V and Relief and Color Distinguishing Characteristics Serpentine MgsSi40 1O(OH)a Mon. (-) n=1.55± Very low B Colorless to green; fibrous or radial; often alters from olivine. Talc Mg3Si40 1O(OH), Mon. (001) (-)2V= (3=1.59 High biref. Difficult to distinguish from 0-30 (.03-.05) muscovite except by its associ- ation and slightly lower indices and 2V. C. Low Relief (Positive and Negative) Feldspar (001) (010) (100) Colorless Variously twinned. Orthoclase KAISi30 a Mon. (001) (010) (-) 2V = (3=1.523 Low (.006) Index lower than 1.54; 2 good 30-60 cleavages; no polysynthetic twinning. Sanidine KAISi30 a (-)2V 0-40 (3=1.523 Low High temperature polymorph c.v of orthoclase. c.v 0 Microcline KAISi30 a Tri. (-)2V=83 (3=1.522 Low (.007) Cross-hatch (plaid) twinning (001) (010) distinctive. Anorthoclase (Na,K)AISi3Oa Tri. (-)2V=50 (3=1.528 Low (.006) Cross-hatch twinning; un- common. Plagioclase Albite NaA1Si30 a, An = 0-10 Tri. (+) 2V=70 (3=1.529 Low (.010) Albite twinning characteristic. Plagioclase end (001) (010) (100) All other plagioclase has n > member 1.54. Oligoclase An 10-30 -do- (-) 2V=86 May be antiperthitic (with exso- lution of Or). May be iridescent (peristerite) . Andesine An 30-50 -do- (+) 2V=87 May be antiperthitic up to about An 40. Labradorite An 50-70 -do- (+) 2V=78 May be iridescent when slowly cooled. Continued Bytownite An 70-90 -do- (-) 2V=83 Fine lamellar exsolution common. Anorthite CaAI2Si20., An 90-100 -do- (-) 2V=77 ~=1.583 Metamorphic and arc volcanic Plagioclase end rocks. member Quartz Si02 Hex. (+) ~=1.553 Low (.009) No color, cleavage, or alter- w=1.544 ation. Often has inclusions; un- dulatory extinction common. Nepheline NaAISi04 Hex. (-) w=1.536- Low (.005) Resembles quartz, but sign 1.549 negative. Told from orthoclase by higher index and uniaxial character; often altered to can- crinite or sodalite (blue). CAl Cordierite Mg.A14Si O,. Orth. (-) 2V ~=1.543 Low (.011) Often confused with quartz but CAl s .... 40-80 biaxial; also may show inclu- sions of sillimanite, yellow pleo- chroic haloes, and characteris- tic twins. Scapolite Marialite -Ab + NaCI Tet. (-) w=1.539- (.002-.039) Alteration of plagioclase, Meionite -An + CaC03 1.596 often zoned mostly contact-metamorphic. int. colors Zoned interference colors char- acteristic. D. Moderate Negative Relief Leucite KAISi2O. Iso. n=1.508 Generally weakly birefracting; inclusions common; trapezo- hedral crystals. Analcite NaAISi2O.·H2O Iso. n=1.489 Trapezohedral form; generally lacks inclusions. Chemical XI. System and Indices Birefringence Mineral Composition Best Cleavage Sign,2V and Relief and Color Distinguishing Characteristics Calcite CaC03 Hex. (1011) (-) 00=1.658 Extreme biref. Differential relief on rotation of E=1.484 (.172) stage; one index below 1.54, other far above; rhombohedral cleavage; often twinned; told from other rhombohedral car- bonates only by index. Cristobalite Si02 Tet. (-) 00=1.487 Low Often looks isotropic; multiple E=1.484 twinning; generally in fibrous balls in cavities; negative U) relief. U) I\) Tridymite Si02 Orth. (+) 2V=35 13=1.469 Low Lath-shaped forms; negative elongation; wedge-shaped grains; slight pinkish color; neg- ative relief. Zeolites Complex hydrous Orth. ( +) or rarely Low-Mod. Generally low index, weak Na,Ca,AI silicate (-) biref.; rarely colored; often fi- brous.