guide to thin section microscopy - Mineralogical Society of America

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Guide to Thin Section Microscopy Extinction Sign of elongation l Crystals with columnar to acicular or platy to flaky habits show elongate sections if cut parallel to their long morphological axis or perpendicular to a dominantly developed basal plane. If such elongate sections are rotated into a diagonal position (i.e., long axis l parallel to NE-SW) they show under crossed polarizers and compensator-in (first-order red plate or λ/4 plate) either addition (Γ Comp + Γ Min = Γ total ) or subtraction (Γ Comp - Γ Min = Γ total ), depending on their optic sign (Fig. 4-45). Addition is observed if the wave with the larger refractive index (n z ') vibrates parallel or at a small angle to the long dimension of the crystal section: l (+) = positive sign of elongation or "length-slow". Examples: acicular-columnar crystals– sillimanite, gedrite, anthophyllite; platy crystals– gehlenite, micas, antigorite, pennine. Subtraction is observed if the wave with the smaller refractive index (n x ') vibrates parallel or at a small angle to the long dimension of the crystal section: l (–) = negative sign of elongation or "length-fast". Examples: acicular-columnar crystals– tourmaline, apatite; platy crystals– åkermanite, brucite, clinochlore. For optically uniaxial mineral species with acicular-columnar habit, the sign of elongation corresponds to the optic sign. For platy uniaxial minerals, the two signs are opposite. Elongate sections in which n y is parallel (or at a small angle) to the morphological long axis of the crystal or perpendicular to the planar dimension of platy crystals show a variable sign of elongation, l(+) or l(–), depending on the exact orientation of the crystal in thin section. Examples of such elongate crystals– wollastonite, epidote. Raith, Raase & Reinhardt – February 2012 Figure 4-45 A. Platy muscovite showing positive sign of elongation. In muscovite and firstorder red plate, fast (n x ) and slow (n y ↔n z ) waves vibrate in identical orientation. Therefore, the retardation adds up and increased interference colours result (Γ res = Γ Red1 + Γ Ms = 1231 nm; second-order blue-green). 109
Guide to Thin Section Microscopy Extinction Figure 4-45 B. Prismatic apatite showing negative sign of elongation. In apatite, the fast wave vibrates parallel to the slow wave of the first-order red plate, and the slow wave in apatite parallel to the fast wave of the first-order red plate. Hence, retardation is reduced and subtraction results (Γ res = Γ Red1 + Γ Ap = 476 nm; first-order orange-yellow). If the extinction direction deviates significantly from the long axis of the crystal section, it is convention to use the extinction position that forms an angle smaller than 45˚ with the long axis, and to rotate it into the diagonal position before the first-order red plate or lambda/4 plate is inserted for determining the sign of elongation. However, if extinction angles are close to 40-50˚, the use of l(+) and l(-) becomes impractical. Determination of the optic sign of optically uniaxial minerals Optically uniaxial minerals that are elongate in c direction (e.g., quartz, apatite, tourmaline) and are present in thin section in a cut that is roughly parallel to that long axis, will have the E-wave (n e ) vibrating parallel to c and the O-wave (n o ) orthogonal to c. If the habit of optically uniaxial minerals is platy (e.g., melilite), the same applies, but the O-wave now vibrates parallel to the elongate section. Raith, Raase & Reinhardt – February 2012 For both crystal habits, the optic sign can be determined easily by compensation, that is, whether n e > n o (= optically uniaxial positive) or n e < n o (= optically uniaxial negative) (Fig. 4-46). The long axis of the crystal section is rotated into diagonal position (NE-SW orientation), and the first-order red plate or lambda/4 plate is inserted. For crystals elongate in c, n e is now parallel to n zComp and n o is parallel to n xComp . For platy minerals, the relation is reverse as c is perpendicular to the long dimension of the crystal sections. The retardation between the waves which are created in the optically uniaxial mineral (Γ Min ) and in the compensator plate will be increased through addition or decreased through subtraction, depending on optical sign. Correspondingly, the crystal section shows higher or lower interference colours if the compensator is inserted. 110
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