Accessory Plates

Accessory plates are anisotropic optical elements that slide into a slot above the objective lens of the petrographic microscope. Their primary function is to determine which of the two vibration directions in a mineral grain carries the fast ray and which carries the slow ray. This information is needed to determine optic sign, sign of elongation, and in some cases to distinguish between orders of interference color. The common accessory plates are the gypsum plate and the mica plate. ^[1]

Each plate is a carefully ground piece of anisotropic material - quartz, muscovite, gypsum, or anisotropic plastic - mounted in a metal or plastic holder. The slow ray vibration direction of the plate runs across the width of the holder; the fast ray vibration direction runs along the length. In most microscopes the plates slide in NW-SE so that the plate’s slow ray vibrates NE-SW and the fast ray vibrates NW-SE. ^[1]

The Gypsum Plate

The gypsum plate is also called the full-wave plate, one-wavelength plate, quartz-sensitive tint plate, or first-order red plate. Common markings on the holder include Gips, Gyps, Rot I, 1λ, Δ = 550 nm, or Δ = 537 nm. Depending on the manufacturer, it produces 537 or 550 nm of retardation. This retardation places the baseline color - with no mineral on the stage - at the transition from first to second order, which appears as a distinctive magenta color. ^[1]

The gypsum plate is also used to distinguish between first-order white and high-order white, and to identify second-order versus higher-order colors - adding or subtracting 550 nm shifts the color up or down by one full order, which is often enough to resolve the ambiguity.

The Mica Plate

The mica plate is also called the quarter-wave plate or quarter-wavelength plate, and may be marked Mica, Glimmer, ¼λ, or Δ = 147 nm. It produces around 150 nm of retardation, which corresponds to a first-order white interference color. ^[1]

How Retardations Add or Subtract

The key principle governing accessory plate use is that the total retardation (Δ_T) depends on the orientation of the mineral relative to the plate: ^[1]

When the mineral’s slow ray is parallel to the plate’s slow ray (and fast is parallel to fast), retardations add:

The interference color increases to a higher order. If the mineral produces 250 nm (first-order white) and the gypsum plate (Δ_A = 550 nm) is inserted with slow on slow, the total retardation is 800 nm, and the color rises to second-order yellow. The mnemonic: retardations add = slow on slow. ^[1]

When the mineral’s fast ray is parallel to the plate’s slow ray (and slow is parallel to fast), retardations subtract:

The interference color decreases. If the mineral produces 250 nm and the gypsum plate (Δ_A = 550 nm) is inserted with fast on slow, the total retardation is |250 − 550| = 300 nm, producing a first-order white with a yellow cast. The mnemonic: retardations subtract = slow on fast. ^[1]

Procedure for Identifying Fast and Slow Rays

The standard procedure to identify which vibration direction in a grain carries the fast or slow ray is: ^[1]

1. Rotate the stage to place the grain at extinction - this aligns its two vibration directions (p and q) N-S and E-W. Mentally label the N-S direction as “p.” ^[1]
2. Rotate the stage 45° clockwise, moving p into the NE-SW position. Record the interference color and read the retardation Δ_M from the chart. ^[1]
3. Insert the accessory plate (its slow ray is NE-SW). Observe the new interference color. If retardations add (color increases) - p is the slow ray. If retardations subtract (color decreases) - p is the fast ray. ^[1]

Using the gypsum plate in the worked example where Δ_M = 400 nm: addition yields second-order yellow-orange (Δ_T = 950 nm); subtraction yields first-order gray (Δ_T = 150 nm). The two outcomes are completely unambiguous.

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