Katlang Pink Topaz – Pakistan
Chromium-coloured pink to champagne topaz from Katlang, Mardan district; Cr³⁺ colouring diagnostic, calcite-vein hosted, rare among topaz globally.
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Introduction
Pink topaz from the Katlang deposit in the Mardan district of Khyber Pakhtunkhwa
is exceptional among topaz varieties worldwide: its pink to pinkish-orange colour
is caused by trace chromium (Cr³⁺) – an extremely rare colouring mechanism for
topaz. Most pink topaz on the market is either irradiation-induced pink (colourless
topaz irradiated and heat-treated) or Mn-coloured. Katlang material is genuinely
Cr-coloured, making it diagnostically distinct and scientifically noteworthy.
Geological Setting
Katlang mine geology:
- Host: Narrow calcite veins in recrystallised limestone (marble); the topaz
crystallises within vein cavities - Location: Katlang, Mardan district, Khyber Pakhtunkhwa; the deposit was
documented by Spengler (1985) and Gübelin, Graziani and Kazmi (1986) - Crystal habit: Well-formed prismatic crystals; sizes typically small (up to
3 cm reported); associated colourless, reddish-brown, and tan topaz also recovered - Topaz species: Al₂SiO₄(F,OH)₂; the F/OH ratio at Katlang influences specific
physical constants (RI, optic axial angle, unit cell dimensions)
Colour and Chromophore
What makes Katlang topaz unique:
Chromium Colouring
- Confirmed chromophore: Gübelin et al. (1986) confirmed "the color is due
to trace elements – principally chromium (Cr³⁺)" - This Cr³⁺ substitution produces pink to pinkish-orange to champagne colour
through absorption in the yellow-green region (~550 nm)
Colour Range
- Pink: Most prized; pure pink with slight orange modifier
- Pinkish-orange (peach): Transition colour between pink and champagne
- Champagne: Golden-yellow with slight pinkish cast
- Colourless, reddish-brown, and tan material is also recovered from the
same vein system but lacks the Cr colouring
Distinguishing Katlang from Other Pink Topaz
| Property | Katlang (Cr-coloured) | Irradiated Pink Topaz | Mn-Coloured Pink Topaz |
|---|---|---|---|
| Chromophore | Cr³⁺ | Colour centres (radiation) | Mn³⁺ |
| Absorption spectrum | Cr bands (~550 nm, ~670 nm) | Different band pattern | Mn band (~490 nm) |
| Chelsea filter | Reddish (Cr response) | Inert or faint | May show weak response |
| Stability | Stable | May fade in light/heat | Generally stable |
| Origin | Pakistan only (major source) | Any country's colourless topaz | Various sources |
| Treatment | Natural colour | Treated | Natural |
Chromium in Topaz
Physical Properties
Properties of Katlang topaz:
- Species formula: Al₂SiO₄(F,OH)₂; orthorhombic; biaxial positive
- RI: 1.619–1.627 (α), 1.620–1.628 (β), 1.627–1.636 (γ); birefringence ~0.008–0.010
- SG: ~3.49–3.57
- Hardness: 8 (Mohs)
- Note: Gübelin et al. noted the F/OH ratio variation at Katlang affects
measured RI and optic axial angle compared to typical high-F topaz values