Tenebrescence
Tenebrescence – reversible photochromic colour change in hackmanite driven by S₂⁻ radical colour centres, mechanism, named species, and diagnostic distinction from the alexandrite effect and irradiation-induced colour changes.
Definition
Tenebrescence is the reversible photochromic colour change exhibited by hackmanite (a
sulfur-bearing variety of sodalite) in which the stone is colourless to pale grey under
incandescent or ambient light, turns violet to deep purple upon brief UV or short-wavelength
visible light exposure, and then returns to colourless when placed in warm light or kept
in darkness.
The effect is driven by the photochemical creation and annihilation of S₂⁻ radical colour
centres within the sodalite framework. It is fully reversible – the same stone can be
darkened and bleached thousands of times without degradation.
Mechanism
Physical and chemical basis of tenebrescence:
Mineral Chemistry
Hackmanite is the chromatic variety of sodalite – Na₈Al₆Si₆O₂₄₂ – in which
some Cl⁻ is replaced by the S₂⁻ disulfide radical anion. The sodalite framework
provides cage-like sites (β-cages) that host the color-active sulfur species.
Kondo & Beaton (2009) described hackmanite from Myanmar and Afghanistan, documenting
the tenebrescent colour change: hackmanite "turns pink to purple when exposed to
sunlight or UV radiation and returns to its original colour when placed in incandescent
light or kept in the dark."
S₂⁻ Colour Centre
The S₂⁻ radical anion absorbs visible light around 530–560 nm (yellow-green), giving
the stone its purple/violet appearance – the complementary colour to yellow-green.
In the colourless state, sulfur is present in a configuration that does not absorb
significantly in the visible range. UV exposure converts some sulfur species to S₂⁻,
creating the colour centre. Visible light or thermal energy reverses the conversion.
Goettlicher et al. (2013) used sulfur K X-ray absorption near-edge structure (XANES)
spectroscopy to confirm the role of the polysulfide radical in the photochromic
mechanism. Radomskaya et al. (2021) further confirmed the crystal chemistry and
luminescence behaviour of natural hackmanite from Russia.
Associated Phosphorescence
After UV exposure, hackmanite shows a yellow-orange persistent afterglow as the excited
S₂⁻ centres partially relax via spin-forbidden transitions. This phosphorescence is an
associated feature of the tenebrescence cycle – both arise from the same colour centre.
See also Fluorescence and Phosphorescence for further detail.
Reversibility
The colour change is fully reversible over thousands of cycles under normal conditions.
This distinguishes tenebrescence from:
- Irradiation-induced colour changes (e.g., maxixe beryl, blue topaz) – these are
permanent or fade only slowly under prolonged light exposure - Heat treatment effects – permanent structural changes
Named Species
| Mineral | Occurrence | Tenebrescence Character |
|---|---|---|
| Hackmanite (sodalite var.) | Myanmar (Sagaing Region), Afghanistan (Badakhshan), Canada (Ontario), Russia (Inagli Massif), Greenland | Colourless/grey → violet/pink in UV; reverses under incandescent light or warming; primary tenebrescent gem species |
| Tugtupite (beryllosodalite) | Greenland | Related photochromic effect; deep red → brighter red under UV; same S₂⁻ mechanism variant [PARTIALLY_SUPPORTED – no separate tugtupite primary paper retrieved] |
| Some synthetic sodalites | Laboratory | Strong engineered tenebrescence; used in persistent luminescence research |
Distinguishing Tenebrescence from the Alexandrite Effect
| Feature | Tenebrescence | Alexandrite Effect |
|---|---|---|
| Reversibility | Fully reversible by light or mild heat | Reversible in the same sense – colour shifts immediately with light source change |
| Cause | Photochemical creation/destruction of S₂⁻ colour centre | Fixed Cr³⁺ (or V³⁺) absorption window; colour depends on illuminant spectral power |
| UV activation required? | Yes – UV or short-wavelength visible light triggers the colour state | No – colour change is passive, driven by visible illuminant composition only |
| Mineral group | Sodalite group (tectosilicate) | Chrysoberyl (alexandrite), garnet, sapphire, diaspore |
| State persistence | Coloured state persists in darkness until reversed by visible light or heat | Colour reverts immediately when illuminant changes; no residual state |
| Test lamp needed? | Yes – SWUV lamp to induce colour | No – observe under daylight and incandescent only |
How to Test for Tenebrescence
Standard diagnostic procedure:
Test Procedure
- Observe the stone under incandescent or ambient light – note colourless to pale grey
body colour. - Illuminate with shortwave UV (SWUV, 254 nm) for 10–30 seconds in subdued conditions.
- Remove UV source and observe immediately under incandescent light – dramatic violet
to purple colour should be visible. - Leave in ordinary light or warm gently – colour fades to near-colourless within
seconds to minutes.
The effect is instantaneous and dramatic in fine material. Even in weak hackmanite,
some visible shift should occur.
Distinguishing Hackmanite from Sodalite
Hackmanite and non-sulfur sodalite are mineralogically the same species differing
only in sulfur content. Sodalite does not respond to UV in the tenebrescent manner.
If UV exposure produces no colour change, the stone is sodalite (or another mineral
entirely). Tenebrescence is diagnostic for hackmanite within the sodalite group.
Diagnostic Relevance
Tenebrescence in gemmological practice:
Tenebrescence is diagnostic for hackmanite; no other commonly encountered gemstone
shows fully reversible UV-triggered colour change of this character.Distinguish from irradiation-induced colour changes (e.g., maxixe beryl, blue topaz,
treated pink diamonds): irradiation-induced colours do not reverse under ordinary
daylight exposure – they are permanent or fade only under prolonged high-intensity light
or high temperature.Hackmanite also shows yellow-orange phosphorescence after UV excitation – a feature
confirmed by both Kondo & Beaton (2009) and Radomskaya et al. (2021).
Sources
Kondo & Beaton (2009)
Hackmanite/Sodalite from Myanmar and Afghanistan. Gems & Gemology 45(1), 38–43. DOI: 10.5741/gems.45.1.38. [VERIFIED] – Primary gemmological source for tenebrescence in hackmanite; documents colour change, phosphorescence, and locality characteristics.
Goettlicher et al. (2013)
Sulfur K X-ray absorption near edge structure spectroscopy on the photochrome sodalite variety hackmanite. Zeitschrift für Kristallographie – Crystalline Materials 228(3), 157–171. DOI: 10.1524/zkri.2013.1587. [VERIFIED] – Confirms S₂⁻ radical as photochromic agent via XANES.
Radomskaya et al. (2021)
Sulfur-Bearing Sodalite, Hackmanite, in Alkaline Pegmatites of the Inagli Massif. Geology of Ore Deposits 63(7). DOI: 10.1134/s1075701521070060. [VERIFIED] – Crystal chemistry, photochromism, and luminescence of Russian hackmanite.