Schiller and Peristerescence
Schiller and peristerescence in feldspar – the mechanism of non-spectral lamellar scattering, named species, and distinction from adularescence, labradorescence, aventurescence, and chatoyancy.
Definition
Schiller (from German "Schiller," shimmer) is a broad, non-spectral, white-to-bluish
internal reflectance sheen produced by lamellar microstructure within feldspar. It arises
from coherent scattering of white light at exsolution lamellae that are too thin to produce
spectral colour separation.
The term covers two closely related varieties:
- Adularescence – the schiller of orthoclase moonstone (albite lamellae in orthoclase host)
- Peristerescence – the schiller of peristerite (albite–oligoclase two-phase plagioclase, An0–An16)
Both produce a single-colour (blue or white) billowy glow, wholly distinct from the polychromatic
colour play of labradorescence.
Mechanism
Physical cause of schiller in feldspar:
Exsolution Lamellae
During slow subsolidus cooling, a single feldspar separates into alternating lamellae of
two compositionally distinct phases. In orthoclase moonstone, albite (Ab) lamellae alternate
with orthoclase (Or) at the Huttenlocher-type intergrowth. In peristerite, albite alternates
with oligoclase.
The periodic layering has spacing on the order of visible wavelengths (approximately 60–500 nm).
Layer Thickness and Colour
- ~60–150 nm layers: Preferential constructive interference in the blue spectral region
→ prized blue schiller - ~200–300 nm layers: Broad white-light scattering → silver-white sheen
- >500 nm layers: No optical effect; stone appears transparent
Thinner lamellae produce bluer schiller; the classic blue moonstone owes its colour to
exceptionally thin albite layers near 100 nm.
Distinction from Labradorescence
Schiller (adularescence/peristerescence) involves non-spectral, single-hue scattering.
Labradorescence, by contrast, arises from the Bøggild intergrowth at a specific plagioclase
composition (An47–An58) and produces spectrally selective, polychromatic colour play –
multiple distinct spectral colours from different zones of the stone.
Miura, Tomisaka & Kato (1975) established the relationship between lamellae thickness and
the selectively reflected wavelengths in labradorite, confirming the structural basis for
the two distinct phenomena.
Named Species
| Species / Variety | Mineral | Layer Structure | Visual Character |
|---|---|---|---|
| Orthoclase moonstone | Orthoclase (KAlSi₃O₈) | Albite exsolution, ~60–150 nm for blue | Blue to white floating billowy glow; moves with viewing angle |
| Sanidine moonstone | Sanidine (high-T K-feldspar) | Similar exsolution; higher formation temperature | White to cream schiller; less common |
| Peristerite moonstone | Albite–oligoclase (An0–An16) | Two-phase plagioclase; broader lamellae | Pale blue to white sheen; body colour often darker than orthoclase moonstone |
| 'Rainbow moonstone' (trade) | Labradorite (An~50) | Bøggild intergrowth – NOT peristerescence | Multicolour spectral flashes; different mechanism – see labradorescence |
Diagnostic Relevance
Gemmological separation of moonstone varieties:
Observation Technique
Examine in reflected light over a dark background while rocking the stone slowly.
A single-colour billowy glow identifies orthoclase or peristerite moonstone (schiller/adularescence).
Multiple spectral colour flashes indicate labradorite (labradorescence).
The schiller should appear to float just below the surface and move smoothly as the stone tilts.
Refractive Index Separation
RI provides a reliable quantitative separation:
- Orthoclase moonstone: RI ~1.518–1.526
- Peristerite: RI ~1.525–1.536
- Labradorite: RI ~1.559–1.573
Any feldspar moonstone with RI above approximately 1.55 is likely labradorite,
not true orthoclase or peristerite moonstone.
Birefringence also assists: orthoclase δ ~0.005–0.008; labradorite δ ~0.009–0.011.
Krzemnicki 2004 Note
Krzemnicki (2004) reported red and green labradorite from the Democratic Republic of Congo
showing spectral colour play, demonstrating that labradorescence colours vary with composition
and confirming that the distinction from schiller rests on mechanism and RI, not colour alone.
Sources
Miura, Tomisaka & Kato (1975)
Labradorescence and the ideal behavior of thicknesses of alternate lamellae in the Bøggild intergrowth. Mineralogical Journal 7(6), 526–541. DOI: 10.2465/minerj1953.7.526. [VERIFIED]
Krzemnicki (2004)
Red and green labradorite feldspar from Congo. The Journal of Gemmology 29(1), 15–23. DOI: 10.15506/jog.2004.29.1.15. [VERIFIED]
Read (2008)
Gemmology (3rd ed.). Butterworth-Heinemann/Routledge. DOI: 10.4324/9780080507224. [APPROXIMATE] – Chapter 'Colour, Lustre and Sheen'; layer-thickness figures and RI values.