STELLERITE
Stellerite – Ca4(Si28Al8)O72.28H2O – is a tectosilicate belonging to the Zeolite Group and the Stilbite Subgroup. It is not a common zeolite, but its extreme resemblance to stilbite-(Ca) and other members of the Stilbite Subgroup makes it very difficult to identify. It is not an ore mineral; at most, it is marketed as a collector’s specimen.
All macroscopic and microscopic characteristics of stellerite are similar to those of stilbite-(Ca). They also resemble those of other members of the Stilbite Subgroup. It is impossible to distinguish stellerite from barrerite. Colorless tabular crystals macroscopically resemble prehnite. Consequently, the precise identification of stellerite requires other analytical techniques.
Stilbite-(Ca) can be recognized by its Na content (1.08 wt% Na₂O), which is absent in stellerite. Furthermore, X-ray diffraction analysis is required to identify its orthorhombic symmetry. It may contain Mn, Mg, Fe, Sr, Ba, Na, and K as impurities.
1. Characteristics
Crystal system: Orthorhombic dipyramidal.
Color: Colorless to white. Hematite inclusions produce orange, pink, salmon, and honey-colored hues. Celadonite inclusions produce green hues.
Habit: Stellerite exhibits a range of habits, such as (1) individual rectangular crystals, (2) individual spearhead-shaped crystals, (3) radial aggregates, (4) bow-tie aggregates, (5) sheaf-like aggregates, and (6) spherical aggregates – the latter reaching up to 12 cm in diameter. All these habits are also typical of stilbite-(Ca).
Cleavage: {010} perfect.
Tenacity: Brittle.
Twinning: No information available.
Fracture: Irregular.
Mohs Hardness: 4.5
Parting: No.
Streak: White.
Lustre: Pearly.
Diaphaneity: Transparent.
Density (g/cm³): 2.13
2. Geology and Deposits
Stellerite occurs in many igneous and metamorphic rocks altered by hydrothermal solutions – such as hornfels, granites, granitic gneisses, pegmatites, amphibolites, and basalts – usually within cavities and fractures.
Well-developed crystals can be found in cavities (vesicles) within basaltic rocks.
3. Mineral Associations
It is associated with some typical secondary minerals in volcanic rocks, such as quartz and calcite.
It occurs with many other zeolites, such as heulandite, laumontite, chabazite, epistilbite, and gmelinite.
Also with clay minerals (celadonite), tridymite, datolite, prehnite, and fluorite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.485 nβ: 1.486 nγ: 1.498
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: Moderate.
Cleavage: {010} perfect, like many other zeolites.
Habits: Individual crystals are rectangular or spearhead-shaped. Aggregates in bundles, radial forms, bow-tie shapes, or spheres (very common!). The same habits are characteristic of stilbite-(Ca).
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.013, corresponding to first-order colors ranging from dark gray to straw-yellow, not reaching orange.
Extinction: Paralell.
Elongation sign: No information available.
Twins: No information available.
Zoning: No.
CONVERGENT LIGHT
Character: B(-)
2V angle: 47º
Alterations: No information available.
May be confused with: Stilbite and with other zeolites. It is not possible to distinguish stilbite from stellerite either macroscopically or microscopically. The spherical aggregate habit, mentioned in some literature, is not exclusive to stellerite but also occurs with stilbite.
5. Reflected Light Microscopy
Reflected-light microscopy is evidently not the recommended analytical method for identifying stellerite. However, preparing a polished section is important for identifying opaque minerals associated with stellerite, such as native copper.
Sample preparation: xx
PLANE POLARIZED LIGHT – PPL
Reflection color: xx
Pleochroism: xx
Reflectivity: xx
Bireflectance: xx
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: xx
Internal reflections: xx
May be confused with: xx
General Characteristics: xx