CELESTINE
Celestine – SrSO4 – is a relatively rare sulfate, yet it is the most common strontium (Sr) mineral and the primary ore mineral for strontium.
It is classified within the Barite Group, with which it forms a series (a complete solid solution). It forms a limited solid solution with anhydrite. It may contain Ca and Ba, and three varieties exist (Ba-bearing, Ba-rich, and Ca-bearing). It is slightly soluble in water.
The bluish celestine specimens found in the gemstone trade originate from Madagascar. The world’s largest geode is a celestine-crystal geode, measuring 10.7 meters in diameter and over 2 meters in height. It is located at the Heinemann Winery in the village of Put-in-Bay, Ohio (USA). The geode’s walls feature celestine crystals up to 46 cm in size, weighing around 135 kg each.
1. Characteristics
Crystal system: Orthorhombic dipyramidal.
Color: Colorless, white, pale blue, pink, pale green, pale brown, black.
Habit: Well-formed celestine crystals are common; they are typically tabular—ranging from thick to thin—but can also be lamellar or elongated with equidimensional cross-sections, reaching lengths of up to 45 cm.
Cleavage: {001} perfect,, two {210} good and one {010} poor. Striations on [001].
Tenacity: Brittle.
Twinning: On {210} and {101}, very rare.
Fracture: Irregular.
Mohs Hardness: 3 – 3.5
Parting: By sliding, via twinning and translation.
Streak: White.
Lustre: Vitreous, pearly on cleavages.
Diaphaneity: Transparent.
Density (g/cm³): 3.95 – 3.97
2. Geology and Deposits
Celestine occurs primarily in sedimentary rocks, originating from sedimentary diagenetic processes and generally forming small deposits. It can form concretions (nodules).
It typically fills fractures or cavities (including caves). It forms in association with marine carbonate sediments (limestones and dolostones), as burial-related dissolution is a primary mechanism for celestine formation. It may precipitate from Sr-bearing groundwater or, more commonly, through the interaction of gypsum or anhydrite with Sr-rich waters.
It can occur in hydrothermal veins and mafic volcanic rocks.
It sometimes occurs as an alteration product of strontianite.
3. Mineral Associations
In cavities within carbonate rocks, it is associated with carbonates (calcite, dolomite, strontianite), other sulfates (gypsum, anhydrite, barite), and sulfur.
In evaporite rocks it occurs with gypsum, anhydrite, and halite.
In mafic volcaniclastic rocks, it occurs with analcime, natrolite, hydroxyapophyllite, and celadonite.
Also with quartz (including the amethyst variety), fluorite, and pyrite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.619 – 1.622 nβ: 1.622 – 1.624 nγ: 1.630 – 1.632
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless; may exhibit pale colors. If blue, it shows weak pleochroism ranging from indigo blue to lavender blue and violet.
Relief: Moderate.
Cleavage: one perfect {001} cleavage, two good {210} cleavages intersecting at an angle of 75°, and one poor {010} cleavage.
Habits: Generally tabular crystals, ranging from thick to thin. Also elongated, with equidimensional cross-sections. It may occur as massive, fibrous, or granular aggregates, or in cleavable masses.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.011: first-order colors generally ranging from gray to white, sometimes reaching a straw-yellow hue.
Extinction:P arallel to the cleavage in sections parallel to the “z” axis,
symmetrical with respect to the cleavages in the (001) sections.
Elongation sign: ES(+) or ES(-), depending on the orientation of the section, it is not diagnostic.
Twins: On {210} e {101}, very rare.
Zoning: No.
CONVERGENT LIGHT
Character: B(+)
2V angle: 50 – 51º
Alterations: Celestine may alter to strontianite.
May be confused with: Other minerals of the paragenesis.
Barite is very similar but has higher refractive indices, slightly higher birefringence, and a smaller 2V angle.
Gypsum exhibits oblique extinction.
Anhydrite exhibits three mutually perpendicular cleavages and very strong, colorful, interference colors.
5. Reflected Light Microscopy
Reflected-light microscopy is evidently not the recommended analytical method for identifying celestine. However, preparing a polished section is important for identifying the opaque minerals associated with the celestine.
Sample preparation: Polishing celestine is simple and yields high-quality results.
PLANE POLARIZED LIGHT – PPL
Reflection color: Dark gray.
Pleochroism: No.
Reflectivity: Very low (<<10%).
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Anisotropy was not perceived.
Internal reflections: Widespread, colorless. Even in bluish celestine, this hue does not appear among the internal reflections.
May be confused with: Other minerals of the paragenesis. It is not possible to identify celestine under reflected light.
General Characteristics:
Polishing scratches occur due to low hardness.
Cleavage may be visible.
Polishing pits may occur.
Substitutions may occur, with celestine being replaced by calcite, quartz, witherite, barite, or native sulfur. Pseudomorphs are possible.