Crystal system: Orthorhombic bipyramidal.          

Color: Colorless, white, gray, pale yellow, pale green, yellowish brown, pale red. May be zoned.     

Habit: Strontianite crystals are typically long to short prismatic, often acicular and fibrous, reaching 8 cm in length. They can be arranged parallel or radially, forming fans to perfectly spherical masses. They are frequently pseudohexagonal and {110} and {010} are horizontally striated.       

Cleavage: {110} almost perfect, {021} poor, {010} in traces. Striations on {110} and {010}.

Tenacity: Brittle.        

Twinning: Twins along the {110} plane are very common, usually as contact twins, and can be interpenetrating, repeating (“trilling”, “fourlings”) or polysynthetic, with twin lamellae.       

Fracture: Irregular, subconchoidal.       

Mohs Hardness: 3.5

Parting: No.         

Streak: White.         

Lustre: Vitreous. Resinous in a recent fracture.          

Diaphaneity: Transparent.           

Density (g/cm³): 3.74 – 3.78

2. Geology and Deposits

Strontianite is found primarily as a low-temperature mineral (probably forming at ~100ºC) forming fibrous masses in veins hosted in limestones, marls, chalk, and clays, in the form of geodes and as concretions. Its crystallization in cavities and fractures suggests that crystallization occurs at low pressures, probably at the hydrostatic pressure of groundwater.

It rarely occurs as a gangue mineral in hydrothermal veins with sulfides, such as in the type locality (Strontian, Scotland), where it occurs in veins in a gneiss. It is known from carbonatites and can occur in igneous rocks, perhaps by alteration of celestine. It is a secondary mineral in kimberlites.

It alters forming celestine a process that can generate pseudomorphs.

3. Mineral Associations

4. Transmitted Light Microscopy

Refraction indices: nα: 1.516 – 1.520 nβ: 1.663 – 1.667 nγ: 1.667 – 1.668.  Increase with the progressive replacement of Sr by Ca and Ba.

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, without pleochroism.   

Relief: The relief varies from low to moderate to high every 90º of rotation of the stage in crystals with well-defined cleavage. This phenomenon has been nicknamed “relief pleochroism” or “chagrin change” and is typical of many carbonates (calcite, dolomite, aragonite, siderite, rhodochrosite, magnesite, etc.).

When microcrystalline, these carbonates do not exhibit “relief pleochroism” or it is very difficult to perceive.

Cleavage: {110} almost perfect, {021} poor, {010} in traces. The directions of the {110} cleavage intersect at an angle of 64º.           

Habits: It generally occurs as granular masses or aggregates of columnar, acicular to fibrous (elongated along the “z” axis) crystals. Columnar crystals are often pseudohexagonal, as is the case with aragonite.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.165: high-order colors, difficult to determine, pearly, as in all carbonates. 

Extinction: Parallel in longitudinal sections; symmetrical in basal sections. 

Elongation sign: ES(-) in longitudinal sections. 

Twins: Common to the {110} plane; they can be simple, polysynthetic, cyclic, or lamellar. 

Zoning: macroscopically it may be zoned.  

CONVERGENT LIGHT

Character: B(-)   

2V angle: 7º – 10º 

Alterations: Strontianite sometimes alters to celestine.          

May be confused with: Other orthorhombic carbonates, like aragonite.

The two good-quality cleavages of strontianite are diagnostic in relation to aragonite. The flame test for strontianite, after being moistened with HCl, results in an intense red flame, which is diagnostic.

Aragonite has almost the same indices and cleavage, but its density is much lower and it has only one good cleavage direction.
Witherite has higher indices and a larger 2V angle of 16º.
Trigonal (rhombohedral) carbonates such as calcite, dolomite, siderite, rhodochrosite, and magnesite are uniaxial and exhibit rhombohedral cleavage.