Aragonite is a polymorph of calcite. In hand sample, calcite can be distinguished by its rhombohedral cleavage and, if the sample is translucent enough, its double refraction. In thin section, calcite can be distinguished by its parallel extinction.
Physical Properties |
|
| Chemical formula | CaCO3 |
| Class | Carbonate |
| Crystal system | Hexagonal (rhombohedral) |
| Habit | Rhombohedral Hexagonal prisms Scalenohedra Tabular Others |
| Color | Colorless to white Gray Red Blue Yellow/orange Green Brown to black when impure |
| Hardness | 3 |
| Specific gravity | 2.71 |
| Cleavage | Perfect rhombohedral {101} |
| Fracture | Conchoidal |
| Luster | Vitreous |
| Transparency | Transparent to translucent |
| Streak | White |
Optical Properties |
|
| PPL | Colorless, but may exhibit pale pastels High relief Visible polysynthetic “calcite” twinning in coarse grains |
| δ | 0.172 |
| XPL | Extremely high birefringence; colors may look pastel, washed out, or almost neon High relief Polysynthetic “calcite” twinning in coarse grains; fine grains appear as a sort of swirl of color |
| Twinning | Polysynthetic; “calcite” twinning |
| Extinction | Parallel |
Special properties |
Reacts with HCl (fizzes) Double refraction |
| after Perkins, 382 |
Calcite in Hand Sample
Calcite
Four small hand samples
Several habits in one hand sample
Euhedral calcite crystals in a cobble of amygdaloidal basalt
Red and white calcite with green diopside
Calcite veins crosscutting faulted sedimentary strata
Calcite filling the gaps between segments of a stretched, broken belemnite fossil
Scanning electron micrograph of calcite
Another scanning electron micrograph of calcite
Calcite displaying double refraction.
The following two scanning electron micrographs come courtesy of Nik Deems.
Brief tangent: classroom chalk
If you’ve taken an intro Geology class (and I bet you have), you’ve probably heard that chalk is made up of the calcite tests of algae called coccolithophores. Well, sometimes it’s true. Behold this scanning electron micrograph of classroom chalk, and click the blue circle to see an intact coccolithosphere.
Some manufacturers make their chalk from a blend of dolomite and gypsum, seen in this scanning electron micrograph. Others make it of straight gypsum, seen in this scanning electron micrograph.
Calcite in Thin Section
Thin Section GigaPans
Calcite in marble, plane polars
Calcite in marble, crossed polars
Sandstone with calcite cement, plane polars
Sandstone with calcite cement, crossed polars
Huge euhedral calcite, plane polars
Huge euhedral calcite, crossed polars
Calcite, Plane Polars
Calcite displaying pleochroism in plane polars.Magnification: 40xRock: MarbleTo view this mineral in XPL, click here: https://youtu.be/HJWvDJUud1gTo explore …
Calcite, Crossed Polars
Calcite displaying its characteristic twinning.Magnification: 40xRock: MarbleTo view this grain in plane polars, click here: https://youtu.be/oVtYS9jWXFwTo e…
Sandstone with Calcite Cement, Plane Polars
Sandstone with calcite cement in plane polarized light.Magnification: 40xTo view these grains in crossed polars, click here: https://youtu.be/KXxt3mIFMcwTo e…
Sandstone with Calcite Cement, Crossed Polars
Sandstone with calcite cement in cross polarized light.Magnification: 40xTo view these grains in plane polars, click here: https://youtu.be/a0yzVfarYlATo exp…
Huge Euhedral Calcite, Plane Polars
Magnification: 40xRock: Precambrian-aged basalt from Castle Danger, MNClick here to watch this calcite rotate in crossed polars: https://youtu.be/lQ6GEnB2LfI…
Huge Euhedral Calcite, Crossed Polars
Magnification: 40xRock: Precambrian-aged basalt from Castle Danger, MNClick here to watch this calcite rotate in plane polars: https://youtu.be/t35RDEfQR8QCl…
Fine-Grained Calcite Vein, Plane Polars
Magnification: 100xRock: MetagranodioriteClick here to watch this vein rotate in crossed polars: https://youtu.be/zZXhBAXMgJg
Fine-Grained Calcite Vein, Crossed Polars
Magnification: 100xRock: MetagranodioriteClick here to watch this vein rotate in plane polars: https://youtu.be/X3Wjuq9dwyI