Optical calcite, known as Iceland spar, is a high-purity, transparent variety of calcite that exhibits strong double refraction. When a single line or image is viewed through a clean rhombohedral cleavage piece, it appears doubled, making it a classic specimen for physical optics study. Collectors should look for large, clear, and perfectly shaped rhombohedrons free of internal clouding or heavy twinning.
Is this optical calcite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch optical calcite with a known reference. Optical Calcite sits at Mohs 3, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: colorless, white.
- 5Look at form & habitCrystal system: trigonal. Typical habit: rhombohedral crystals.
Often confused with
Optical Calcite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Optical Calcite is noticeably harder (Mohs 3 vs. 1.5-2); luster reads vitreous on Optical Calcite and subvitreous, pearly on {010} cleavages, silky if fibrous on Gypsum.
How to tell apart: Quartz is the harder of the two (Mohs 7 vs. 3).
Often found alongside optical calcite
Minerals reported to co-occur with optical calcite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaCO₃
- Mohs hardness
- 3
- Density
- 2.71 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Trigonal
- Crystal habit
- Rhombohedral crystals
- Cleavage
- Perfect rhombohedral in 3 directions
- Fluorescence
- Often fluorescent under UV
- Rarity
- Common
- Uses
- Collector, Scientific, Optical
- Host rock
- sedimentary environments and hydrothermal veins
Where rockhounds find optical calcite
Classic worldwide localities
- Helgustadir, Iceland
- Chihuahua, Mexico
- Sonora, Mexico
- Tennessee, USA
Field-hunting tip
Where it forms: sedimentary environments and hydrothermal veins. If you start seeing quartz, fluorite, dolomite in float, you are in the right ground. Typical habit: rhombohedral crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify optical calcite?+
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Find optical calcite on the map
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