Apophyllite Identification Guide

What Apophyllite Looks Like

Apophyllite is a group of hydrous calcium (and potassium/sodium) phyllosilicate minerals that form striking, well-developed crystals in cavities of volcanic rock. Crystals are commonly colorless or white, but the prized green apophyllite (from India) ranges to mint and apple-green; pink, yellow, and gray varieties also occur. Crystals are typically cubic-looking prisms capped by a steep pyramid (a "tipped cube" look) or tabular plates, often glassy and water-clear.

• Color: colorless, white, green, pink, pale yellow
• Luster: vitreous on prism faces but distinctly pearly to nacreous on the basal (flat top/bottom) face
• Transparency: transparent to translucent
• Crystal habit: pseudo-cubic prisms with pyramidal terminations, or flat tabular crystals; often in clustered druses

Step-by-Step Field ID Checklist

1. Examine crystal shape: look for square-cross-section prisms topped by a four-faced pyramid, giving a capped-cube outline.
2. Check the two lusters: a glassy side face but a pearly, slightly milky basal face is a strong apophyllite signature.
3. Test the cleavage: apophyllite has one perfect basal cleavage; thin flakes peel cleanly from the flat face.
4. Note the host and company: crystals sitting in basalt cavities with zeolites (stilbite, scolecite) and white prehnite or calcite strongly suggest apophyllite.
5. Gauge hardness: it is fairly soft; a steel knife scratches it.

Key Diagnostic Tests

• Mohs hardness: about 4.5 to 5; scratched by a knife, will not scratch glass
• Streak: white
• Cleavage: perfect basal (one direction), giving micaceous-looking flat flakes (but flakes are brittle, not flexible like mica)
• Fracture: uneven
• Specific gravity: about 2.3 to 2.4, light
• Acid: essentially no fizz in dilute cold HCl (unlike calcite, which fizzes briskly), though it gelatinizes/decomposes in stronger acid
• Heat test (lab): the name comes from its tendency to flake/exfoliate when heated under a blowpipe

Common Look-Alikes and How to Tell Them Apart

• Calcite: also colorless and in basalt cavities, but calcite fizzes vigorously in dilute acid and has rhombohedral cleavage in three directions; apophyllite barely reacts and has one basal cleavage.
• Quartz: much harder (7, scratches glass), with hexagonal prisms and no basal pearly cleavage; apophyllite is soft.
• Zeolites (stilbite, heulandite): often associated; distinguished by habit (sheaf-like stilbite) and different cleavage; apophyllite's capped-cube form and pearly basal face are distinctive.
• Fluorite: forms true cubes and octahedral cleavage, similar hardness (4), but fluorite has four perfect octahedral cleavages versus apophyllite's single basal cleavage.
• Prehnite: botryoidal green masses rather than sharp prisms.

Where Apophyllite Is Typically Found

The world's finest apophyllite comes from the basalt quarries of the Deccan Traps near Pune and Nashik, Maharashtra, India, where green and clear crystals sit on zeolites. Other sources include Brazil, Mexico, Iceland, the Faroe Islands, Germany (Harz), and the United States (New Jersey trap rock, Lake Superior). Look in gas cavities (vugs/amygdules) of basalt.