Apophyllite
Apophyllite is a group, not a species: six valid names on the IMA list. Hardness 4.5 to 5, crystals to 20 cm, and a perfect basal cleavage that breaks them.
Apophyllite is a group name covering six valid mineral species on the IMA list, not a single species. The commonest is fluorapophyllite-(K), tetragonal, hardness 4.5 to 5, density 2.33 to 2.37, in pseudocubic prisms to 20 cm on zeolite matrix, with a perfect {001} cleavage that makes every specimen fragile.
In short
- “Apophyllite” has not been a species name since 1978. The IMA-CNMNC list carries six valid species under the group: fluorapophyllite-(K), fluorapophyllite-(Na), fluorapophyllite-(Cs), fluorapophyllite-(NH4), hydroxyapophyllite-(K) and hydroxyapophyllite-(NH4).
- The green Indian material is fluorapophyllite-(K) where it has been analysed. The Handbook of Mineralogy is blunt that distinguishing it from hydroxyapophyllite requires chemical analysis, so a specimen without one is a presumption, not a determination.
- The perfect {001} cleavage at hardness 4.5 to 5 is why apophyllite arrives broken. The cleavage runs flat across the top of the crystal, at right angles to the prism, and a knock in transit takes the termination off cleanly.
- Density 2.33 to 2.37 is remarkably low — lighter than quartz at 2.65 and lighter than calcite at 2.71. A large, glassy, colourless crystal that feels too light for its size is behaving correctly.
- The name records a laboratory observation. From the Greek for away from and leaf, for the way the mineral exfoliates when heated. Crystals reach 20 cm, and the very large ones come from Rio Grande do Sul in Brazil.
| Species | Formula | IMA status | Country of first description |
|---|---|---|---|
| Fluorapophyllite-(K) | KCa4(Si8O20)F(H2O)8 | Renamed, 1978 | India |
| Fluorapophyllite-(Na) | NaCa4(Si8O20)F(H2O)8 | Renamed, 1976-032 | Japan |
| Fluorapophyllite-(Cs) | CsCa4(Si8O20)F(H2O)8 | Approved, 2018-108a | Tajikistan |
| Fluorapophyllite-(NH4) | (NH4)Ca4(Si8O20)F(H2O)8 | Approved, 2019-083 | Slovakia |
| Hydroxyapophyllite-(K) | KCa4Si8O20(OH,F)(H2O)8 | Renamed, 1978 | USA |
| Hydroxyapophyllite-(NH4) | (NH4)Ca4(Si8O20)(OH)(H2O)8 | Approved, 2025-064a | Yemen |
What to write on the label when the species cannot be determined
Almost every apophyllite specimen in private hands has never been analysed, and the group cannot be resolved by eye. The Handbook of Mineralogy's distribution note for fluorapophyllite says it plainly: many localities worldwide, but distinction from hydroxyapophyllite requires chemical analysis.
That leaves three defensible options and one indefensible one.
Defensible: write “apophyllite group”. Accurate, honest, and says exactly what is known. Defensible: write “fluorapophyllite-(K), by locality and habit, unanalysed” on material from a locality where the species has been established — the Indian Deccan quarries being the obvious case. That records both the determination and its basis. Defensible: keep the original label as written and record the modern position separately.
Indefensible: writing “fluorapophyllite-(K)” flat, with no qualifier, on an unanalysed specimen. That is a species determination you did not make, and it is indistinguishable in the catalogue from one that was made properly. The same argument applies across this site to heulandite, to uvite, and to the lead apatites on pyromorphite against mimetite. Our cataloguing conventions set out the three-state convention that handles all of them.
The cleavage, and why apophyllite breaks in the post
Apophyllite has perfect cleavage on {001} — the basal plane, flat across the top of the crystal and at right angles to the prism — and imperfect cleavage on {110}. Hardness is 4.5 to 5, and the mineral is brittle.
That geometry is the worst possible combination for a crystal that stands proud of a matrix. A pseudocubic apophyllite sitting on basalt has a perfect cleavage plane running horizontally through it at every level, and any lateral knock applies a shearing force across those planes. The result is the failure everyone who collects the group has seen: the top of the crystal comes away as a flat, bright, pearly plate, leaving a mirror-smooth surface where the termination was.
The pearly lustre on the {001} face is how you tell a cleaved crystal from an undamaged one. A grown face is vitreous; a cleavage plane is pearly and often slightly iridescent, and it is perfectly flat in a way a growth face rarely is. Check the top of every apophyllite before buying it, under a light held at a low angle.
For packing, this is a species that needs the gap around it to compress rather than to hold — the sequence is on how to pack and ship mineral specimens. For display, it means never gripping a crystal to lift a plate.
Where the specimens come from
The apophyllite group is worldwide, but collecting is dominated by one region. The Deccan basalt quarries of Maharashtra in India — the Handbook names the Poona, Nasik and Bombay districts — have produced, for decades, the exceptional groups that define what a good apophyllite looks like: glassy colourless or apple-green pseudocubic crystals to several centimetres, on stilbite, on heulandite, on chalcedony, with mesolite sprays and green celadonite coatings.
The other named sources are Teigarhorn at Berufördur in Iceland for fine large crystals, the Alpe di Siusi in Trentino-Alto Adige, St Andreasberg in the Harz, Korsnäs in Finland, Guangdong in China, and in the USA the classic traprock localities: French Creek and Cornwall in Pennsylvania, Paterson in Passaic County and Bergen Hill in New Jersey, and the Fairfax quarry in Virginia. Very large crystals come from near Bento Gonçalves in Rio Grande do Sul, Brazil.
The green colour of the best Indian material is not a separate species and does not indicate a different chemistry in any way the group nomenclature recognises. It is a colour, it fades under prolonged strong light on some material, and a saturated green apophyllite should be kept out of a sunlit window — see which minerals fade in light for how to think about that risk.
Separating it from the zeolites it sits on
Apophyllite arrives on a plate with three or four other species, and collectors routinely mislabel the lot. Three properties do almost all the work.
Crystal system. Apophyllite is tetragonal, so crystals have a square cross-section and the pseudocubic habit that gives the group its look. Stilbite is monoclinic and forms sheaves; heulandite is monoclinic and forms coffin-shaped tablets; mesolite and natrolite form needles.
Hardness. Apophyllite at 4.5 to 5 is distinctly harder than stilbite and heulandite at 3.5 to 4, and distinctly softer than the prehnite at 6 to 6.5 that sometimes accompanies it. A needle probe under the microscope, used on an inconspicuous edge, sorts it.
The cleavage plane. Apophyllite's is basal and flat; stilbite's and heulandite's are on the side faces. A broken apophyllite shows a flat pearly top; a broken stilbite shows a pearly side.
And the density: apophyllite at 2.33 to 2.37 is lighter than heulandite and stilbite at about 2.1 to 2.2 — only slightly, and not a usable field test between them, but a marked difference from prehnite and from calcite. Material from the classic zeolite localities that we are looking for is listed on the wanted list.