Prehnite
Prehnite is Ca2Al(Si3Al)O10(OH)2, hardness 6 to 6.5, density 2.80 to 2.95. Crystals are uncommon and reach 4.5 cm; most specimens are botryoidal aggregates.
Prehnite is a calcium aluminium silicate, orthorhombic, hardness 6 to 6.5 and density 2.80 to 2.95. It is uncommon as free crystals, which reach 4.5 cm; the normal habit is fanlike, reniform, globular or stalactitic aggregates. It keeps company with zeolites but is not one.
In short
- Prehnite is not a zeolite. It occurs with them, in the same basalt cavities, on the same specimens, and is catalogued with them by almost everybody. It is a sheet silicate without the open framework and exchangeable water that define the zeolite group, and it does not dehydrate the way a zeolite does.
- Hardness 6 to 6.5 is the surprise. Botryoidal green material looks soft and is harder than the steel of a knife blade. It is harder than apophyllite at 4.5 to 5, harder than the stilbite and heulandite it sits with at 3.5 to 4, and close to feldspar.
- Free crystals are genuinely uncommon and reach only 4.5 cm, tabular on {001} or prismatic to steep pyramidal. Nearly every specimen on the market is an aggregate, and the aggregates are what the species is collected for.
- It names a metamorphic facies. The prehnite-pumpellyite facies is the standard term for the conditions between burial diagenesis and greenschist metamorphism, which is why prehnite turns up in altered basalt worldwide rather than in one or two famous places.
- The name is a person's. The Handbook of Mineralogy gives it for Colonel Hendrik von Prehn, 1733 to 1785, who discovered the mineral; the IMA records 1788 as the year of description and South Africa as the country.
| Species | Hardness | Density | Habit | Is it a zeolite? |
|---|---|---|---|---|
| Prehnite | 6–6.5 | 2.80–2.95 | Fanlike, reniform, globular; crystals to 4.5 cm | No — a sheet silicate |
| Apophyllite | 4.5–5 | 2.33–2.37 | Tabular to prismatic, pseudocubic, to 20 cm | No — a related sheet silicate |
| Stilbite | 3.5–4 | 2.1–2.2 | Sheaf-like, bow-tie bundles | Yes |
| Heulandite | 3.5–4 | 2.10–2.20 | Tabular, coffin-shaped, to 12 cm | Yes |
| Datolite | 5–5.5 | 2.8–3.0 | Equant wedge-shaped crystals; porcelainous masses | No — a borosilicate |
| Pectolite | 4.5–5 | 2.7–2.9 | Radiating acicular sprays; sharp needles | No — a chain silicate |
Why it is filed with the zeolites and why that is wrong
The Handbook of Mineralogy gives prehnite's association as zeolites, datolite, pectolite, calcite, epidote, copper, and its occurrence as a secondary or hydrothermal mineral in veins and cavities in mafic volcanic rocks. That is the zeolite setting exactly — the amygdaloidal basalts of the Deccan Traps, the New Jersey traprock, Nova Scotia, the Scottish and Irish Tertiary basalts.
So prehnite arrives in the same crate, sits on the same matrix, and is sold alongside stilbite and apophyllite. But it is a sheet silicate, not a framework silicate. Zeolites are defined by an open aluminosilicate framework holding loosely bound, exchangeable water; heat one gently and it gives up water reversibly, which is the origin of the name. Prehnite's hydroxyl is structural.
Does the distinction matter to a collector? For storage, yes. Some zeolites dehydrate and craze in very dry air, which is why zeolite collections are kept away from radiators and out of sealed cases with desiccant. Prehnite is indifferent to that. For cataloguing, it matters because a species collection ordered by structural class puts prehnite nowhere near stilbite, and anyone searching your records for zeolites should not find it there.
Hardness 6.5 on something that looks soft
The most common practical error with prehnite is underestimating it. Botryoidal apple-green prehnite reads as soft to the eye — it has the rounded, waxy, faintly translucent look of a mineral in the 3 to 4 range — and it is 6 to 6.5, above the hardness of a steel knife blade and of window glass.
Two consequences. First, in identification: if a green botryoidal crust will not take a knife scratch, that alone rules out most of what it might otherwise be. Malachite is 3.5 to 4, wavellite 3.5 to 4, smithsonite 4 to 4.5, variscite 3.5 to 4.5. Prehnite and chrysoprase-type chalcedony are the hard ones. Second, in cleaning: prehnite tolerates handling that would ruin a softer species, though the good {001} cleavage means a fan of tabular crystals can still split along the flat.
The density of 2.80 to 2.95 is the other check. That is heavier than quartz at 2.65 but much lighter than any of the copper carbonates — malachite is about 4. A fist-sized green botryoidal mass that feels ordinary in weight and refuses a knife is prehnite far more often than it is anything else.
Where the good material comes from
The Handbook says numerous localities worldwide and then names the ones worth knowing. For collectors the short list is:
The Jeffrey mine at Asbestos, Quebec, which the Handbook singles out for fine crystals and which produced the best of the sharply crystallised material. Bourg d'Oisans in Isère, France, from La Combe-de-la-Selle — the classic Alpine occurrence, and the one most likely to appear with epidote. The Khandivali quarry at Bombay in Maharashtra, and the wider Deccan basalt quarries, which supply most of the green botryoidal material in circulation. Paterson in Passaic County and Bergen Hill in Hudson County, New Jersey, the traprock localities, with the Roncari quarry in Connecticut and the Hampden quarry in Massachusetts. The Brandberg Mountains in Namibia, at Copper Valley. Also Namibia, Antarctica, Australia around Coonabarabran, and the Cape of Good Hope, which is where von Prehn's original material came from.
Prehnite epimorphs after other species — hollow casts where prehnite grew over a crystal that later dissolved — are a recognised speciality, particularly from Bourg d'Oisans. An epimorph is not a pseudomorph and should not be catalogued as one: a pseudomorph replaces the original, an epimorph coats it and is left as a shell. Getting that right on a label is the difference between a description and a mistake. Specific material we are looking for is on the wanted list, and the rest of the silicate suite is under species notes.
Storage, and the one thing that does damage it
Prehnite is an easy species to keep. It is hard, not light-sensitive, not hygroscopic, chemically stable in normal room conditions, and unaffected by the humidity swings that damage sulphides and copper sulphates. It does not need the 45 to 60 per cent relative humidity regime that a pyrite-bearing collection needs.
The exception is mechanical damage to crystal fans. The good {001} cleavage runs across the flat of a tabular crystal, and a fan of blades standing proud of the matrix is vulnerable in exactly the way a stilbite sheaf is. Botryoidal masses have no such weakness and can be handled freely.
The other thing to watch is the associates, not the prehnite. A prehnite plate carrying a zeolite or a sulphide inherits that species's requirements: a specimen with pyrite on it must be kept below 60 per cent relative humidity even though the prehnite could not care less. The rule is that a composite specimen takes the storage conditions of its most demanding component. That principle is set out in storing a collection in a damp house.