Overview
Clinohedrite is a rare hydrous calcium-zinc silicate cherished above all by collectors of fluorescent minerals. It is one of the signature species of the Franklin and Sterling Hill zinc deposits of New Jersey, USA, where it first came to light in 1898. Although unremarkable to the naked eye, where it tends to form pale, glassy veinlets and crusts, clinohedrite is transformed under short-wave ultraviolet light into a glowing orange that has made it a centrepiece of the fluorescent-mineral cabinet. Its name comes from the Greek words for “inclined” and “face,” a reference to the slanted, hemihedral faces of its monoclinic crystals.
Composition & structure
Clinohedrite has the formula CaZn(SiO4)·H2O, placing it among the rare hydrated nesosilicates in which isolated silicate tetrahedra are linked through calcium and zinc. The structure is monoclinic, and the mineral belongs to the unusual domatic crystal class, which lacks a centre of symmetry. This polar, low-symmetry framework is part of why clinohedrite is piezoelectric and fluoresces so vividly, the activator being the zinc together with manganese impurities inherited from the surrounding ore.
| Formula | CaZn(SiO4)·H2O |
| Crystal system | Monoclinic (domatic class) |
| Mohs hardness | 5.5 |
| Lustre | Vitreous (glassy), pearly on {010} |
| Colour | Colourless, white, to pale pink or amethystine |
| Type locality | Franklin, Sussex County, New Jersey, USA |
Formation & occurrence
Clinohedrite is a secondary mineral that crystallised during the late, low-temperature alteration of the metamorphosed zinc orebodies at Franklin and Sterling Hill. There it lines fractures and fills veinlets cutting the massive ore, typically intergrown with hardystonite, esperite, willemite, franklinite and calcite. These deposits are geologically extraordinary: a zinc-rich marble metamorphosed to high grade, producing a roster of species found in abundance almost nowhere else on Earth. Clinohedrite owes its existence to that singular chemistry, which combined ample zinc with calcium, silica and water in the right conditions for the mineral to form.
Identification & similar species
In daylight clinohedrite is easy to overlook, resembling other pale, glassy vein minerals. The defining test is ultraviolet fluorescence: under short-wave UV it glows a rich, warm orange, often side by side with the blue of hardystonite, the green of willemite and the cream-yellow of esperite, a combination that lets collectors identify it at a glance. Its hardness of about 5.5, monoclinic habit and association with the classic Franklin suite further distinguish it. Hemimorphite and other zinc silicates may look similar but do not share clinohedrite’s orange response, and many associated species fluoresce in quite different colours.
Notable localities & collecting
The Franklin Mine and the nearby Sterling Hill Mine in New Jersey remain the world’s premier source of clinohedrite and the type locality; essentially all fine display specimens originate there. The mines are now closed to production, so material on the market is largely old stock recovered before closure, which keeps good fluorescent pieces in steady demand. Beyond New Jersey the mineral is genuinely rare: it has been reported in small quantity from the Christmas mine in Gila County, Arizona, and from a handful of other scattered localities worldwide. Collectors prize clinohedrite less as a single crystal than as part of a glowing multi-coloured Franklin specimen, ideally viewed with a short-wave UV lamp that brings its orange brilliance to life.