Mendipite
Mendipite is Pb3O2Cl2, hardness 2.5, density 7.240, in cleavable masses to 12 cm. It heads a suite of four lead oxychlorides with British type localities.
Mendipite is a lead oxychloride, Pb3O2Cl2, orthorhombic, hardness 2.5, with a measured density of 7.240 — about 2.7 times that of calcite. It occurs in columnar or fibrous aggregates and cleavable masses to 12 cm, in nodules within manganese oxide ores in the Mendip Hills of Somerset, which gave the mineral species its name in 1839.
In short
- Density 7.240. Mendipite is one of the heaviest minerals a British collection is likely to hold — 2.7 times the density of calcite, and it feels wrong in the hand at hardness 2.5.
- The name is the locality. The Handbook of Mineralogy derives Mendip from the Old English Myne-deepes, and the IMA list gives the United Kingdom as the country of first description, 1839.
- It heads a suite of four. Mendipite, chloroxiphite, diaboleite and parkinsonite all have British type localities and all come out of the same lead-manganese pods at Higher Pitts Farm and Merehead quarry in Somerset.
- Chloroxiphite's entire recorded world distribution is two Somerset sites. The Handbook's distribution paragraph for it names Higher Pitts Farm near Priddy and the Merehead quarry near Shepton Mallet, and nothing else at all.
- Hardness will not separate the suite — density will. Mendipite, chloroxiphite and diaboleite are all hardness 2.5, but their densities are 7.240, 6.76 to 6.93 and 5.42.
| Species | Formula | Hardness | Density | Colour | IMA country / year |
|---|---|---|---|---|---|
| Mendipite | Pb3O2Cl2 | 2.5 | 7.240 | Colourless to white, brownish cream, tinged yellow, pink, red or blue | United Kingdom, 1839 |
| Chloroxiphite | Pb3CuO2Cl2(OH)2 | 2.5 | 6.76 to 6.93 | Dull olive-green to pistachio-green | United Kingdom, 1923 |
| Diaboleite | CuPb2Cl2(OH)4 | 2.5 | 5.42 | Deep blue, pale blue streak | United Kingdom |
| Parkinsonite | Pb7MoO9Cl2 | Not stated | Not stated here | Described from the Mendips | United Kingdom, 1991 |
| Hydrocerussite | Pb3(CO3)2(OH)2 | 3.5 | 6.8 | White | Sweden, 1877 — the common associate |
What a Mendip nodule actually is
The occurrence line in the Handbook of Mineralogy is one sentence long: in nodules in manganese oxide ores (Somerset, England). That is the whole geological setting, and it is unusual enough to be worth unpacking.
The Mendips were worked for lead from Roman times onward. In a small number of places the lead mineralisation sits in pods within manganese-iron deposits developed along fissures in dolomitic conglomerate and limestone — the phrasing the Handbook uses on the chloroxiphite sheet. Inside those pods, lead, copper, chlorine and oxygen produced a set of oxychloride minerals that occur almost nowhere else on earth in collectable quantity.
The associated species listed for the Somerset material are hydrocerussite, cerussite, malachite, pyromorphite, calcite, chloroxiphite, diaboleite and parkinsonite. A single nodule can carry several of them, which is why intact nodules are worth more to a collector than a cleaned-out fragment. Our Mendip Hills locality notes cover the wider orefield.
Density is the test, and hardness is the trap
Mendipite, chloroxiphite and diaboleite are all hardness 2.5. Scratching will tell you nothing except that all three are soft, which you could have guessed. Density does all the work: 7.240, 6.76 to 6.93, and 5.42 respectively. Those are separated by more than any plausible measurement error on a specimen of reasonable size.
Colour is the second test and it is quicker. Mendipite runs colourless to white or brownish cream, sometimes tinged yellow, pink, red or blue. Chloroxiphite is dull olive-green to pistachio-green, in bladed crystals often curved, to 4 cm. Diaboleite is deep blue with a pale blue streak, in tabular crystals to about 2 cm. White and heavy, olive-green and bladed, deep blue and tabular — in the Mendip context those three descriptions are nearly diagnostic on their own.
One thing to be careful about: mendipite's very high refractive indices — 2.24 to 2.31 — give it a resinous to adamantine lustre on fracture surfaces and a pearly to silky one on cleavages. That sharp difference between two faces of the same specimen sometimes reads as two minerals when it is one. The cleavage or fracture distinction is the thing to apply here.
Chloroxiphite, and a distribution list with two entries
It is worth pausing on the companion species, because it carries a fact that is unusually clean. The Handbook of Mineralogy's distribution paragraph for chloroxiphite reads, in full: in England, at the Higher Pitts Farm, near Priddy, and in the Merehead quarry, near Shepton Mallet, Mendip Hills, Somerset. That is the entire world distribution as recorded there.
Compare that with mendipite itself, which is genuinely international: the Kunibert mine near Brilon in Germany, Långban in Sweden, Tarnowitz in Silesia, Laurium in slag, and the Elura zinc-lead-silver deposit northeast of Cobar in New South Wales, alongside the Somerset sites at Higher Pitts Farm, Priddy Hill Farm, the Wesley mine, near Churchill, and Merehead quarry.
The practical consequence for a collector is that mendipite can be bought as foreign material and chloroxiphite effectively cannot. If a chloroxiphite specimen is genuine, it came out of two fields in Somerset, and a locality that says anything else on a chloroxiphite label deserves a question. That is a rare case where the locality is testable against the species rather than the other way round — the situation our note on specimens without a locality describes from the opposite direction.
Handling: soft, heavy, lead-bearing and perfectly cleaved
Four properties combine badly here. Hardness 2.5 means a fingernail is close to marking it. The {010} cleavage is perfect. The density of 7.240 means a small specimen has real mass and real momentum if dropped. And it is a lead mineral.
On the last point, the NatSCA and Icon conservation leaflet groups some lead minerals with arsenic and antimony species as ones to handle with gloves and store in lidded clear polystyrene boxes lined with Plastazote — while noting that galena itself is not toxic because it is insoluble. Mendipite is an oxychloride rather than a sulphide, so the insolubility argument does not transfer, and gloves plus washed hands is the sensible default. Our toxic minerals page covers the wider set.
Support the specimen from underneath. A 12 cm cleavable mass at density 7.240 weighs several times what a visitor will expect, and the commonest way these get broken is somebody taking the weight wrongly with one hand. Mendip nodule material is not something that can be replaced from the next parcel, so if a particular associated species is what you are after, say so on a wanted list rather than waiting for it to appear.