Plancheite
Plancheite is a fibrous blue copper silicate, Cu8(Si4O11)2(OH)4·H2O: orthorhombic, satiny, hardness 6, density 3.65–3.80. Its water check, associates, places.
Plancheite is a fibrous blue copper silicate, Cu8(Si4O11)2(OH)4·H2O on the September 2026 IMA list: orthorhombic, satiny, pale to deep blue, hardness 6, density 3.65–3.80. Worked, it is 43.41% copper. The sheet’s calculated density, 3.85, needs one full water molecule; Evans and Mrose (1977) put the water nearer 0.4.
In short
- The species. The Handbook of Mineralogy sheet gives orthorhombic crystals fibrous by extension on [001], striated, commonly as compact radial aggregates; hardness 6; density 3.65–3.80; pale blue to deep blue or pale greenish blue; satiny; biaxial (+). The September 2026 IMA-CNMNC Master List row reads Cu8(Si4O11)2(OH)4·H2O, status Rd, 1967 s.p., Republic of the Congo, with American Mineralogist 62 (1977), 491 as its second reference.
- The 3.85 water check. The printed cell is 19.043 × 20.129 × 5.269 = 2019.7 ų with Z = 4. With one H2O per formula (1171.07 g/mol) it gives 3.851, the sheet’s bracketed [3.85]. Evans and Mrose (1977) write the water as xH2O with x < 0.43, “probably variable”, and print 3.815 at x = 0.4; worked, 0.4 H2O gives 3.816. With no water at all the cell still gives 3.792.
- Documented failure: the measured density. The sheet’s measured 3.65–3.80 sits below every cell figure. 3.80 would need 0.13 H2O; 3.65 would need less than none (−2.40). So a low reading on a plancheite aggregate is not explained by its water, and this page does not guess the cause. In water a 20.00 g piece reads 14.52–14.74 g on the measured range, 14.81 g at the calculated 3.85.
- The associate ledger. The sheet names chrysocolla, dioptase, malachite, conichalcite and tenorite. Of the six in the table below, plancheite prints the highest hardness, 6, one step above dioptase’s 5. Quartz is not on plancheite’s association line. The blue look-alike is shattuckite: the separation is on plancheite vs shattuckite.
- Where it is found. The sheet’s Distribution opens with Mindouli, Renéville and Tshiniama in the Congo Republic, then Tantara, Mindigi and M’sesa near Kambove in Katanga, on the Katanga Copperbelt page; Tsumeb, Guchab and Okatumba in Namibia; three English places, the Driggith mine in Caldbeck Fells, the Gunheath china clay pit at St. Austell in Cornwall and the Engine vein, Alderley Edge; and Arizona, Michigan, Japan and La Rioja, Argentina.
| Species | IMA formula; status | System (sheet) | Colour (sheet) | Hardness | Density (sheet) | Cu by mass | 20.00 g in water |
|---|---|---|---|---|---|---|---|
| Plancheite | Cu8(Si4O11)2(OH)4·H2O; Rd, 1967 s.p. | Orthorhombic | Pale blue to deep blue, pale greenish blue | 6 | 3.65–3.80 meas.; [3.85] calc. | 43.41% | 14.52–14.74 g |
| Chrysocolla | (Cu2-xAlx)H2-xSi2O5(OH)4·nH2O; A, 1980 s.p. | Orthorhombic (?) | Blue, blue-green, or green; brown to black when impure | ~2–4 | 1.93–2.4 | Varies (x, n) | 9.64–11.67 g |
| Dioptase | CuSiO3(H2O); G, 1798 | Hexagonal | Emerald-green, blue-green | 5 | 3.28–3.35 | 40.31% | 13.90–14.03 g |
| Malachite | Cu2(CO3)(OH)2; G, year printed as ? | Monoclinic | Bright green, dark green, blackish green | 3.5–4 | 4.05 | 57.48% | 15.06 g |
| Conichalcite | CaCu(AsO4)(OH); G, 1849 | Orthorhombic | Grass-green to yellowish green, pistachio-green, emerald-green | 4.5 | 4.33 | 24.48% | 15.38 g |
| Tenorite | CuO; A, 1962 s.p. | Monoclinic | Steel-gray, iron-gray, black | 3.5 | 6.45 (synthetic) | 79.89% | 16.90 g |
What is plancheite? A fibrous copper silicate of oxidised copper deposits
Plancheite is Cu8(Si4O11)2(OH)4·H2O, an orthorhombic copper silicate that the sheet calls a rare secondary mineral in the oxidized portion of copper deposits. The Handbook of Mineralogy plancheite sheet heads it Cu8Si8O22(OH)4·H2O, the same atoms written without the silicate unit, and describes crystals fibrous by extension on [001], striated, commonly as compact radial aggregates. Hardness is 6, measured density 3.65–3.80, and the calculated density is given in brackets as [3.85]. It is translucent, pale blue to deep blue or pale greenish blue, with a satiny lustre. The sheet prints no streak and no cleavage, and this page gives neither.
Optics and a published disagreement. The sheet gives biaxial (+), α = 1.697, β = 1.718, γ = 1.741 and a measured 2V of 88.5°, the same figures as the Table 1 of Evans and Mrose (1977), American Mineralogist 62, 491–502, which they determined on crystals from Rioja, Argentina. The sheet prints pleochroism as marked, X very pale blue and Y = Z blue; Evans and Mrose found no appreciable pleochroism on clear single crystals and suggest that observations on aggregates or very thin fibres may have been misread. Both are printed here because both are published.
What the lists record. The September 2026 IMA-CNMNC Master List row is Cu8(Si4O11)2(OH)4·H2O, status Rd (redefined), 1967 s.p., country Republic of the Congo, first reference Comptes Rendus de l’Académie des Sciences de Paris 146 (1908), 722, and second reference the 1977 paper. Evans and Mrose describe plancheite’s structure as very similar to shattuckite’s, except that pyroxene-type chains are replaced by amphibole-type chains; the Si4O11 in the IMA formula is that chain written out. The sheet gives type material as n.d. and marks no type locality, so this page names none (what a type locality is).
How much water is in plancheite? The 3.85 water check
The sheet’s bracketed density, 3.85, is what the printed cell gives with exactly one water molecule; the structure paper puts the water nearer 0.4. An orthorhombic cell has a volume of a × b × c: 19.043 × 20.129 × 5.269 = 2019.7 ų, against the 2019.5 ų printed by Evans and Mrose. Four formula units of Cu8Si8O22(OH)4·H2O, 1171.07 g/mol on standard atomic weights, give 4 × 1171.07 ÷ (0.60221 × 2019.7) = 3.851 g/cm³.
The 1977 figure, reproduced. Evans and Mrose write the formula Cu8(Si4O11)2(OH)4·xH2O and say there is evidence for free water in the structure, but x < 0.43 and is probably variable; their Table 1 gives D(x-ray) 3.815 at x = 0.4. Worked on the same cell, x = 0.4 gives 3.816, x = 0.43 gives 3.818, and x = 0 gives 3.792. The IMA list and the Handbook both print a whole H2O.
Documented failure: the measured range sits below every cell figure. Solving the same equation backwards, a density of 3.80 needs 0.13 H2O and 3.65 needs −2.40, which is impossible. So no water content in the formula reaches the bottom of the measured range, and the gap is not explained by water. This page does not offer a reason. For the balance it means a solid 20.00 g piece should read 14.52–14.74 g in water on the measured densities and 14.81 g at the calculated one; the method is how to measure specific gravity at home.
What grows with plancheite? Dioptase, chrysocolla and the associate ledger
The sheet’s association line is chrysocolla, dioptase, malachite, conichalcite, tenorite, and the table above puts their numbers side by side. chrysocolla is the light one: 1.93–2.4, so 20.00 g reads 9.64–11.67 g in water against plancheite’s 14.52–14.74 g. dioptase is close in weight at 13.90–14.03 g, but its sheet prints emerald-green to blue-green and hardness 5. malachite and conichalcite are green on their sheets, and tenorite is steel-gray to black with a density of 6.45 (synthetic). Plancheite prints the highest hardness of the six, 6.
“Plancheite with dioptase” and “plancheite in quartz”. Dioptase is on the association line, and Evans and Mrose (1977) also note the close association of plancheite and shattuckite with each other and with chrysocolla and dioptase. Quartz is not on plancheite’s association line; the shattuckite sheet does name quartz among its associates. This page therefore makes no claim about plancheite in or on quartz.
The look-alike. Shattuckite, Cu5(SiO3)4(OH)2, is the blue copper silicate most often confused with plancheite; Evans and Mrose say the two are so similar in appearance that they have often been confused. The shattuckite page works their copper-to-silicon ratios; the separating tests, led by refractive index, are on plancheite vs shattuckite.
Where is plancheite found? Congo, Katanga, Namibia and three English places
The plancheite sheet’s Distribution, as printed. From Mindouli, Renéville and Tshiniama, Congo Republic. In Congo (Zaire), fine material from Tantara, Mindigi and M’sesa, near Kambove, Katanga (Shaba) Province. From Tsumeb, Guchab and Okatumba, about 80 km east of Windhoek, Namibia. In England, at the Driggith mine, Caldbeck Fells, Cumbria; the Gunheath china clay pit, St. Austell, Cornwall; and the Engine vein, Alderley Edge, Cheshire. In Arizona, the Table Mountain mine, Galiuro Mountains, and the Azurite mine, Tortolita Mountains, Pinal Co.; the Bighorn district, south of Aguila, Maricopa Co.; the Harquehala Mountains, Yuma Co.; and elsewhere. In Michigan, the Algomah mine, Ontonagon Co. The Kurokawa mine, Gifu Prefecture, and the Kisanmori mine, Akita Prefecture, Japan. La Rioja, Argentina. We add no other locality.
Two Congos. Mindouli, Renéville and Tshiniama are printed in the Congo Republic, and Tantara, Mindigi and M’sesa in Katanga, in the Democratic Republic; why a label reading only “Congo” does not say which is explained on the Katanga Copperbelt page. The sheet’s analysis is from Mindouli; its X-ray pattern and the 1977 crystals are from La Rioja.
Plancheite is a copper compound; handling questions belong to toxic minerals in a collection, HSE EH40, COSHH 2002 or a clinician, and this page is not advice. Related species are at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.