Plancheite vs Shattuckite
Plancheite vs shattuckite, two blue fibrous copper silicates: every plancheite index is below 1.75, every shattuckite index above; hardness 6 against about 3.5.
They are different minerals. Plancheite is Cu8(Si4O11)2(OH)4·H2O and shattuckite Cu5(SiO3)4(OH)2: both blue, fibrous, orthorhombic copper silicates, solved in the same 1977 paper. Every printed plancheite refractive index (1.697–1.741) is below 1.75 and every shattuckite index (1.753–1.815) above it. Without optics, hardness, 6 against about 3.5, points the same way.
In short
- Two species, one paper. The September 2026 IMA-CNMNC Master List lists both as Rd, redefined, 1967 s.p., each with American Mineralogist 62 (1977), 491 as second reference: Evans and Mrose (1977), American Mineralogist 62, 491–502. They found plancheite’s structure very similar to shattuckite’s, except that pyroxene-type chains are replaced by amphibole-type chains, and its cell like shattuckite’s with the a axis doubled.
- The 1.75 rule. On the two Handbook sheets plancheite reads α 1.697, β 1.718, γ 1.741 and shattuckite α 1.753, β 1.782, γ 1.815. Every plancheite index is below 1.75 and every shattuckite index above it; the gap between plancheite’s highest and shattuckite’s lowest is 0.012. Evans and Mrose: the optical properties “serve readily to distinguish the two species”.
- Without a refractometer. Hardness 6 against about 3.5. Density 3.65–3.80 against 4.11, so a solid 20.00 g piece reads 14.52–14.74 g in water against 15.13 g, about 0.4 g apart at the nearest. Shattuckite prints perfect cleavage on {010} and {100}; plancheite prints none. By formula plancheite is 43.41% copper and shattuckite 48.43%.
- Documented failure: colour, lustre and locality. The plancheite sheet allows pale blue to deep blue and shattuckite’s is deep to medium blue, so deep blue is on both. Satiny against silky is not a test. Both are biaxial (+) with a 2V near 88°. Counted by script, 3 named places are on both Distribution fields: Tantara and M’sesa near Kambove, Katanga, and Tsumeb.
- The decision. Index first: below 1.75 is plancheite, above is shattuckite. Without optics, hardness and weight in water point the same way once a solid, single-mineral piece is in hand. A mixed crust needs X-ray powder work. The species pages are plancheite and shattuckite; chrysocolla, which both sheets name as an associate, is on shattuckite vs chrysocolla.
| Property | Plancheite | Shattuckite | Separates? |
|---|---|---|---|
| Formula (IMA); status | Cu8(Si4O11)2(OH)4·H2O; Rd, 1967 s.p. | Cu5(SiO3)4(OH)2; Rd, 1967 s.p. | Yes, by analysis |
| Silicate unit; O per Si | Si4O11; 2.75 | SiO3; 3.00 | Yes, by structure |
| Chain type (Evans and Mrose) | Amphibole-type | Pyroxene-type | Yes, by X-ray |
| Cu:Si; Cu by mass | 1.00; 43.41% | 1.25; 48.43% | Yes, by analysis |
| Water-equivalent (IMA formula) | 4.62% | 2.75% | By analysis only |
| System; space group | Orthorhombic; Pcnb | Orthorhombic; Pcab | No (X-ray only) |
| Cell a, b, c (Å) | 19.043, 20.129, 5.269 | 9.885, 19.832, 5.3825 | Yes, by X-ray: a doubled |
| Habit (sheet) | Crystals fibrous by extension on [001], striated; compact radial aggregates | Rarely prismatic crystals to 2 mm; spherulitic masses of acicular crystals | Partly |
| Cleavage | None printed | {010} and {100}, perfect | Partly |
| Hardness | 6 | ~3.5 | Yes |
| Density (measured) | 3.65–3.80 | 4.11 | Yes: 1.08 to 1.13 times |
| 20.00 g piece in water | 14.52–14.74 g | 15.13 g | Yes: about 0.4 g apart at the nearest |
| Refractive indices | α 1.697, β 1.718, γ 1.741 | α 1.753, β 1.782, γ 1.815 | Yes: 0.012 gap at 1.75 |
| Birefringence (γ − α) | 0.044 | 0.062 | Partly |
| Optic sign; 2V | Biaxial (+); 88.5° measured | Biaxial (+); 88° calculated | No |
| Colour (sheet) | Pale blue to deep blue, pale greenish blue | Deep to medium blue | No: deep blue on both |
| Lustre (sheet) | Satiny | Silky | No |
| Places on both sheets | Tantara, M’sesa (Kambove), Tsumeb | Tantara, M’Sesa (Kambove), Tsumeb | No |
Is plancheite the same as shattuckite? No: two chain structures from one 1977 paper
Plancheite and shattuckite are separate species, both redefined in 1967 and both fixed by the same structure paper. The September 2026 IMA-CNMNC Master List gives plancheite as Cu8(Si4O11)2(OH)4·H2O and shattuckite as Cu5(SiO3)4(OH)2, each status Rd, 1967 s.p., each with American Mineralogist 62 (1977), 491 as its second reference. That paper, Evans and Mrose (1977), American Mineralogist 62, 491–502, says the two are so similar in appearance that they have often been confused, and even said at times to be identical, but that optical properties, X-ray powder data and chemical analysis have amply proven their validity as distinct species.
The chain check, worked from the formulas. Evans and Mrose describe shattuckite as brucite-like layers with pyroxene-type (SiO3) chains joined to their surfaces, and plancheite as very similar except that the pyroxene-type chains are replaced by amphibole-type chains. The IMA formulas carry that difference in their silicate units: SiO3 is 3.00 oxygen per silicon and Si4O11 is 2.75. The plancheite cell is like shattuckite’s but with the a axis doubled: 19.043 Å against 9.885 Å, with b and c close (20.129 and 5.269 against 19.832 and 5.3825). None of this can be seen in the hand; it is why X-ray powder data settle the question.
Copper and water. Worked on standard atomic weights, plancheite is 43.41% copper with 4.62% water-equivalent, and shattuckite 48.43% with 2.75%; their copper-to-silicon ratios are 1.00 and 1.25. How shattuckite’s 1.25 compares with every other copper silicate on the IMA list, and why one old plancheite analysis lands nearer it, is on the shattuckite page.
How do you tell plancheite from shattuckite? The 1.75 rule
Refractive index is the one property where the two sheets do not overlap at all: plancheite 1.697 to 1.741, shattuckite 1.753 to 1.815. The plancheite sheet prints α 1.697, β 1.718 and γ 1.741; the shattuckite sheet prints α 1.753, β 1.782 and γ 1.815. The highest plancheite index and the lowest shattuckite index are 0.012 apart, and 1.75 falls between them. An index below 1.75 on a blue fibrous copper silicate points to plancheite, above it to shattuckite.
Where the numbers come from. Evans and Mrose’s Table 1 gives the same six figures: shattuckite’s taken from Larsen and Berman (1934), plancheite’s measured by them on crystals from Rioja, Argentina. They write that the optical properties serve readily to distinguish the two species. They also write that wider variation in refractive indices has been reported for plancheite, possibly because of variation in water content. So the rule rests on one plancheite measurement set, and a reading close to 1.75 is not decisive on its own.
The other optical figures do not help. Both are biaxial (+), with plancheite’s 2V measured at 88.5° and shattuckite’s calculated at 88°. Birefringence differs, 0.044 against 0.062, but fibrous aggregates rarely give a clean reading. The sheets print pleochroism for both, while Evans and Mrose found no appreciable pleochroism on clear single crystals of either, so pleochroism is not used here.
What if you cannot measure an index? Hardness, cleavage and weight in water
Hardness is the plainest hand test: 6 for plancheite against about 3.5 for shattuckite. The method is on how to test mineral hardness properly. Shattuckite’s sheet prints perfect cleavage on {010} and {100}; plancheite’s prints none.
Weight in water. On the measured densities a solid 20.00 g piece reads 14.52–14.74 g in water as plancheite and 15.13 g as shattuckite. At the nearest that is about 0.4 g; shattuckite is 1.08 to 1.13 times as dense. The method is how to measure specific gravity at home. Plancheite’s measured density sits below what its own cell gives at any water content, a point worked on the plancheite page, so readings at the low end of 3.65–3.80 are within the sheet’s own range.
Mixtures defeat the balance. Both sheets name chrysocolla as an associate, plancheite’s names dioptase and malachite, and shattuckite’s names quartz and malachite. A piece containing any of them reads between or outside these figures. A weight in water that agrees with one species shows only that the piece is consistent with it.
Why do colour, lustre and locality fail? Three documented failures
Colour. Evans and Mrose write that the two species are usually distinguishable by colour: on the ISCC-NBS charts shattuckite deep blue to medium blue, plancheite light greenish blue to very light greenish blue. The Handbook plancheite sheet, though, prints pale blue to deep blue as well as pale greenish blue, so a deep blue plancheite is within its own sheet. Colour suggests; it does not decide.
Lustre and habit. Plancheite is printed satiny, shattuckite silky; plancheite as fibrous, striated crystals in compact radial aggregates, shattuckite as spherulitic masses of acicular crystals and, rarely, prismatic crystals to 2 mm. Evans and Mrose say both generally occur in silky, fibrous needles, though shattuckite from Ajo sometimes has well-developed prismatic crystal forms.
Locality. A script over the two Distribution fields finds 3 named places on both: Tantara and M’sesa, near Kambove, Katanga (see the Katanga Copperbelt), and Tsumeb. In Arizona both sheets list mines, but none by the same name; shattuckite’s type locality is the Shattuck mine at Bisbee. A Katanga or Tsumeb label does not say which species is present. Both are copper compounds; handling questions belong to toxic minerals in a collection, HSE EH40 or a clinician, and this page is not advice. More identification pages are in the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.