Natrolite vs scolecite
Natrolite and scolecite crystals share hardness, cleavage and weight. Optics split them: natrolite below 1.50, straight extinction; scolecite above, inclined.
Natrolite or scolecite? Not by hardness or weight: both are 5 to 5.5, and a 20.00 g piece reads 10.91 to 11.15 g in water as natrolite and 11.11 to 11.27 g as scolecite. The Handbook sheets separate them optically: natrolite’s indices stay below 1.50 and its needles go dark parallel to their length; scolecite’s start at 1.507 and go dark up to 15 to 18° off it.
In short
- The criteria first. Hardness, density and cleavage fail on this pair, as the natrolite and scolecite species pages already show. Three things are left on a loose needle: its refractive index against 1.50, its extinction angle, and its sign of elongation. Habit and twinning are set out on the scolecite page; chemistry needs a laboratory.
- The extinction rule. The natrolite sheet prints Z = c on prisms of {110}, so a needle extinguishes parallel to its length and the slow ray vibrates along it. The scolecite sheet prints X ∧ c = 15° to 18° on needles elongated along [001], so extinction is inclined by up to that angle and the fast ray lies near the length. Both are read from the rendered sheets.
- The 1.50 line. Natrolite’s indices run 1.473 to 1.496, scolecite’s 1.507 to 1.521. A grain reading below 1.50 is not scolecite; above 1.507 it is not natrolite. Mesolite, 1.5048 to 1.5053, falls in the gap, only 0.0017 below scolecite.
- Where the rules fail. The full scolecite angle shows only in one orientation, so a single straight-looking fragment proves nothing. The mesolite sheet prints X ∧ c of about 8° on a mineral it calls orthorhombic, an inconsistency this page does not use. Hairlike tufts may be too fine to read, and intergrowths read as neither.
- Who each answer is wrong for. Natrolite is wrong for a needle with clearly inclined extinction or an index above 1.507. Scolecite is wrong for one that reads below 1.50, or extinguishes straight on every fragment. Either is wrong for a near-black hairlike tuft, which behaves like mesolite.
| Criterion | Natrolite | Scolecite | Mesolite | Does it choose? |
|---|---|---|---|---|
| IMA formula | Na2(Si3Al2)O10·2H2O | Ca(Si3Al2)O10·3H2O | Na2Ca2(Si9Al6)O30·8H2O | Only by analysis |
| Crystal system, point group | Orthorhombic, mm2 | Monoclinic, m | Orthorhombic, mm2 | Not by eye |
| Orientation on the sheet | X = a; Y = b; Z = c | Z = b; X ∧ c = 15° to 18°; Y ∧ a = −14° to −17° | X ∧ c ≈ 8°; Z = b | Yes, under a polarising microscope |
| Extinction against the needle’s length | Parallel | Inclined, up to 15° to 18° | Printed as about 8°; not used here | Yes, natrolite against scolecite |
| Sign of elongation | Length-slow (Z along the length) | Length-fast (X near the length) | Length-fast (X near the length) | Yes, natrolite against the other two |
| Optic sign | (+) | (−) | (+) | Scolecite against the other two, with an interference figure |
| Refractive indices | 1.473 to 1.496 | 1.507 to 1.521 | 1.5048 to 1.5053 | Yes at 1.50; mesolite to scolecite, 0.0017 apart, barely |
| Hardness | 5 to 5.5 | 5 to 5.5 | 5 | No |
| Density; 20.00 g piece in water | 2.20 to 2.26; 10.91 to 11.15 g | 2.25 to 2.29; 11.11 to 11.27 g | 2.26; 11.15 g | No: all three overlap |
| Na2O; CaO; H2O by mass, worked | 16.30%; 0%; 9.48% | 0%; 14.29%; 13.78% | 5.32%; 9.63%; 12.37% | Only by analysis |
| UV on the sheet | Commonly fluoresces orange to yellow | May fluoresce yellow to brown, SW and LW | None printed | A pointer only |
| Twinning on the sheet | On {110}, {011}, {031} | On {100}, twin axis [001], common | Characteristically on {010} or {100} | Set out on the scolecite page |
How do I tell natrolite from scolecite? The criteria
What fails. Both sheets give hardness 5 to 5.5 and perfect {110} cleavage; scolecite’s sheet adds {1-10}. Worked as 20 − 20/D, a 20.00 g piece reads 10.91 to 11.15 g in water as natrolite and 11.11 to 11.27 g as scolecite, so the ranges overlap between 11.11 and 11.15 g. Both are white to colourless, vitreous, and silky when fibrous. The natrolite page shows the weight overlap across the whole family and the scolecite page the habit and twin laws; neither is repeated here.
What is left. Natrolite is a sodium zeolite and scolecite a calcium one: 16.30% Na2O with no calcium, against 14.29% CaO with no sodium, on standard atomic weights. That is decisive, but only with an analysis. The 1997 IMA zeolite report gives all three of natrolite, mesolite and scolecite the same NAT framework and keeps them as separate species because their compositions are distinctive. On a loose needle, without chemistry, the sheets leave optics: the extinction angle, the sign of elongation, and the index.
The tool. All three optical tests need a polarising microscope and a grain or a needle small enough to lie on a slide; the equipment question is on best microscope for mineral specimens. None of them is a hand test, and this page does not pretend otherwise.
The extinction rule: straight for natrolite, slanted for scolecite
The extinction rule: a natrolite needle goes dark between crossed polars when its length is parallel to a polariser; a scolecite needle can go dark as much as 15° to 18° away from it. The natrolite sheet describes prisms of {110} terminated by {111}, striated along the elongation, so the length is the c axis, and its Orientation field reads X = a, Y = b, Z = c. With the slow vibration direction Z along the length, extinction is parallel and the needle is length-slow. The scolecite sheet describes slender prisms elongated along [001] and an Orientation of Z = b, X ∧ c = 15° to 18°. The fast direction X lies 15° to 18° off the length: extinction is inclined and the needle is length-fast.
Worked as a test. Rotate a single needle between crossed polars and note the angle between its length and the cross-hair at extinction. Zero on every needle examined points to natrolite; a clear angle up to 15° to 18° points to scolecite. The sign of elongation, read with an accessory plate, is a second, independent check: slow along the length is natrolite; fast is scolecite or mesolite.
Why one fragment is not enough. Because scolecite’s Z is the b axis, the full 15° to 18° appears only when the needle lies with b upright; in other orientations the measured angle is smaller, and a needle lying with its b axis flat on the slide extinguishes straight. The rule is therefore read on several fragments, and only an inclined one proves anything. These readings are worked from the sheets’ Orientation fields; they are not measurements of our own.
The 1.50 line: refractive index
The 1.50 line: every natrolite index on its sheet is below 1.50, and every scolecite index is at or above 1.507. Natrolite prints α 1.473 to 1.483, β 1.476 to 1.486, γ 1.485 to 1.496. Scolecite prints α 1.507 to 1.513, β 1.516 to 1.520, γ 1.517 to 1.521. The gap between natrolite’s highest and scolecite’s lowest is 0.011.
The optic sign agrees. The natrolite sheet gives biaxial (+), the scolecite sheet biaxial (−). A grain that reads below 1.50 and is length-slow is consistent with natrolite on both counts; one above 1.507 and length-fast with scolecite on both.
The catch is mesolite. Its sheet prints α 1.5048, β 1.5050 and γ 1.5053: above the 1.50 line, and 0.0017 below scolecite’s lowest α. An index reading near 1.505 cannot choose between mesolite and a low-index scolecite. The mesolite page covers what does separate it; its birefringence arithmetic is not repeated here. See mesolite.
Where the rules fail: mesolite, fine fibres and old labels
A documented inconsistency. The mesolite sheet calls the mineral orthorhombic, point group mm2, yet prints an Orientation of X ∧ c ≈ 8°, Z = b. An orthorhombic crystal is expected to show extinction parallel to its axes, so the printed 8° and the printed symmetry do not sit together. This page prints the value as the sheet gives it and does not use it as a test. What the sheet does support is that mesolite, with X near the length, is length-fast like scolecite, not length-slow like natrolite.
Fine fibres and mixtures. A hairlike tuft may be too fine to give a clean extinction position or index, and an intergrowth of two species reads as neither. The 1997 report notes that Fuchs, in 1816, clarified the distinctions among natrolite, scolecite and mesolite; old labels from before that, or using names like mesotype, are decoded on the natrolite page.
Localities do not settle it. A script over the Distribution fields finds none of scolecite’s ten named places on the natrolite sheet: the scolecite sheet names Teigarhorn, Iceland, Ben More on Mull and Talisker Bay on Skye, and the Poona, Nasik and Bombay districts of India; the natrolite sheet names the Dean quarry, St Keverne, Cornwall, White Head, Co. Antrim, and Kola. That reflects what the sheets chose to list, not where each species can occur. Both are fibrous on their sheets; what the law calls asbestos is on is scolecite asbestos, and nothing here is safety advice.
Who each answer is wrong for
“Natrolite” is wrong for a needle that extinguishes clearly off its length, is length-fast, or reads above 1.507. On the sheet it is also the one of the two that commonly fluoresces orange to yellow, but a lamp reading is a pointer; the general case is on fluorescent minerals explained.
“Scolecite” is wrong for a needle that reads below 1.50, is length-slow, or extinguishes straight on every fragment examined. Either name is wrong for a hairlike tuft that stays nearly dark between crossed polars and reads about 1.505; that is mesolite’s behaviour. Where none of the optical tests can be made, the honest label is “natrolite-group zeolite”, and the old label stays with the specimen.
On this site, the zeolites are set out on which minerals are zeolites, and the other identification pages are at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.