Gyrolite
Gyrolite is NaCa16(Si23Al)O60(OH)8·14H2O: white pearly balls to 5 cm in basalt, hardness 3 to 4, flexible flakes. How to tell it from apophyllite and calcite.
Gyrolite is a hydrous sodium calcium silicate, NaCa16(Si23Al)O60(OH)8·14H2O, grandfathered by the IMA (1851, United Kingdom). It forms white, pearly, spherical masses to 5 cm in basalt cavities: hardness 3 to 4, density 2.388 to 2.390, flakes flexible. Apophyllite and calcite are on its Handbook association line; quartz is not.
In short
- The name is Greek for round, after its common habit: spherical to radial masses to 5 cm, with platy or plumose aggregates and fibrous layers. Colour is colourless to white, and the lustre vitreous to pearly.
- Apophyllite and calcite are recorded associates; quartz is not. The Handbook of Mineralogy association line reads apophyllite, stilbite, laumontite, thomsonite, okenite, tobermorite, xonotlite and calcite. It does not include quartz.
- The three-way rule. Hardness 3 to 4 separates gyrolite from apophyllite (4.5 to 5) and quartz (7). Density 2.39 separates it from calcite (2.7102, 13.4% denser). A flake that bends separates it from all three, which their sheets record as brittle.
- Flexible flakes are uncommon. Of the 176 sheets in this site's reference set, 18 record any flexibility (ligature-aware search). None of the 9 zeolite-group sheets is among them, and neither apophyllite sheet is.
- Three localities are in Britain and Ireland: Portree on the Isle of Skye, ’S Airde Beinn on the Isle of Mull, and Ballhenry, Co. Antrim. The Handbook’s first analysis is Skye material. None of the 176 sheets names gyrolite, so the pairing with apophyllite is recorded on gyrolite’s sheet only.
| Species | Formula (Handbook) | Hardness | Density (measured) | Mass of 10 cm³ | Tenacity | Habit on the sheet | Optical sign |
|---|---|---|---|---|---|---|---|
| Gyrolite | NaCa16(Si23Al)O60(OH)8·14H2O | 3 to 4 | 2.388 to 2.390 | 23.9 g | Flakes flexible | Spherical to radial masses to 5 cm; platy, plumose | Uniaxial (−) |
| Fluorapophyllite | KCa4Si8O20(F,OH)·8H2O | 4.5 to 5 | 2.33 to 2.37 | 23.7 g | Brittle | Tabular to prismatic crystals to 20 cm, commonly pseudocubic | Uniaxial (+) |
| Hydroxyapophyllite | KCa4Si8O20(OH,F)·8H2O | 4.5 to 5 | 2.37 | 23.7 g | [Brittle] | Equant or tabular crystals to 15 cm | Uniaxial (+) |
| Calcite | CaCO3 | 3 | 2.7102 | 27.1 g | Brittle | Well-formed crystals common; perfect {10-11} cleavage | Uniaxial (−) |
| Quartz | SiO2 | 7 | 2.65 | 26.5 g | Brittle, tough when massive | Prismatic crystals; conchoidal fracture | Uniaxial (+) |
| Okenite | Ca5Si9O23·9H2O | Printed as .5 to 5; not quoted | 2.28 to 2.33 | 23.3 g | Elastic | Compact radial fibrous balls to 4 cm | Biaxial (−) |
| Stilbite | NaCa2Al5Si13O36·14H2O | 3.5 to 4 | 2.19 | 21.9 g | Brittle | Thin tabular; sheaflike or globular aggregates | Biaxial (−) |
What is gyrolite?
Gyrolite is NaCa16(Si23Al)O60(OH)8·14H2O: one sodium and sixteen calcium to twenty-three silicon and one aluminium, with hydroxyl and molecular water. The September 2026 IMA-CNMNC Master List gives the same formula, writing the water as (H2O)14, and records gyrolite as grandfathered, 1851, United Kingdom. The list names a country, not a mine; the Handbook sheet marks no type locality, but its first locality is Portree on the Isle of Skye and its first analysis is Skye material.
It is triclinic and pseudohexagonal, and on a microscopic scale it is twinned by rotation of 120° and 240° perpendicular to (001). What a collector sees is the habit the name records, from the Greek for round: spherical to radial masses to 5 cm, also massive, platy or plumose aggregates, and fibrous layers. It is colourless to white. The Handbook says it may be greyish, tan, yellowish, brownish or pale green from inclusions, and its lustre is vitreous to pearly.
The cleavage is perfect basal and poor prismatic, and the flakes are flexible. The sheet gives no mineral group, and it does not call gyrolite a zeolite. It occurs with zeolites, though: stilbite, laumontite and thomsonite are on its association line. The zeolite group itself is covered on which minerals are zeolites.
Gyrolite with apophyllite: how to tell them apart
Apophyllite is the first name on gyrolite’s association line, and the two share localities. The gyrolite sheet lists Nasik, Poona and Bombay in Maharashtra, India. Both apophyllite sheets in this site’s reference set, fluorapophyllite and hydroxyapophyllite, print the same three names. The link is one-way in the Handbook, though. A case-insensitive search of all 176 sheets for gyrolite finds it on none, including both apophyllite sheets.
Density cannot separate them. Gyrolite is 2.388 to 2.390 and fluorapophyllite 2.33 to 2.37, a gap of under 1% at the top of each range. Hardness can: gyrolite is 3 to 4 and both apophyllites 4.5 to 5. So can habit and tenacity. Apophyllite forms tabular to prismatic crystals to 20 cm, commonly pseudocubic and brittle. Gyrolite forms round, platy masses whose flakes bend. In a polarising microscope, gyrolite is uniaxial negative and fluorapophyllite uniaxial positive.
Flexible flakes are a strong clue. Method: all 176 sheets were converted with pdftotext -layout and searched case-insensitively for flexib. The search matched either the letters fl or the stray glyph that replaces the fl ligature on older sheets. It finds 18 sheets; a literal search finds only 14. The positive controls, muscovite and talc, print °exible and both match. They range from the micas and talc to stibnite, gypsum and vivianite. No zeolite-group sheet and neither apophyllite sheet is among them. Gyrolite’s own sheet is not in the set and is not counted. The species page is apophyllite.
Gyrolite on calcite, and gyrolite with quartz
Calcite is the last name on gyrolite’s association line. It is the harder case for a scratch test, because calcite is 3 and gyrolite 3 to 4. Density works better: calcite measures 2.7102, 13.4% denser than gyrolite’s 2.390. A 10 cm³ piece weighs 27.1 g in calcite and 23.9 g in gyrolite. Calcite is brittle, and it breaks on its perfect {10-11} rhombohedral cleavage. Gyrolite forms pearly balls that shed bendable flakes.
Quartz is not on gyrolite’s association line, so this page does not say how often the two occur together. It can say how to tell them apart. Quartz is hardness 7 and gyrolite 3 to 4, so quartz scratches gyrolite easily. Quartz measures 2.65, 10.9% denser than gyrolite, and has a conchoidal fracture and no flexible flakes. See how to tell calcite from quartz and how to test mineral hardness properly.
The white ball that is not gyrolite is okenite, which is also on the association line. Its sheet, which is not in the set, describes compact radial fibrous balls, to 4 cm, with an elastic tenacity and a density of 2.28 to 2.33. Okenite is fibrous and gyrolite platy; that is the difference to look for under a loupe. Okenite’s printed hardness, .5–5, is garbled on the sheet itself, so it is not used here.
What is gyrolite made of? A worked composition
On standard atomic weights, NaCa16(Si23Al)O60(OH)8·14H2O has a formula mass of 2685.38. As oxides that is SiO2 51.46%, CaO 33.41%, H2O 12.08%, Al2O3 1.90% and Na2O 1.15%. The charges balance: sodium, calcium, silicon and aluminium supply +128, and sixty oxygens and eight hydroxyls supply −128. The sheet prints no ideal column, so these figures are ours, from the formula shown.
The two analyses on the sheet sit close for silica and lime. The Isle of Skye analysis reads SiO2 50.70%, CaO 33.24% and H2O 14.18%, with no sodium reported. The sheet recalculates it to Ca15.80Mg0.12(Si23.20Al0.80)O60(OH)7.04·18.12H2O. The Qarusait, Greenland, analysis by XRF and AA reads SiO2 50.88%, CaO 31.34%, Na2O 1.34% and H2O 13.68%, which is 16.72 H2O to the formula. Both analyses carry more water than the ideal 14 H2O. The sheet does not say why, and we do not guess.
Against apophyllite: fluorapophyllite, KCa4Si8O20F·8H2O written as its fluorine end member, is 15.89% H2O and 24.73% CaO on the same method. Gyrolite has more lime and less water. That is a difference for an analysis, not for the hand.
Where gyrolite is found, including Skye, Mull and Antrim
The Handbook records gyrolite as replacements of wall rocks and in vugs, amygdules and veinlets in basalts. It also occurs in hydrothermally altered rhyolites and sediments, and in some ore deposits.
The distribution, in the sheet’s order: Portree, Isle of Skye, and ’S Airde Beinn, Isle of Mull, Scotland; Ballhenry, Co. Antrim, Ireland; Ortano, Elba, and Monte Biaena, Pannone, Trentino-Alto Adige, Italy; Ljósá, Eysturoy, and Gjónoyri, Streymoy, Faeroe Islands; Qarusait and Tapaursak, Greenland; Nasik, Poona, Bombay Island and around Bombay, Maharashtra, India. In Japan it is recorded at Sawayama Lake, Nagano Prefecture, at Irakawa, Yamagata Prefecture, and at Otemo, Niigata Prefecture. In the USA it is recorded at Fort Point, San Francisco Co., and the New Almaden mine, Santa Clara Co., California; Yellowstone National Park, Wyoming; and the Goose Creek quarry, Leesburg, Virginia. The last entry is 40 km southwest of Cape Blomiden, between Margaretville and Port George, Nova Scotia. The sheet adds that a few other localities are known.
Against the 176 sheets in the set (string search of each Distribution field): Skye is on the gonnardite and scolecite sheets, at Fiskavaig Bay and Talisker Bay, not Portree. The Maharashtra localities are on 9 sheets, among them stilbite, heulandite and both apophyllites. The rest of the silicates are indexed at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.