Thomsonite
Thomsonite is a zeolite series of two IMA species, thomsonite-Ca and thomsonite-Sr: hardness 5–5.5, density 2.23–2.39 for the common Ca member. The Sr line.
Thomsonite is a zeolite series of two IMA species: thomsonite-Ca, NaCa2(Al5Si5)O20·6H2O, and thomsonite-Sr, NaSr2(Al5Si5)O20·6-7H2O. Thomsonite-Ca, the common one, is orthorhombic, hardness 5–5.5, density 2.23–2.39, and may be concentrically zoned. The name follows Sr/(Sr + Ca): 0 in the Scottish analysis, 0.696 in the Kola one.
In short
- Two species, one series. A script over the September 2026 IMA-CNMNC Master List finds thomsonite-Ca, NaCa2(Al5Si5)O20·6H2O (Rn, 1997 s.p., United Kingdom), and thomsonite-Sr, NaSr2(Al5Si5)O20·6-7H2O (A, 2000-025, Russia), and no unsuffixed thomsonite row. Each Handbook of Mineralogy sheet says the two form an isomorphous series, and both print Zeolite group.
- The Sr line. The sheets say the suffix indicates the Ca-dominant member and alludes to the predominance of Sr rather than Ca. Worked as Sr/(Sr + Ca), the Old Kilpatrick and Goble analyses on the thomsonite-Ca sheet are 0, with no SrO line, and the Mt. Rasvumchorr analysis on the thomsonite-Sr sheet is 0.696 (Sr1.42 against Ca0.62). Above 0.5 the name is thomsonite-Sr.
- Documented failure: the 2.47 problem. For thomsonite-Ca, Z = 4 formula units in the printed cell give 2.371 against the printed calculated 2.366: it reproduces. For thomsonite-Sr the same check gives 2.623 for the sheet’s own analysis formula and 2.641–2.694 for 6 to 7 waters, against a printed calculated 2.61 and a measured 2.47(2). The measured density is about 0.15 below every calculation.
- What a specimen usually is. The thomsonite-Sr sheet records it only as prismatic crystals to 1 mm, or as zones in crystals of thomsonite-Ca to 0.02 mm, at two Khibiny localities. The thomsonite-Ca sheet calls its mineral a common zeolite with many localities known, in radiated, globular and compact habits that may be concentrically zoned. Why those zoned pieces are not agate is on is thomsonite an agate.
- Where it is found. The thomsonite-Ca sheet names good crystals from Old Kilpatrick and Bishopton, Scotland; Mt. Monzoni, Italy; the Czech Republic; Saxony; the Faeroe Islands; New Jersey, Colorado and Oregon; and Flinders, Victoria. It does not name Lake Superior, Michigan or Minnesota, and this page adds no locality from a gem-trade source.
| Formula or analysis | Sheet | Ca | Sr | Si | Sr/(Sr + Ca) | Name it takes | D measured | D printed calc. | D from cell, worked | 20.00 g in water |
|---|---|---|---|---|---|---|---|---|---|---|
| NaCa2(Al5Si5)O20·6H2O (IMA) | Thomsonite-Ca | 2 | 0 | 5 | 0 | Thomsonite-Ca | 2.23–2.39 | 2.366 | 2.371 | 11.03–11.63 g |
| Old Kilpatrick, Scotland | Thomsonite-Ca (1) | 2.00 | Not printed | 4.97 | 0 | Thomsonite-Ca | — | — | 2.371 | — |
| Goble, Oregon (microprobe) | Thomsonite-Ca (2) | 1.70 | Not printed | 5.52 | 0 | Thomsonite-Ca | — | — | 2.343 | — |
| NaSr2(Al5Si5)O20·6-7H2O (IMA) | Thomsonite-Sr | 0 | 2 | 5 | 1 | Thomsonite-Sr | 2.47(2) | 2.61 | 2.641 (6 H2O) to 2.694 (7 H2O) | 11.90 g |
| Mt. Rasvumchorr, Kola (microprobe, TGA) | Thomsonite-Sr (1) | 0.62 | 1.42 | 5.05 | 0.696 | Thomsonite-Sr | — | — | 2.623 | — |
What is thomsonite? Two IMA species in one series
Thomsonite is the series name for two zeolite species, thomsonite-Ca and thomsonite-Sr, with the same framework of five aluminium and five silicon tetrahedra per formula unit. The Handbook thomsonite-Ca sheet describes orthorhombic, pseudotetragonal crystals, commonly prismatic, acicular or bladed, flattened on {010}, elongated and striated ∥ [001], to 12 cm, in radiated spherical or columnar aggregates, and smooth, globular, botryoidal, compact or massive material. Cleavage is perfect on {010} and good on {100}; it is brittle, hardness 5–5.5, density 2.23–2.39, and pyroelectric. Colour is white, yellowish, pink, brown or greenish and may be concentrically zoned; lustre vitreous to pearly; biaxial (+), α = 1.497–1.530, β = 1.513–1.533, γ = 1.518–1.544.
What the lists record. The September 2026 IMA-CNMNC Master List has thomsonite-Ca, status Rn, 1997 s.p., United Kingdom, and thomsonite-Sr, approved as 2000-025, Russia, and no unsuffixed row. The Handbook likewise has no unsuffixed thomsonite sheet. The sheet names the mineral for Dr. Thomas Thomson (1773–1852), Scottish chemist and mineralogist of Glasgow, who first analyzed it. Neither sheet marks a type locality (what a type locality is).
Where it grows. In amygdules and fractures in mafic igneous rocks, typically basalts; in some alkalic igneous rocks, contact metamorphic zones and hypabyssal rocks; and as an authigenic cement in some sandstones. Its associates are zeolites, calcite, prehnite, datolite and quartz. That it is a zeolite, and how its silicon share compares with natrolite and scolecite, is set out on is thomsonite a zeolite.
Thomsonite-Ca or thomsonite-Sr? The Sr line
A specimen takes the suffix of whichever of calcium and strontium is dominant, so Sr/(Sr + Ca) above 0.5 makes it thomsonite-Sr. The thomsonite-Ca sheet’s Name line says its suffix indicates the Ca-dominant member of the series; the thomsonite-Sr sheet’s says its suffix alludes to the predominance of Sr rather than Ca. Both are the same rule read from either end.
Worked on the three printed analyses. Old Kilpatrick, Scotland, corresponds to Na1.04Ca2.00Al5.03Si4.97O20·5.95H2O and has no SrO line: Sr/(Sr + Ca) is 0. Goble, Oregon, by electron microprobe, Na1.09Ca1.70Al4.48Si5.52O20·6H2O: also 0. Mt. Rasvumchorr, Khibiny massif, Kola Peninsula, Russia, with SrO 16.27% and CaO 3.85%, corresponds to (Sr1.42Ca0.62Ba0.01)(Na0.94K0.03)[Si5.05Al4.91O20]·7.09H2O: 0.696, worked from the formula and again from the oxides. Even the Sr species carries about three-tenths of its large-cation site as calcium.
What a label should say. The thomsonite-Sr sheet records only prismatic crystals to 1 mm, or zones in crystals of thomsonite-Ca to 0.02 mm, and names only Mt. Rasvumchorr and Mt. Yuksporn in the Khibiny massif. Outside an analysed piece, then, the name a label can defend is thomsonite, the series name, or thomsonite-Ca with the basis stated. The Sr line cannot be read without an analysis: the two species overlap in hardness (5–5.5 against 5) and their indices overlap (thomsonite-Sr’s α 1.528 lies inside thomsonite-Ca’s 1.497–1.530).
Documented failure: why thomsonite-Sr’s density does not check
The calcium member’s density reproduces from its cell; the strontium member’s measured density sits about 0.15 below every calculation. Thomsonite-Ca’s printed cell, 13.088 × 13.052 × 13.229 Å, holds 2259.8 ų. Four formula units of NaCa2(Al5Si5)O20·6H2O, 806.55 g/mol, give 4 × 806.55 ÷ (0.60221 × 2259.8) = 2.371, against the printed 2.366; the Old Kilpatrick formula gives the same 2.371 and Goble’s 2.343, both inside the measured 2.23–2.39.
The strontium member. Thomsonite-Sr’s cell, 13.050 × 13.123 × 13.241 Å, is only 0.34% larger, while replacing two calcium with two strontium makes the formula 11.79% heavier. Worked the same way, the Rasvumchorr analysis formula gives 2.623, and the ideal formula 2.641 with 6 waters and 2.694 with 7. The sheet prints 2.61 calculated and 2.47(2) measured. This page cannot say why the measurement is low and does not offer a cause; it quotes both printed figures and uses the measured one for the balance.
On the balance. A 20.00 g piece reads 11.03–11.63 g in water as thomsonite-Ca and 11.90 g as thomsonite-Sr at the measured 2.47; at the printed 2.61 it would be 12.34 g. The measured gap between the species, 0.27 g at the top of the Ca range, is too small to name a specimen. The method is on how to measure specific gravity at home.
What is thomsonite stone? The zoned, globular habit
The polished “stone” is the compact, zoned habit of thomsonite-Ca. The sheet records radiated spherical aggregates and smooth, globular, botryoidal, compact and massive material, in colours that may be concentrically zoned. The International Gem Society thomsonite page describes a pinkish gray eyelike pattern in some cabochons and gives hardness 5–5.5 and specific gravity 2.25–2.40, in line with the sheet. The concentric pattern is why the question is thomsonite an agate gets asked; the answer is no, and the separation is on that page.
Thomsonite and its look-alikes. natrolite, mesolite and scolecite overlap thomsonite on the balance, as is thomsonite a zeolite shows; cleavage, perfect on {010} and good on {100} for thomsonite-Ca, is the better clue on a crystal large enough to show it. Whether a zeolite specimen is a hazard is a question this page does not answer; it is routed to toxic minerals in a collection, and nothing here is advice.
Where is thomsonite found? What the sheets print
The thomsonite-Ca sheet’s Distribution, as printed. A common zeolite with many localities known; a few for good crystals are from Old Kilpatrick, Dumbartonshire, and Bishopton, Renfrew, Scotland. At Mt. Monzoni, Val di Fassa, Trentino-Alto Adige, Italy. From Ostroh (Seeberg), Kadan (Kaaden) and Ústí nad Lábem (Aussig), Czech Republic. At Hammer-Unterwiesenthal, Saxony, Germany. From Nolsoy, Essuroy and Streymoy, Faeroe Islands. In the USA, at Paterson, Passaic Co., New Jersey; on North and South Table Mountain, Jefferson Co., Colorado; in Oregon, at Springfield, Drain, Goble, Ritter Hot Springs and elsewhere. At Flinders, Victoria, Australia. The thomsonite-Sr sheet adds Mt. Rasvumchorr and Mt. Yuksporn, Khibiny massif, Kola Peninsula, Russia.
Thomsonite and Lake Superior. People search for thomsonite from Lake Superior, Michigan and Minnesota beaches. Neither Handbook sheet names any of them, and this page does not add a locality from a gem-trade source. Other zeolites are in the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.