Pectolite
Pectolite is NaCa2Si3O8(OH): triclinic, hardness 4.5 to 5, density 2.84 to 2.90, radiating needles. Larimar is its blue form. Natrolite reads 1.8 g lighter.
Pectolite is a sodium calcium silicate, NaCa2Si3O8(OH): triclinic, hardness 4.5 to 5, density 2.84 to 2.90, usually white radiating needles in basalt cavities. Larimar is its blue variety from the Dominican Republic, its colour attributed by Gems & Gemology to traces of copper. Serandite is its manganese analogue; natrolite, which grows with it, is a look-alike.
In short
- One formula, two sources agreeing. The September 2026 IMA-CNMNC Master List lists pectolite as NaCa2Si3O8(OH), status G, 1828, Italy, and the Handbook of Mineralogy sheet prints the same formula. Worked on standard atomic weights it is 9.32% Na2O, 33.74% CaO, 54.23% SiO2 and 2.71% H2O, exactly the average composition GIA’s Summer 2013 Gem News note gives. GIA places it in the wollastonite group. By script, the IMA list has no row for larimar.
- The 1.8 g gap. Pectolite and natrolite both form white radiating needles in basalt, and their sheets name four of the same places, by script. A 20.00 g piece reads 12.96 to 13.10 g in water as pectolite and 10.91 to 11.15 g as natrolite: at least 1.81 g apart. Pectolite’s lowest index, 1.592, is 0.096 above natrolite’s highest.
- Where the gap fails, documented. Woodruff and Fritsch (Gems & Gemology, 1989) measured a green Dominican pectolite with numerous natrolite inclusions at specific gravity 2.62, which is 12.37 g in water for 20.00 g. Natrolite is a common inclusion in the blue material, so a mixed piece reads between the two species.
- Larimar is blue pectolite. The same paper describes it from a single volcanic deposit near Baoruco, Barahona province, Dominican Republic; it was reportedly found in 1974. It attributes the colour to traces of copper, measured at 46 to 47 ppm, and found the colour unchanged after more than 50 hours under a FiberLite lamp. The Handbook sheet names no Dominican locality.
- The manganese series, worked. On their sheet analyses, manganese is 0% of the calcium-manganese site in the Thetford mines pectolite, 68.7% in the Rouma Isle serandite and 91.8% in the Tanohata serandite. Serandite’s density, 3.34, reads 14.01 g in water.
| Material | Crystal system | Hardness | Density or SG | Refractive indices | 20.00 g piece in water | Luminescence recorded |
|---|---|---|---|---|---|---|
| Pectolite, NaCa2Si3O8(OH) | Triclinic | 4.5 to 5 | 2.84 to 2.90 | α 1.592 to 1.610; γ 1.630 to 1.645 | 12.96 to 13.10 g | Sheet: may be triboluminescent |
| Natrolite, Na2(Si3Al2)O10·2H2O | Orthorhombic | 5 to 5.5 | 2.20 to 2.26 | α 1.473 to 1.483; γ 1.485 to 1.496 | 10.91 to 11.15 g | Sheet: commonly fluoresces orange to yellow under UV |
| Serandite, NaMn2+2Si3O8(OH) | Triclinic | 5 to 5.5 | 3.34 | α 1.668; γ 1.703 | 14.01 g | Sheet: none printed |
| Blue pectolite (Larimar), Woodruff and Fritsch | Polycrystalline | Somewhat over 5 to 6, by scratch tests | 2.87 fine blue; 2.90 with chalcocite; 2.62 green with natrolite | 1.59 to 1.63 | 13.03 g; 13.10 g; 12.37 g | Long-wave UV: moderate, zoned, mostly chalky green |
| Ceramic imitation of Larimar (GIA, 2013) | 1.87 | Spot 1.53 | 9.30 g | |||
| Chrysocolla-stained chalcedony (Woodruff and Fritsch) | 2.24 or lower | 1.46 to 1.57 | 11.07 g or less | |||
| Pectolite against natrolite (sheets) | Different systems | Ranges touch at 5 | Pectolite 25.7% denser at the closest | Gap of 0.096 | At least 1.81 g apart | Natrolite’s sheet records fluorescence; pectolite’s has no fluorescence line |
What is pectolite? A silicate named for being compact
The names. The September 2026 IMA-CNMNC Master List gives pectolite as NaCa2Si3O8(OH), status G for grandfathered, with the year 1828 and the country Italy. The Handbook sheet prints the same formula and the Name field from the Greek for compact, in allusion to its resistance to pulverization; the International Gem Society instead derives it from pektos, congealed. Both derivations are printed because the two sources differ. The GIA Gem News note of 2013 calls pectolite a mineral of the wollastonite group; the sheet prints no group line.
What it looks like, per the sheet. Triclinic. Crystals may be tabular but are commonly acicular, to 15 cm; also radiating fibrous, spheroidal or columnar, and fine-grained massive. Twinned with twin axis [010] and composition plane near {100}, commonly. Perfect cleavage on {100} and {001}; brittle, but tough when compact. Colourless, whitish, greyish or yellowish, with a silky to subvitreous lustre. Hardness 4.5 to 5; density 2.84 to 2.90. The only luminescence on the sheet is may be triboluminescent; a ligature-aware search finds no fluorescence line, which is a blank field, not a tested absence.
The composition check. On standard atomic weights NaCa2Si3O8(OH) has a formula mass of 332.40 and is 9.32% Na2O, 33.74% CaO, 54.23% SiO2 and 2.71% H2O. GIA’s 2013 note gives the average composition of pectolite as 54.23% SiO2, 33.74% CaO, 9.32% Na2O and 2.71% H2O: the same four numbers. The sheet’s Thetford mines analysis, 54.18 SiO2, 33.36 CaO, 8.72 Na2O and 2.74 H2O, sits close to them. The 1997 IMA zeolite report does not mention pectolite, by a text search in which natrolite is the positive control.
Pectolite or natrolite? The 1.8 g gap
The 1.8 g gap: a 20.00 g piece of pectolite weighs 12.96 to 13.10 g in water, and one of natrolite 10.91 to 11.15 g, so a reading of 12 g or more rules natrolite out. The densities are 2.84 to 2.90 and 2.20 to 2.26; at the closest ends pectolite is 25.7% denser. The weighing method is on how to measure specific gravity at home. Hardness does not help: pectolite is 4.5 to 5, natrolite 5 to 5.5, and the ranges touch at 5.
Why the two get confused. Both are white to colourless, both grow as acicular or radiating fibrous aggregates, and both occur in cavities in basalt. Of 23 place names searched by script in the two Distribution fields, four appear on both: the Lovozero and Khibiny massifs of the Kola Peninsula, Bergen Hill in New Jersey and Mont Saint-Hilaire in Quebec. Optically they do not overlap: pectolite’s α starts at 1.592, natrolite’s γ ends at 1.496, a gap of 0.096. The natrolite sheet says it commonly fluoresces orange to yellow under UV; the pectolite sheet has no fluorescence line, so the lamp is at most a pointer. The species page for the zeolite is natrolite; its old-label decoder is not repeated here.
Where the gap fails, documented. Woodruff and Fritsch, “Blue Pectolite from the Dominican Republic”, Gems & Gemology, Winter 1989 measured five Dominican stones hydrostatically at 2.62 to 2.87, and a specimen with numerous chalcocite inclusions at 2.90. The lowest, a green stone, contained numerous natrolite inclusions, which they give as SG 2.20 to 2.25. At 2.62 a 20.00 g piece reads 12.37 g in water, between the two pure species. The same paper describes natrolite as a common inclusion, in light grey sprays of elongated square crystals that leave square or lozenge-shaped pits on a cabochon cut across them. A mixed piece is not one species, and no scale reading will name it.
Is larimar pectolite? What Gems and Gemology found
Larimar is blue pectolite. Woodruff and Fritsch identified six samples as pectolite by X-ray powder diffraction. They place the deposit near Baoruco, a village south-southwest of Barahona, in a single volcanic deposit of about 0.15 km², the material first found as waterworn pieces in the Rio Baoruco. They say Norman Rilling, a Peace Corps volunteer, reportedly found blue stones there in 1974, that many unverified stories exist about the find, and that the trade name Larimar, used since 1975, was coined by Miguel Mendez from his daughter’s nickname Lari and the Spanish mar, sea. GIA’s 2013 note says the light blue variety has only been reported from the Dominican Republic.
The colour. The paper measured 46 to 47 ppm copper and attributes the blue to traces of it, adding that the details of the electronic transitions involved were not known. In their test, a slice kept at the end of a FiberLite lamp for more than 50 hours showed no noticeable change against its unexposed half. A jeweller’s torch whitened another slice as small cracks spread, and gamma irradiation turned a slab mostly violet. These are the paper’s laboratory observations, not care instructions.
The imitations on record. Woodruff and Fritsch name “Victoria stone” and “Imori stone”, both devitrified glasses, and separate chrysocolla-stained chalcedony by index (1.46 to 1.57 against 1.59 to 1.63) and SG (2.24 or lower against 2.87). GIA’s 2013 note describes a ceramic imitation at SG 1.87 and spot index 1.53, with gas bubbles under the microscope. The separation from hemimorphite is on is hemimorphite the same as larimar and is not repeated here. Hobbs (Gems & Gemology, 1982) records “Alaska jade” and “Pectolite jade” as names for pectolite; the jade names are on is jade a mineral.
Pectolite and serandite: the manganese series, worked
The series. The pectolite sheet says it forms a series with serandite, and the serandite sheet says the same in reverse. The IMA list writes serandite NaMn2+2Si3O8(OH), status Rn, 1931, Guinea: manganese in the place of calcium. Its sheet gives hardness 5 to 5.5, density 3.34, indices α 1.668 to γ 1.703, and colours of rose-red, orange, salmon-pink, light pink, colourless, brown and black. As an end member it would be 39.18% MnO by mass.
The manganese share, worked from the sheets’ analyses as molar MnO over MnO plus CaO: the Thetford mines pectolite prints no MnO, so 0%; the Rouma Isle, Guinea serandite, MnO 28.99 and CaO 10.42, gives 68.7%; the Tanohata mine, Japan serandite, MnO 37.33 and CaO 2.62, gives 91.8%, against the 91.7% implied by the sheet’s own formula line, Mn 1.88 and Ca 0.17. Woodruff and Fritsch describe pectolite and serandite as an isostructural series and tie a violet colour after irradiation to trace manganese.
In the hand. At 3.34, a 20.00 g serandite reads 14.01 g in water, about 0.9 g heavier than the densest pectolite on its sheet. Of the place names searched by script, three appear on both sheets: the Lovozero massif, Mont Saint-Hilaire and Granite Mountain near Little Rock, Arkansas. Pink manganiferous material on old labels is a reason to measure rather than assume.
Where pectolite is found: Ayrshire, Quebec, New Jersey and Kola
The Occurrence field: a primary mineral in nepheline syenites; a hydrothermal mineral in cavities in basalts and diabases; in serpentinites and peridotites; and from metamorphosed high-calcium rocks. Its associates are zeolites, datolite and prehnite.
The Distribution field opens numerous localities and names: Mt Baldo and Mt Monzoni, Trentino-Alto Adige, Italy; Niederkirchen near Wolfstein and Rauschermühle, Rhineland-Palatinate, Germany; Lendalfoot, Ayrshire, Scotland; Zlto, Sweden; the Lovozero and Khibiny massifs, Kola Peninsula, Russia; Paterson and Bergen Hill, New Jersey; Magnet Cove and Granite Mountain near Little Rock, Arkansas; fine crystals from the Jeffrey mine, Asbestos, and Mont Saint-Hilaire, Quebec; Ahmadnagar, Maharashtra, India; Bou Agrao, High Atlas Mountains, Morocco; and Pilansberg, South Africa.
What the sheet does not name is the Dominican Republic, by script; the Larimar locality comes from Gems & Gemology, as above. The one British locality on the sheet is Lendalfoot, Ayrshire, and we add none. Fibrous minerals in general, and what the law calls asbestos, are covered on toxic minerals in a collection; nothing here is safety advice. The reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.