Mesolite
Mesolite is a natrolite-group zeolite, hardness 5, density 2.26, in hairlike tufts. Birefringence 0.0005 is the lowest of 132 measurable sheets in our set.
Mesolite is a sodium-calcium zeolite of the natrolite family, Na2Ca2Al6Si9O30·8H2O, orthorhombic, hardness 5 and measured density 2.26, in hairlike tufts and prismatic crystals to 20 cm in basalt cavities. Its birefringence, 0.0005, is the lowest of the 132 sheets in this site's 176-sheet reference set that print both extreme refractive indices.
In short
- Mesolite's birefringence is 0.0005, the lowest of 132 measurable sheets in this site's reference set. Next come hydroxyapophyllite at 0.001, fluorapophyllite at 0.002 and gonnardite at 0.002 to 0.005. Scolecite is 0.008 to 0.010 and natrolite 0.012 to 0.013 — sixteen to twenty-six times mesolite's figure.
- Under crossed polars that is the difference between near-black and grey-white. At 30 µm thickness, retardation is 15 nm for mesolite against 240 to 300 nm for scolecite and 360 to 390 nm for natrolite. The worked calculation is below.
- The mesolite cell is one natrolite plus two scolecites long. Its b edge is 56.655 Å; natrolite's b plus twice scolecite's is 18.613 + 2 × 18.981 = 56.575 Å, within 0.14%. That matches the formula arithmetic on our scolecite page.
- Hardness and density do not separate it from its neighbours: mesolite 5 and 2.26, natrolite 5 to 5.5 and 2.20 to 2.26, scolecite 5 to 5.5 and 2.25 to 2.29. Habit, twinning and optics do.
- The sheet records no fluorescence for mesolite, where natrolite's says it commonly fluoresces orange to yellow and scolecite's that it may fluoresce yellow to brown. That is a pointer, not a test: a blank field is not proof of absence.
| Species | Formula | Hardness | Density | Refractive indices | Birefringence | Optic sign | Fluorescence on the sheet |
|---|---|---|---|---|---|---|---|
| Mesolite | Na2Ca2Al6Si9O30·8H2O | 5 | 2.26 | 1.5048 to 1.5053 | 0.0005 | + | None recorded |
| Natrolite | Na2Al2Si3O10·2H2O | 5 to 5.5 | 2.20 to 2.26 | 1.473 to 1.496 | 0.012 to 0.013 | + | Commonly orange to yellow under UV |
| Scolecite | CaAl2Si3O10·3H2O | 5 to 5.5 | 2.25 to 2.29 | 1.507 to 1.521 | 0.008 to 0.010 | − | May fluoresce yellow to brown, SW and LW |
| Gonnardite | Na2CaAl4Si6O20·7H2O | 5 | 2.25 to 2.36 | 1.497 to 1.513 | 0.002 to 0.005 | + or −; commonly zoned | None recorded |
Is it mesolite, natrolite or scolecite?
This is the question every white radiating zeolite from a basalt raises, and the answer starts with what does not work. Hardness and density overlap across all three, and all three have perfect {110} cleavage; our scolecite page sets out that comparison in full and concludes that twinning and habit are what remain in the hand.
Habit first. The Handbook describes mesolite as prismatic crystals to 20 cm but commonly in hairlike tufts and aggregates of fibres, divergent radiating compact masses and fibrous stalactites. Natrolite forms short to long prisms to a metre, in stellate and interlacing groups; scolecite to slender prisms, often square in section, to 30 cm. Mesolite is characteristically twinned on {010} or {100}.
Optics second, and here mesolite is distinctive. Its refractive indices, 1.5048 to 1.5053, sit between natrolite's lower values and scolecite's higher ones, its optic sign is positive where scolecite's is negative, and its birefringence is so low that a fibre under crossed polars stays nearly dark. Anyone with a polarising microscope can make that observation on a detached fibre; see the best microscope for mineral specimens.
Mesolite has the lowest birefringence in this site's reference set
Method. Each of the 176 Handbook sheets in the reference set was converted with pdftotext -layout and the Optical Class field parsed for α and γ (biaxial) or ω and ε (uniaxial). Birefringence is the difference, taken at both ends of any printed range. 132 sheets print both indices as numbers: 130 were parsed by script and two, chalcophyllite and scheelite, read by hand because of their layout. The remaining 44 are opaque, isotropic, or print one index or none.
Result: mesolite, at 1.5053 − 1.5048 = 0.0005, is the lowest of the 132. Then hydroxyapophyllite 0.001, fluorapophyllite 0.002, gonnardite 0.002 to 0.005, vesuvianite and pharmacosiderite from 0.003. Isotropic species such as analcime, fluorite and halite have no birefringence at all and are outside this comparison; mesolite is the lowest among species that are birefringent and have both indices on their sheet.
Two of the four lowest, mesolite and gonnardite, are zeolites, and the two between them are the apophyllites, which share the Poona localities. That is worth knowing for the reason below: low birefringence is a practical identification aid in exactly the material where hardness and density fail.
What 0.0005 looks like under crossed polars: a worked calculation
The interference colour a grain shows between crossed polars depends on its retardation, which is thickness × birefringence. Take a standard 30 µm section or a fibre of that thickness, which is 30,000 nm:
Mesolite: 30,000 × 0.0005 = 15 nm. Gonnardite: 60 to 150 nm. Scolecite: 240 to 300 nm. Natrolite: 360 to 390 nm.
On the Michel-Lévy chart, 15 nm is effectively black to dark grey, while 240 to 390 nm falls in first-order grey-white to white. A white zeolite fibre that stays almost dark in every orientation between crossed polars is behaving like mesolite; one that lights up grey-white is behaving like scolecite or natrolite. Substitute your own fibre thickness — retardation scales linearly, so a 10 µm fibre of mesolite gives 5 nm and of natrolite 120 to 130 nm, and the contrast holds.
It is a pointer, not a determination: orientation, thickness and intergrowth all affect what you see, and a confirmed name still needs analysis.
Why the mesolite cell is one natrolite and two scolecites long
Our scolecite page shows that the mesolite formula is exactly one natrolite plus two scolecites, atom for atom, and that its name, from the Greek for middle, says so. The unit cell says the same thing independently.
From the three Handbook sheets: natrolite b = 18.613 Å, scolecite b = 18.981 Å, mesolite b = 56.655 Å. One natrolite b plus two scolecite b is 18.613 + 37.962 = 56.575 Å, 0.080 Å or 0.14% short of the measured mesolite edge. The a edges agree too: mesolite 18.4049 against natrolite 18.272 and scolecite 18.508. All three share Z = 8 and the Fdd2-type or Cc cells of the family.
A cell three repeats long, built of one sodium repeat and two calcium repeats, is what an ordered 1:2 arrangement looks like in the cell data — and it is why mesolite is a species in its own right rather than a midpoint on a continuous series.
Where mesolite forms, and the localities that matter
The Handbook places mesolite in cavities in volcanic rocks, typically basalt, and also in andesites, porphyries and hydrothermal veins, with natrolite, scolecite, other zeolites and calcite. It notes there are many localities but only a few provide large crystals or rich masses.
Those named are: North Mountain, Gates Mountain and Cape Blomidon in Nova Scotia; Table Mountain in Colorado; large crystals from Skookumchuck Dam near Bucoda, Washington; Goble, Dollar, Shotgun Creek, Springfield and Ritter Hot Springs in Oregon; Puy de Marman in the Puy-de-Dôme; the Breiddalur–Berufjord area of Iceland; Naalsoy, Streymoy and others of the Faeroe Islands; and exceptional crystals from the Pashan Hills and other localities around Poona, Maharashtra. The IMA list gives Iceland, with a question mark, as the type country.
Handle tufts by the matrix, never the fibres. The sheet records mesolite as brittle, though compact masses are tough, and a hairlike tuft has no mechanical strength at all. Our handling and storage note covers the general case. The rest of the zeolites 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.