NATROLITE

Natrolite is a relatively common tectosilicate, a mineral of the Zeolite Group, Natrolite Subgroup, which typically occurs as a secondary mineral in vesicles and fractures of basic igneous rocks. It has no economic importance other than for mineral collectors.

In the Natrolite Subgroup there are three very common zeolites: scolecite (of Ca), natrolite (of Na) and mesolite (intermediate term). Macroscopically the three are very similar to each other; all form radiated aggregates of long prismatic crystals, usually colorless to white.

Natrolite can contain both Ca and K. There are six varieties, based on different colors or anomalous levels of certain elements. Natrolite is pyroelectric and piezoelectric, as is scolecite as well. Under ultraviolet light, it may fluoresce from orange to yellow, but it is not a diagnostic property as it occurs only sporadically.

1. Characteristics

Crystal system: Orthorhombic piramidal, pseudo-tetragonal.

Color: Usually colorless to white. Rarely red, yellow, brown, bluish or green.

Habit: Long prismatic, acicular to fibrous, forming radiated aggregates.

Cleavage: {110} perfect.

Tenacity: Brittle.

Twinning: On {110}, {011}, {031}, rare.

Fracture: Irregular.

Mohs Hardness: 5 – 5.5

Parting: On {010}.

Streak: White.

Lustre: Vitreous.

Diaphaneity: Transparent.

Density (g/cm³): 2.2 – 2.26

 

2. Geology and Deposits

Natrolite typically occurs as a secondary mineral filling cavities (vesicles) and fractures in intermediate and basic rocks (basalts, gabbros), especially in volcanic rocks. More rarely occurs in veins in granites, gneisses and nepheline-syenites. It can form by deuteric alteration in alkaline plutonic rocks such as nepheline-syenites, in this case commonly forming pseudomorphoses on plagioclase, nepheline and minerals of the Sodalite Group. It ocurrs autigenic in lake sediments and marine sediments, in this case cryptocrystalline.

A detailed account of the occurrence, forms and other aspects of natrolite can be found in Rudy Tschernich’s book “Zeolites of the World”, available for download on the internet. Detailed information on the various ways that natrolite occurs can be found on the website of the Commission on Natural Zeolites: http://www.iza-online.org/.

 

3. Mineral Associations

Natrolite occurs associated with the minerals that form the host rocks, generally mafic and ultramafic igneous rocks: olivine, pyroxene (aegirine), plagioclase, nepheline, sanidine, magnetite, ilmenite, chalcopyrite, pyrite, native copper and others.

It composes the secondary mineral assemblage, with calcite, macrocrystalline quartz, chalcedony, nepheline, sodalite, analcime, benitoite, neptunite, prehnite, fluoroapophyllite-(K), rhodochrosite, joaquinite-(Ce) and zeolites such as thomsonite-Ca, scolecite, mesolite, heulandite, stilbite and laumontite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.473 – 1.483   nβ: 1.476 – 1.486       nγ: 1.485 – 1.496

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, never shows color much less pleochroism.

Relief: Moderate.

Cleavage: {110} perfect, parallel to the length of the prisms.

Habits: Long prismatic to acicular, may be fibrous. Typically forms radial aggregates.

The basal sections are square (diagnostic!).

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.013, corresponding to colors between dark gray, light gray, white and yellow. It is usually light gray.

Extinction: Parallel to the cleavage in the longitudinal sections.

Symmetrical in the basal sections. (diagnostic!)

Elongation sign: SE(+)

Twins: On {110}, {011}, {031}, rare.

Zoning: No.

CONVERGENT LIGHT

Character: B(+)

2V angle: 58 – 64º

Alterations: may alter to other zeolites or clay minerals.

May be confused with: other zeolites and associated minerals.

Nepheline occur in the same paragenesis and also displays square or rectangular basal sections.

 

In XPL, longitudinal sections of natrolite, elongated, very similar to sections of other zeolites. Sometimes they are well-developed prisms; sometimes they are more irregular. In PPL they are completely colorless.
In XPL, basal sections of natrolite, square and very diagnostic. In PPL, they are colorless.
Natrolite 01: In sanidine-nephelinite, nosean (at right in the images) and a vertical vein filled with sprays of natrolite crystals. The natrolites are very luminous, sharp and clear.
Natrolite 01: same section as of the previous figure, now in XPL.
Natrolite 01: in XPL, detail of the natrolites in the sanidine-nephelinite. Scolecite and mesolite also form sprays of this kind.
Natrolite also develops as alteration of nosean crystals, as in this image. Nosean is isotropic (black). Image in XPL.
In some rocks, natrolite may host very small (few microns) inclusions of idiomorphic crystal of minerals of the Sodalite Group, frequently as rhombododecahedrons. Here, in this basal section of natrolite in XPL, are several of these inclusions. Leucite can show the same kind of inclusions.
In XPL, another basal section of natrolite with very small inclusions of Sodalite Group minerals.
In PPL, image of a basal section of natrolite with one of those very small idiomorphic inclusions of minerals of the Sodalite Group, this one with clear outlines of the rhombododecahedron. Around it, dozens of much smaller, very tiny, almost submicroscopic inclusions probably of the same type. This much smaller generation of inclusions was perceived only looking the picture in the laptop screen.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of natrolite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with natrolite, like copper.

Sample preparation: natrolite accepts polishing very easily, despite cleavages and long prism habit. Even with a quick polish, it ends up with a polishing quality much superior to that of quartz and feldspars, for example.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, like quartz and feldspars.

Pleochroism: No.

Reflectivity: Low (<<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anistropy was not observed.

Internal reflections: Widespread clear, milky, whitish, can be multicolored.

May be confused with: many other light colored transparent minerals. It is not possible to identify natrolite under Reflected Light. In some cases it is possible to observe some prismatic features and even some square basal sections, but this is not the rule and it is not a common diagnostic feature.

In PPL, natrolite on the top and the basaltic rock below. Natrolite shows good polishing and the absence of cleavages or other diagnostic features. In black, holes in the polished section.
The same section of the previous figure, now in XPL. There are no diagnostic features.
<