LAWSONITE

Lawsonite – CaAl2(Si2O7)(OH)2.H2O – is a very rare sorosilicate, typical of very specific metamorphic rocks. It has no economic importance.

It is classified in the Lawsonite Group, has an Al analogue (cortesognoite) and has a variety with Cr.

1. Characteristics

Crystal system: Orthorhombic bipiramidal.

Color: Colorless, white, greyish blue, pale blue.

Habit: Prismatic to tabular, usually in small crystals. It can be massive or granular.

Cleavage: {001} perfect, {110} poor.

Tenacity: Brittle.

Twinning: Common on {101}, lamellar and polysynthetic.

Fracture: Irregular.

Mohs Hardness: 7.5

Parting: No.

Streak: White.

Lustre: Vitreous, resinous.

Diaphaneity: Transparent.

Density (g/cm³): 3.1

 

2. Geology and Deposits

Lawsonite is a mineral that occurs in high-pressure and low-temperature metamorphic rocks such as blueschists, being an index mineral for the blueschist facies.

It is common in low-grade mafic metamorphic rocks such as metagabbros, metabasalts and metadiorites, where it forms by saussuritization of plagioclases.

Sometimes it occurs in greenschists (with chlorite), in marbles and in amphibolites.

 

3. Mineral Associations

Lawsonite occurs with quartz, feldspars (albite), amphiboles (glaucophane, riebeckite), micas (muscovite, chlorite, sericite), titanite, epidote, clinozoisite, pumpellyite, jadeite and chalcopyrite.

In marbles with calcite and garnet.

 

4. Transmitted Light Microscopy

Refraction indices: nα:  1.663 – 1.665  nβ: 1.672 – 1.676      nγ: 1.684 – 1.686

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Usually colorless. May be pleochroic in:

X = pale brownish yellow blue,

Y = deep bluish green, yellowish green and

Z = colorless, yellowish.

In thick thin sections (>30mμ) it presents pale blue, bluish-gray, blue-yellow and green-yellow colors. In these cases, it presents pleochroism between X = blue, Y = yellow and Z = colorless.

Relief: Moderate to high.

Cleavage: Perfect at {100} and {010}, which intersect at 90º.

In addition, there is imperfect cleavage according to {101}, which intersect at 67º from each other. Therefore, depending on the section, there will be cleavages with one angle or another.

Habits: Usually euhedral, tabular and prismatic crystals according to z. Rhombohedral, rarely acicular. Basal sections are rhombic or square.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.019 to 0.021: upper 1st order orange colors to maximum 2nd order blue, in standard thickness thin sections (30 mμ).

Extinction: Parallel in longitudinal sections, symmetrical in basal sections. May show undulating extinction.

Elongation sign: ES(-) in the longitudinal sections, the major diagonal of the rhombic basal section is ES(+). It’s not diagnostic.

Twins: Commonly simple (010) and lamellar. Often polysynthetic twins on {101}. Parquet patterns with undulating extinction are possible.

Zoning: No.

CONVERGENT LIGHT

Character: B(+)

2V angle: 76 – 87º, tends to 84º

Alterations: sericitization and chloritization may occur. There is a substitution of lawsonite for epidote or pumpellyite in the transition to greenschist facies.

May be confused with: several other metamorphic minerals.

Clinozoisite and zoisite tend to have lower and/or anomalous interference colors.

Andalusite is similar, but has lower birefringence and occurs in other paragenesis.

Pumpellyite is very similar but has oblique extinction. Lawsonite is usually intercropped or occurs with pumpellyite.

Prehnite has other habits, lower relief, higher birefringence and sometimes anomalous colors.

Scapolite is uniaxial and usually shows only gray interference colors.

Tremolite has amphibole cleavage intersection angles (124º or 56º), better cleavage and oblique extinction.

Cancrinite has other habits, is U(-) and occurs in other paragenesis.

 

Lawsonite 01: in the center of the image, in PPL, several lamellar crystals of lawsonite, colorless and with moderate relief.
Lawsonite 01: the same section as in the previous figure, now in XPL. Lawsonite shows strong 1st -order end colors.
Lawsonite 02: In PPL, in blueschist, glaucophane (blue), chlorite (greenish) and colorless lawsonite lamellae.
Lawsonite 02: the same section as in the previous figure, now in XPL. Glaucophane has dark bluish colors, chlorite is gray and lawsonite has strong colors. One of the lamellae has thin polysynthetic twins.
In XPL, lamellar lawsonite crystals, with glaucophane and chlorite around them, in blue shale. The two bluish colored crystals show polysynthetic twins.
Lawsonite 03: Lawsonite rhombic basal sections in PPL . Its shape is similar to those of amphiboles, but they do not show the typical two cleavages of the amphiboles.
Lawsonite 03: the same section as in the previous figure, now in XPL. Interference colors are between pale yellow, strong yellow, orange and late 1st order blue and the medium relief is well defined. Cleavage is difficult or impossible to see, much different from the basal sections of amphibole.
Lawsonite rhombic basal sections in XPL. Interference colors here reach green, indicating that the thin section is just above the standard 30 micra thickness. Top left, a basal section showing its symmetrical extinction.

5. Reflected Light Microscopy

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

Sample preparation: The polishing of lawsonite does not offer difficulties, but the glaucophanes that can occur in association, if in large crystals, can generate holes due to the excellent cleavage of the amphiboles they bear.

PLANE POLARIZED LIGHT – PPL

Reflection color: Very light gray, almost white, to medium grey.

Pleochroism: Distinct, between off-white gray and medium gray.

Reflectivity: Low (<<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Distinct anisotropy between light and dark gray, sometimes masked by internal reflections.

Internal reflections: Generalized, very luminous, colorless, milky and white, standing out a lot.

May be confused with: titanite, which shows similar reflection habits and color, but has internal reflections in different colors (caramel, yellow, reddish) and much stronger anisotropy.

Lawsonite 01: In PPL, center of image, cluster of several lawsonite crystals showing rhombic basal sections and a square basal section. Around them, glaucophane. In lawsonites, the two possible colors of pleochroism are distinctly perceived, varying between white and light gray.
Lawsonite 01: the same section of the previous figure, now in XPL. Lawsonite shows intense, luminous and very conspicuous internal reflections in white and occasionally some colored reflections. Anisotropy is not so easy to perceive. Around it, glaucophane with internal reflections in dark blue.
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