SERPENTINE

Serpentine – Mg3Si2O5(OH)4 – is a very common phyllosilicate, occurring mainly in metamorphic rocks. It is a component of rocks with varied applications, such as soapstone (steatite) and agalmatolite.

“Serpentine” is actually not a mineral, but just a generic term used for minerals in the Kaolinite-Serpentine Group. The Group is composed of:

Clinochrysotile – Mg3(Si2O5)(OH)4 monoclinic/triclinic,

Orthochrysotile – Mg3Si2O5(OH4) orthorhombic/pseudohexagonal,

Lizardite – Mg3(Si2O5)(OH)4 hexagonal,

Antigorite – (Mg,Fe2+)3(Si2O5(OH)4 monoclinic.

In volumetric terms the lizardite is the most common. Group minerals are usually only differentiated by X-Ray Diffractometry and other analytical techniques.

1. Characteristics

Crystal system: Hexagonal, pseudo-ortorhombic.

Color: Green, pale yellow, white.

Habit: Usually in extremely small flakes. Massive, tabular crystals up to 2 mm.

Cleavage: {0001} perfect.

Tenacity: Flexible.

Twinning: No.

Fracture: No information available.

Mohs Hardness: 2.5

Parting: No.

Streak: White.

Lustre: Waxy.

Diaphaneity: Transparent.

Density (g/cm³): 2.55

 

2. Geology and Deposits

Serpentine is a mineral that occurs in mafic to ultramafic rocks that have undergone hydrothermal metamorphism, with serpentine replacing olivine and pyroxene. In these rocks, olivine and pyroxene crystals partially substituted by serpentine may occur. Rocks composed of serpentine are known as serpentinites.

Lizardite is a product of retrometamorphism, typically from lower temperatures (<250º), replacing olivine, orthopyroxene and other minerals in ultramafic igneous rocks. It has three varieties and two polytypes.

Chrysotile also is from lower temperatures (<250º), has three varieties and three polytypes.

Antigorite is the high temperature serpentine (>250º) and has eight varieties.

 

3. Mineral Associations

Serpentine occur associated with common minerals such as quartz, feldspars (albite), pyroxenes (diopside), amphiboles (actinolite, hornblende-(Fe)), garnet (grossular, andradite), titanite, epidote and olivine.

With other phyllosilicates such as talc and chlorite (clinochlore), as well as with “asbestos” (acicular or fibrous minerals, these can be pyroxenes or, more commonly, amphiboles).

It occurs also with carbonates (calcite, dolomite, magnesite, hydromagnesite, hydrotalcite), oxides (hematite, magnetite, ilmenite, chromite, chromite-(Mg)) and hydroxides (brucite).

With rarer minerals such as chondrodite, deweylite, bultfonteinite and stichite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.583 – 1.554     nβ: 1.546 – 1.560     nγ: 1.546 – 1.560

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless to faint green. Pleochroism absent or weak in pale green tones:

X,Y = yellow-green,

Z = pale green.

Relief: Low.

Cleavage: Antigorite: {001} perfect; lizardite: basal cleavage; chrysotile: fibrous, has no cleavage. Usually the cleavage of antigorite and lizardite is not visible under the microscope.

Habits: Anhedral, very small grains, asbestiform habit (fibers ± parallel), foliate, mesh texture (mesh texture in minerals altering serpentine), no pleochroic halos. Chrysotile is fibrous, lizardite and antigorite tend to be tabular, laminar crystals, but lizardite can also be fibrous.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.001 to 0.01: corresponds to 1st order medium colors, gray in various shades and white, at most pale yellow, does not reach orange.

Extinction: Parallel to fibers, cleavage or crystalline boundaries.

Elongation sign: ES(+) or ES(-), itis not diagnostic.

Twins: Rare.

Zoning: No.

CONVERGENT LIGHT

Character: B(-). Almost always the crystals are so small that it is impossible to obtain interference figures.

2V angle: 10 – 90º, it can be almost zero. Simulates being uniaxial!

Alterations: chrysotile alters to antigorite under low-grade metamorphism conditions. Antigorite alters to talc.

May be confused with: with the exception of chrysotile when it occurs as parallel fibers in veins (fibers perpendicular to the direction of the veins), the 3 members can only be differentiated by X-Ray Diffractometry. As serpentine is often a product of alteration, it occurs in bands or zones, mixed with other minerals, which makes their identification difficult.

Chlorite shows weaker plechroism and, in CPL, generally anomalous interference colors.

Low birefringence and low relief differentiate serpentine from fibrous amphiboles and micas.

Serpentine 01: Serpentine in serpentinite in PPL. Serpentine is colorless and with low relief, rarely pale green.
Serpentine 01: The same section as of the previous image, now in XPL. Serpentine shows this typical texture – not large individual crystals, but clusters of extremely small crystals forming irregular alignments.
Serpentine in XPL. viewed with the lowest magnification objective (2.5x). In some cases, the serpentine clusters have a better development, in other cases their development is not so defined.
Again, as in the previous image, serpentine viewed with the lowest magnification objective (2.5x).
Serpentine in marble with large calcite crystals and serpentine pseudomorphoses on olivine. The original olivine grains have been completely replaced with serpentine. XPL.
Detail of the olivine pseudomorphosis from the previous images. Olivine can form very complex geometric designs. XPL.

5. Reflected Light Microscopy

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

Sample preparation: polishing of serpentine is simple and acquires a very good quality, but coarse-grained abrasives should be avoided during polishing due to the low hardness of the mineral.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, like quartz and feldspar. It is not as dark as pyroxenes, amphiboles and chlorite.

Pleochroism: No.

Reflectivity: Low (<10%).

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy was not observed.

Internal reflections: Generalized white, clear to milky. They can be grayed out.

May be confused with: when serpentine is well developed and with a typical habit (see images below), its identification is not difficult. When it occurs as alteration material in the midst of other minerals, identification is difficult to impossible, as white to light reflections occur in many minerals.

General Characteristics: 

Polishing scratches are practically unavoidable.

Serpentine 01: In PPL, altered pyroxene on the left (darker gray) and, on the right, serpentine (lighter gray, polishing grooves). In the serpentine, very small grains of magnetite (much lighter).
Serpentine 01: The same section as of the previous image, now in XPL. Pyroxene has yellow internal reflections, magnetite is isotropic (black) and serpentine has white internal reflections and the characteristic habit.
Serpentine 02: In PPL, serpentine (dark, grooves) and magnetite (almost white, high relief). Magnetite is a very common opaque associated to serpentine in serpentinites.
Serpentine 02: The same section as of the previous image, now in XPL. Magnetite is isotropic and serpentine shows a well-developed habit.
In XPL, seprentine with the typical habit and whitish internal reflections.
In XPL, serpentine with its habit amd clear to white internal reflections.
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