PHLOGOPITE

Phlogopite – K2Mg6Al2Si6O20(OH,F)4 – is a rarer phyllosilicate, but relatively common in parageneses with carbonates. It is an important industrial mineral. It is the second most mined mica, after muscovite.

Phlogopite is part of the biotite series, which consists of: phlogopite (Fe 0-20%, Mg 100-80%), meroxene (Fe 20-50%, Mg 80-50%), lepidomelane (Fe 50-80%, Mg 50-20%) and siderophyllite (Fe 80-100%, Mg 20-0%). The term biotite applies to the meroxene, lepidomelane and siderophyllite members.

It may contain, as impurities, Ti, Zn, Ni, Na, Mn, Ba, Cr, Ca, Li, Rb and H2O. There are nine varieties, based on higher levels of certain elements (Ba, Cr, F, Mn, etc.).

1. Characteristics

Crystal system:  Monoclinic.         

Color: Brown to reddish-brown. It can also be gray, green, or yellowish.     

Habit: Micaceous, tabular.       

Cleavage:  {0001} perfect, like all micas.      

Tenacity: Flexible.        

Twinning: No.       

Fracture: Micaceous.       

Mohs Hardness: 2 – 3

Parting: No.         

Streak: White.        

Lustre: Vitreous, pearly.          

Diaphaneity: Transparent.           

Density (g/cm³):  2.78 – 2.85

 

2. Geology and Deposits

Phlogopite is common in basic and ultrabasic igneous rocks (mafic and ultramafic) such as peridotites, mellilites, lamproites, and lamprophyres.

It occurs in metamorphic rocks that have undergone regional or contact metamorphism, such as dolomitic marbles, fenites, skarns, and schists (phlogopite-schists).

It also occurs in carbonatites and kimberlites.

 

3. Mineral Associations

It is associated with the following minerals:

– in kimberlites, olivine, diopside, calcite, and pyrope;

– in silica-undersaturated volcanic rocks, olivine, melilite, leucite, and nepheline;

– in siliceous marbles, calcite, quartz, tremolite, diopside, forsterite, spinel, members of the Chondrodite Group, pyrite, and lazurite;

– in carbonatites, calcite, fluorapatite, magnetite, ilmenite, pyrite, chalcopyrite, tremolite, actinolite, allanite, spinel, corundum (ruby), coppita, and other less common minerals. Carbonatites can contain more than 80 different minerals, including 16 types of carbonates.

Also erythrite and anthophyllite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.530 – 1.573      nβ: 1.557 – 1.617      nγ: 1.558 – 1.618

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Depending on the Fe content, colorless to yellowish, greenish to pale brown, rarely pink.

Sections // to (001) almost without pleochroism.
Oblique or perpendicular sections to (001) with strong pleochroism:
X = colorless, pale yellow, pale orange, pale pink;
Y = Z = brownish yellow, leather brown, reddish orange, reddish brown, pale reddish brown.

Relief: Moderate.           

Cleavage: {001} perfect. Typical basal cleavage of micas, easily seen in thin section as a perfect cleavage direction in longitudinal sections.

Habits: Lamellar and tabular, prismatic to pseudo-hexagonal.

May contain radioactive inclusions with dark (black) halos around them (zircon, monazite, apatite, etc.).

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.027 to 0.045: intense and colorful colors of the 2nd and 3rd order.
Basal sections with black to gray colors.  

Extinction: Parallel and mottled.
Rarely oblique with an angle of 0 to 5º. 

Elongation sign: ES(+), as all micas.            

Twins: Can occur, according to (001).         

Zoning: Rare, with a greenish core and lighter or darker edges. 

CONVERGENT LIGHT

Character: B(-), simulates being uniaxial.          

2V angle: 2-8º, rarely up to 15º         

Alterations: May alter to vermiculite.         

May be confused with: Diagnostic features include lamellar habit, perfect cleavage, and parallel and mottled extinction.

Biotite has higher relief, stronger colors and pleochroism, and a smaller 2V angle.

Colorless varieties of phlogopite can be distinguished from muscovite by their smaller 2V angle and paragenesis.         

Phlogopite 01: In PPL, in the center of the image, phlogopite showing, in this position, its first pleochroism color. Around it, calcite. The next image shows the second color.
Phlogopite 01: The same section as of the previous image, now rotated 90 degrees to show the second pleochroism color. Depending on the section, pleochroism is either absent or strong, and also depends on the iron content. Basal sections do not exhibit pleochroism.
In XPL, phlogopite showing strong and vivid interference colors. Around it, calcite.
Phlogopite 02: In PPL, phlogopite surrounded by calcite. It exhibits the typical habit of micas: perfect cleavage parallel to elongation, parallel and mottled extinction. Therefore, it is important to pay attention to the details for correct identification.
Phlogopite 02: The same section as of the previous image, now in XPL.
Phlogopite 03: In PPL, in carbonatite, phlogopite showing its first pleochroism color.
Phlogopite 03: The same section as of the previous image, now rotated 90 degrees to show the second pleochroism color.
Phlogopite 03: The same section as of the previous image, now in XPL. Interference colors resemble the ones of biotite.

5. Reflected Light Microscopy

Reflected light microscopy is clearly not the recommended analytical method for identifying phlogopite. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with phlogopite.

Sample preparation: Phlogopite, like all micas, is difficult to polish well. Due to its low hardness and excellent cleavage, the mineral always ends up with many grooves and polishing defects, which is, in a way, diagnostic, along with its other characteristics under reflected light.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Very dark gray, much darker than quartz, feldspars, amphiboles, and pyroxenes. It is even darker than the darkest color of calcite pleochroism, with which it is frequently associated.       

Pleochroism:  Weak.     

Reflectivity: Low (<<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not perceived.        

Internal reflections: Common in the colors of phlogopite in hand specimen, generally reddish-brown. The shades vary according to the grain thickness at the point considered: the finer the grain, the lighter the shade.      

May be confused with: Other dark-colored micas, such as biotite. Considering the paragenesis is important.     

General Characteristics: 

Parallel extinction is characteristic.

Polishing grooves are always present.

Phlogopite 01: In PPL, diagonally on the image, phlogopite surrounded by calcite and some actinolite (top). Dark gray colors are typical for phlogopite.
Phlogopite 01: The same section as of the previous image, now in XPL. Phlogopite shows very dark, brownish, internal reflections.
Phlogopite 02: In PPL, in carbonatite, idiomorphic phlogopite (at left), apatite (right, high relief) and calcite (between them, strong pleochroism).
Phlogopite 02: The same section as of the previous image, now in XPL. Phlogopite shows deep reddish brown, almost black, internal reflexions. Calcite is milky, apatite yellowish.
In XPL, in carbonatite, phlogopite with typical internal reflections in dark reddish-brown and with alteration along the cleavage planes. Around it, calcite with typical anisotropy.