GLAUCONITE

Glauconite – (K,Na)(Mg,Fe+2,Fe+3)(Fe+3,Al)(Si,Al)4O10(OH)2 – is a common phyllosilicate in marine sediments and sedimentary rocks. It has no economic importance.

For decades it was believed that glauconite and celadonite were the same mineral. Therefore, care must be taken with old samples, which may have the wrong designation.

Celadonite may contain Ti, Ca, and P as impurities. There are 5 varieties of glauconite.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: Blue-green, yellow-green, green, rarely colorless.     

Habit: Very small micaceous flakes, earthy aggregates.
Frillate to greasy.       

Cleavage: {001}  perfect.      

Tenacity: Elastic.        

Twinning: No.       

Fracture: No information available.       

Mohs Hardness: 2

Parting:  No.        

Streak: No information available.         

Lustre: Dull, earthy.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.4 – 2.95

 

2. Geology and Deposits

Glauconite is a synsedimentary to diagenetic mineral of marine sediments. It forms by alteration of detrital biotites and other minerals through marine diagenesis under shallow water conditions and reducing environments. It is especially common in unconsolidated sands (“green sands”), but also occurs in siltstones, shales, and impure carbonate rocks. Until the late 1990s, the term “green sand” was applied interchangeably to aggregates of glauconite and celadonite.

True glauconites may have been found in environments other than marine sediments, but were classified as celadonites until 1998.

 

3. Mineral Associations

It is associated with quartz, feldspar, clay minerals, carbonates (dolomite, siderite, calcite, ankerite), pyrite, zircon, aegirine and “limonite” (iron oxides and hydroxides).

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1,592 – 1,610    nβ: 1,614 – 1,641    nγ: 1,614 – 1,641

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Green, yellow, yellow-green, olive-green to blue-green.

Distinct pleochroism between:

X = yellow-green to green, dark blue-green,
Y and Z = stronger yellow, dark olive-green, blue-green.

This pleochroism is generally not visible in very fine-grained aggregates.

Relief: Moderate.           

Cleavage: {001} perfect, usually not visible in thin section due to the very small size of the grains.

Habits: Glauconites typically occur as rounded to elliptical aggregates (“pellets”) composed of very fine-grained particles with a micaceous habit.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: 0,020 a 0,032: cores fortes de 1ª ordem até início de 2ª ordem, geralmente mascaradas pela cor forte do mineral. A birrefringência aumenta com o aumento do teor em Fe.

Extinction: 2-3º oblique (simulates parallel extinction!)           

Elongation sign: ES(+), but the crystals are usually so small that they do not allow this identification.            

Twins: No.         

Zoning: No.             

CONVERGENT LIGHT

Character: B(-), generally impossible to determine due to the very small size of the particles.

2V angle: 0 – 20º (calculated)         

Alterations: It alters easily, with the formation of limonite.          

May be confused with: Few other minerals. The habit of glauconite—in rounded aggregates of microcrystals—is very typical.

Ortho- and leptochlorite exhibit lower birefringence.

Green biotite has much higher birefringence.

Celadonite (a clay mineral) is extremely similar, but occurs in other paragenesis.

Fragments of volcanic rocks, when chloritized and occurring in sandstones, look very much like glauconite.         

Glauconite 01: In PPL, glauconite in microcrystalline fossiliferous limestone, observed with the lowest magnification objective (2.5x). The rounded green aggregates of glauconite change color distinctly between PPL and XPL.
Glauconite 01: The same section as of the previous image, now in XPL. The color of the glauconites have changed from a somewhat citrusy green in PPL to a deep green with yellowish undertones in XPL.
Glauconite 02: In PPL, glauconite pellets in a fossiliferous calcareous sandstone, observed with a medium magnification objective (10x).
Glauconite 02: The same section as of the previous image, now in XPL. The interference colors in glauconite are clearly visible, appearing as a darker green with yellowish hues, in contrast to the creamy colors of the calcite and the gray and white colors of the quartz grains.

5. Reflected Light Microscopy

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

Sample preparation: It is not possible to polish celadonite aggregates due to the low hardness and submicroscopic size of the crystals that form the aggregates. During polishing, the aggregates are worn down deeply (negative relief), forming a blurred surface in relation to the surrounding grains. In XPL, however, the green colors become very conspicuous through their internal reflections.

PLANE POLARIZED LIGHT – PPL

Reflection color: Blurred surface, usually displaying greenish colors in various dark shades. 

Pleochroism: No.      

Reflectivity:  Very low, diffuse.      

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: No anisotropy is observed.        

Internal reflections: Widely distributed in various shades of light green, whitish or not.      

May be confused with: Celadonite is very similar. 

Glauconite 01: In PPL, in sandstone ("green sand"), quartz grains and rounded, spherical aggregates of glauconite are found. The rock, shape, and color of the glauconite pellets are very diagnostic.
Glauconite 01: The same section as of the previous image, now in XPL. The green internal reflections of the glauconite pellets are very distinctive and diagnostic. Only celadonite is similar.