Glauconitic Sandstone

Description

Glauconitic sandstone is a type of sandstone characterized by a significant proportion (typically >5-10%) of the mineral glauconite. Glauconite is a phyllosilicate mineral of the mica group, specifically a dioctahedral mica, with a characteristic green to bluish-green color. The sandstone matrix is typically composed of quartz grains, with varying amounts of feldspar, chert, and other detrital minerals. The presence of glauconite imparts a distinctive green, olive-green, or bluish-green hue to the rock. The texture can range from fine-grained to coarse-grained, and it is often well-sorted. The cement can be siliceous, calcareous, or ferruginous.

How to Identify

Color

Typically green, olive-green, bluish-green, or greenish-gray due to the presence of glauconite. The intensity of the green color is directly proportional to the glauconite content.

Luster

Dull to earthy in the rock, individual glauconite grains can have a dull to sub-vitreous luster.

Texture

Clastic, granular. Grains are typically sand-sized (0.0625 mm to 2 mm). Can be fine- to coarse-grained, often well-sorted. Glauconite grains are typically rounded to ovoid, often pelletal or vermicular in shape.

Crystal Form

Glauconite itself forms as small, rounded, ovoid, or vermicular grains within the sandstone matrix. It does not typically exhibit macroscopic crystal forms within the rock.

Cleavage

Glauconite, as a mica-group mineral, has perfect basal cleavage (one direction). However, in the form of small grains within sandstone, this cleavage is rarely observable without microscopic examination.

Geological Environment

Shallow marine environments, continental shelves, areas of slow sedimentation, often associated with transgressive sequences or condensed sections. Requires specific redox conditions for glauconite formation.

Key Facts

• Hardness: Glauconite: 2-3 (Mohs). The overall hardness of the sandstone will depend on the cementation, but typically 6-7 for quartz grains.
• Specific Gravity: Glauconite: 2.2-2.8 g/cm³. The bulk rock density will vary based on porosity and cementation.
• Crystal System: Monoclinic (for glauconite mineral)
• Color: Green, olive-green, bluish-green
• Luster: Dull to earthy
• Transparency: Opaque (rock), translucent to opaque (glauconite grains)
• Fracture: Conchoidal to uneven (for quartz grains), splintery (for glauconite grains)
• Cleavage: Not typically observed in the rock; glauconite has perfect basal cleavage (001)
• Composition: Predominantly quartz (SiO2) grains, with 5-50% or more glauconite (a complex hydrous potassium iron aluminum silicate: (K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2·nH2O). May also contain feldspar, chert, and other accessory minerals. Cement can be silica, calcite, or iron oxides.

Quick Check

• Color: Green, olive-green, bluish-green
• Luster: Dull to earthy (rock), dull to sub-vitreous (glauconite grains)
• Streak: Pale green to greenish-white (for glauconite, not typically tested for the whole rock)

Physical Characteristics

• Crystal Habit: Glauconite typically occurs as small, rounded, ovoid, or vermicular grains; rarely as platy aggregates.
• Cleavage Type: Perfect basal cleavage (001) for glauconite, but not typically visible in the rock.
• Fracture Type: Uneven to conchoidal (for quartz grains), splintery (for glauconite grains).
• Tenacity: Brittle.
• Luster Type: Dull to earthy (rock), dull to sub-vitreous (glauconite grains).

Formation

Glauconitic sandstone forms in shallow marine environments, typically on continental shelves, where sedimentation rates are low and there is a sufficient supply of iron-rich clay minerals and organic matter. Glauconite forms authigenically (in situ) through the alteration of biotite, fecal pellets, or other clay minerals under reducing conditions, followed by oxidation. The presence of glauconite indicates marine depositional environments with slow sedimentation, often associated with transgressive or condensed sections.

Usage

Historically, glauconite-rich sands (greensands) have been used as a natural fertilizer due to their potassium content. They are also used as a natural water softener and filter medium. In geology, glauconite is a valuable indicator of marine depositional environments and can be used for radiometric dating (K-Ar and Ar-Ar methods) of sedimentary rocks, though its reliability can be affected by post-depositional alteration.

Age Distribution

Common in Phanerozoic marine sedimentary sequences, particularly abundant in Cretaceous and Paleogene strata.

Where to Find

Finding Tips

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Scientific Classification

Mineral Class

Phyllosilicate (for glauconite)

Group

Mica Group (for glauconite)

Crystal System

Monoclinic (for glauconite)

Chemical Formula

(K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2·nH2O (for glauconite)

Composition

Sedimentary rock composed primarily of quartz and glauconite, with variable amounts of other detrital minerals and cement.

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