Dendritic Manganese Oxide on Sandstone

Description

Dendritic Manganese Oxide on Sandstone refers to the distinctive, often fern-like or tree-like patterns of black to dark brown manganese oxide minerals that have precipitated onto or within the surface and internal fractures of sandstone. The sandstone acts as the host rock, providing a substrate for the manganese oxide growth. The patterns are not fossils, but rather mineral growths that mimic organic forms. The manganese oxides involved are typically cryptocrystalline or amorphous mixtures, often referred to historically as 'psilomelane', which is now an obsolete term for a group of hard, massive manganese oxides, primarily Romanechite, Hollandite, and Cryptomelane.

How to Identify

Color

The dendrites are typically black, dark gray, or dark brown. The host sandstone can vary widely in color (white, tan, red, brown, gray) depending on its composition and cementing agents.

Luster

The manganese oxide dendrites typically have a submetallic to dull luster. The sandstone host rock will have a dull to vitreous luster depending on its grain size and cement.

Texture

The dendrites themselves are typically very fine-grained, forming delicate, branching patterns. The sandstone host rock will have a clastic texture, ranging from fine-grained to coarse-grained, with individual sand grains often visible.

Crystal Form

The manganese oxides form dendritic (tree-like, fern-like) or arborescent (branching) patterns rather than distinct macroscopic crystals. The individual mineral grains are cryptocrystalline. The host sandstone consists of detrital quartz grains, often sub-angular to rounded.

Cleavage

The manganese oxides do not exhibit macroscopic cleavage due to their cryptocrystalline nature and dendritic growth habit. The quartz grains in the sandstone have poor to indistinct cleavage.

Geological Environment

Found in sedimentary environments where manganese-rich fluids have permeated porous sandstone. This can occur in various settings, including arid regions where groundwater evaporation concentrates minerals, or in areas with active hydrothermal systems or weathering of manganese-rich source rocks. Often found in fractures, along bedding planes, or within the matrix of sandstones.

Key Facts

• Hardness: Variable, typically 5-6 for the manganese oxides (e.g., Romanechite), 7 for quartz in the sandstone.
• Specific Gravity: Variable, typically 3.7-4.7 for the manganese oxides. The bulk specific gravity of the composite rock will be lower, closer to that of sandstone (2.2-2.8).
• Crystal System: Manganese oxides can be monoclinic (Romanechite), tetragonal (Hollandite, Cryptomelane), or amorphous. Quartz is trigonal.
• Color: Black, dark gray, dark brown (dendrites); variable (sandstone).
• Luster: Submetallic to dull (dendrites); dull to vitreous (sandstone).
• Transparency: Opaque (dendrites); translucent to opaque (sandstone grains).
• Fracture: Uneven to conchoidal (manganese oxides); conchoidal to uneven (quartz).
• Cleavage: None observed macroscopically for dendrites; poor/indistinct for quartz.
• Composition: Manganese oxides (e.g., (Ba,H2O)2(Mn4+,Mn3+)5O10 for Romanechite, K(Mn4+,Mn3+)8O16 for Cryptomelane) on a substrate primarily composed of SiO2 (quartz) grains, often with other detrital minerals and cementing agents.

Quick Check

• Color: Black to dark brown patterns on a lighter sandstone background.
• Luster: Submetallic to dull for the dendrites, dull to vitreous for the sandstone.
• Streak: Black to brownish-black (for the manganese oxide).

Physical Characteristics

• Crystal Habit: Dendritic, arborescent, fern-like, moss-like patterns. Cryptocrystalline aggregates.
• Cleavage Type: Not applicable for the dendritic growth; quartz in sandstone has poor/indistinct cleavage.
• Fracture Type: Uneven to conchoidal for the manganese oxides.
• Tenacity: Brittle.
• Luster Type: Submetallic to dull.

Formation

Dendritic manganese oxides form as secondary mineral growths within fractures, bedding planes, or pore spaces of sandstone. They typically precipitate from low-temperature, manganese-rich aqueous solutions percolating through the rock. The dendritic (tree-like or fern-like) patterns are a result of crystal growth in a two-dimensional environment, often along micro-fractures or between mineral grains, where the rate of crystal growth is faster along certain crystallographic axes. The manganese is sourced from the weathering of manganese-bearing minerals in surrounding rocks or from hydrothermal fluids.

Usage

Primarily of interest to mineral collectors, geologists, and artists due to their aesthetic appeal and unique growth patterns. They have no significant industrial or commercial use as a composite material, though the manganese oxides themselves are important ores.

Age Distribution

Can occur in sandstones of various geological ages, from Precambrian to Cenozoic, wherever conditions for manganese precipitation are met.

Where to Find

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Similar Rocks

Scientific Classification

Mineral Class

Oxides

Group

Manganese Oxides (e.g., Romanechite group, Hollandite group)

Crystal System

Variable (monoclinic, tetragonal, or amorphous for the manganese oxides); Trigonal (for quartz).

Chemical Formula

Variable, e.g., (Ba,H2O)2(Mn4+,Mn3+)5O10 (Romanechite) on SiO2 (Quartz)

Composition

Hydrated barium manganese oxide, potassium manganese oxide, etc., on silicon dioxide (quartz) with minor accessory minerals and cementing agents.

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