How to identify Dendritic Sandstone

To correctly identify Dendritic Sandstone (Sedimentary rock (primarily quartz) with manganese oxide inclusions), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Host sandstone color varies (white, buff, tan, red, brown, grey) with distinct black, dark brown, or dark grey dendritic (tree-like or fern-like) patterns.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dendrites typically have a submetallic to dull luster; the sandstone matrix is dull to vitreous (if quartz grains are prominent).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, granular texture of sandstone, ranging from fine-grained to coarse-grained. The dendrites themselves are typically very fine-grained to cryptocrystalline, appearing as flat, painted-on patterns.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   The manganese oxides form arborescent (tree-like) or fern-like patterns, not distinct macroscopic crystals. The quartz grains in the sandstone are typically anhedral to subhedral.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Sandstone itself does not exhibit cleavage, but individual quartz grains may show conchoidal fracture. The manganese oxide dendrites do not exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in sedimentary environments where porous sandstones are exposed to groundwater flow rich in dissolved manganese. This often occurs in arid or semi-arid regions, or in areas with significant weathering of manganese-bearing rocks in the source area.

Key Facts

Hardness

Sandstone matrix: 6.5-7 (due to quartz). Manganese oxides: 2-6 (variable, depending on specific mineral and crystallinity).

Specific Gravity

Sandstone matrix: 2.6-2.7. Manganese oxides: 3.5-5.0 (variable).

Crystal System

Sandstone: Quartz is trigonal. Manganese oxides: Pyrolusite is tetragonal, Romanechite is monoclinic.

Color

Host rock: variable (white, buff, tan, red, brown). Dendrites: black, dark brown, dark grey.

Luster

Host rock: dull to vitreous. Dendrites: submetallic to dull.

Transparency

Opaque (both sandstone and dendrites).

Fracture

Sandstone: granular, conchoidal for individual quartz grains. Dendrites: uneven to splintery.

Cleavage

None for the rock as a whole. Quartz has no cleavage. Manganese oxides typically lack distinct cleavage.

Composition

Primarily quartz (SiO2) grains cemented by silica, calcite, iron oxides, or clay minerals. Dendrites are composed of manganese oxide minerals, predominantly pyrolusite (MnO2) and romanechite ((Ba,H2O)2(Mn4+,Mn3+)5O10), with minor iron oxides.

Physical characteristics

• Crystal Habit: Sandstone: granular, clastic. Manganese oxides: dendritic, arborescent, fern-like, often cryptocrystalline.
• Cleavage Type: Not applicable for the rock. Individual quartz grains lack cleavage. Manganese oxides typically lack distinct cleavage.
• Fracture Type: Sandstone: granular, conchoidal for quartz. Manganese oxides: uneven to splintery.
• Tenacity: Brittle.
• Luster Type: Dull to submetallic for dendrites; dull to vitreous for sandstone.

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