How to identify Scoria with secondary mineralization

To correctly identify Scoria with secondary mineralization (Scoria with secondary alteration/mineralization (likely iron oxides/hydroxides or clay minerals)), 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

   Original scoria is typically black, dark brown, or reddish-brown. Secondary mineralization introduces a range of colors: yellows, oranges, reds, and browns (iron oxides/hydroxides); whites, pale greens, or earthy tones (clay minerals); white or colorless (zeolites, calcite, quartz).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Original scoria is dull to earthy. Secondary minerals can exhibit various lusters: earthy (clays, iron oxides), vitreous (quartz, some zeolites), silky (some zeolites), or dull.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Highly vesicular (porous) with irregularly shaped, often interconnected vesicles. The secondary minerals will fill or coat these vesicles, creating a more solid or encrusted appearance within the pores. The overall texture remains rough and abrasive.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   The primary scoria is amorphous to microcrystalline. Secondary minerals may exhibit microcrystalline to macroscopic crystal forms within the vesicles, such as botryoidal, reniform, radiating, or drusy habits for zeolites, calcite, or quartz, or earthy/massive for clays and iron oxides.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No cleavage in the scoria matrix. Individual secondary minerals may exhibit cleavage (e.g., calcite, some zeolites), but this is typically not observable in the bulk rock.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic terrains, particularly around cinder cones, stratovolcanoes, and basaltic lava flows. Found in areas where post-eruptive hydrothermal activity, groundwater circulation, or prolonged weathering has occurred.

Key Facts

Hardness

Variable. Scoria matrix is generally 5-6 (Mohs), but the overall hardness can be reduced if the matrix is extensively altered to soft clay minerals. Secondary minerals vary widely (e.g., clays 1-2.5, zeolites 3.5-5.5, quartz 7, calcite 3).

Specific Gravity

Variable, typically 1.0-2.0 for scoria (due to high porosity). Secondary mineralization can increase the bulk density if vesicles are filled with denser minerals, but it will still be relatively light compared to solid basalt.

Crystal System

Scoria matrix is amorphous to microcrystalline. Secondary minerals can be various crystal systems (e.g., monoclinic for many zeolites and clays, trigonal for calcite, hexagonal for quartz).

Color

Dark (black, brown, reddish-brown) with secondary colors (red, yellow, white, green, pale).

Luster

Dull to earthy for scoria; variable for secondary minerals (earthy, vitreous, silky, dull).

Transparency

Opaque for scoria matrix. Secondary minerals can be opaque, translucent, or rarely transparent (e.g., small quartz crystals).

Fracture

Conchoidal to irregular for scoria. Secondary minerals may exhibit their own fracture patterns.

Cleavage

None for scoria matrix. Some secondary minerals may exhibit cleavage.

Composition

Primary scoria is mafic (basaltic) in composition, rich in pyroxenes, olivine, and plagioclase feldspar, with a glassy groundmass. Secondary mineralization involves the deposition of iron oxides/hydroxides (e.g., FeO(OH), Fe2O3), clay minerals (e.g., hydrated aluminosilicates), zeolites (e.g., hydrated tectosilicates of alkali and alkaline earth metals), and sometimes carbonates (e.g., CaCO3) or silica (SiO2).

Physical characteristics

• Crystal Habit: Scoria is massive, highly vesicular. Secondary minerals can form coatings, infillings, botryoidal, reniform, radiating, or drusy aggregates within vesicles.
• Cleavage Type: Not applicable for the scoria matrix. Individual secondary minerals may have distinct cleavage (e.g., perfect rhombohedral in calcite, good in some zeolites).
• Fracture Type: Conchoidal to irregular for the scoria matrix. Secondary minerals may exhibit their own fracture types.
• Tenacity: Brittle for scoria. Secondary minerals can be brittle, earthy, or sectile depending on the mineral.
• Luster Type: Dull to earthy for scoria. Secondary minerals can be earthy, vitreous, silky, or dull.

Explore Scoria with secondary mineralization