ROCK CHEMISTRY AND WEATHERING

Table 1 lists 13 of the most common rock-forming minerals in order of increasing resistance to chemical weathering. For each mineral, the chemical reaction(s) that cause it to decompose is(are) also identified.

TABLE 1: CHEMICAL WEATHERING OF COMMON MINERALS

MINERAL NAME

CHEMICAL FORMULA

RELATIVE RESISTANCE

CHEMICAL WEATHERING REACTION(S)

Halite

NaCl

Very low

Dissolution

Gypsum

CaSO₄ 2H₂O

Very low

Dissolution

Pyrite

FeS₂

Low

Dissolution, Oxidation

Calcite

CaCO₃

Low

Dissolution

Dolomite

CaMg(CO₃)₂

Low

Dissolution

Olivine

(Fe, Mg)SiO₄

Moderately low

Oxidation, Hydroloysis

Pyroxene

Ca, Mg, Fe, Al Silicates

Moderate

Oxidation, Hydrolysis

Plagioclase

CaAl₂Si₂O₈ NaAlSi₃O₈

Moderate

Hydrolysis

Hornblende (Amphibole)

Na, Ca, Mg, Fe, Al Silicates

Moderate

Oxidation, Hydrolysis

Biotite

K, Fe, Mg, Al Silicate

Moderate

Oxidation, Hydrolysis

K-Feldspar

KAlSi₃O₈

Moderately high

Hydrolysis

Muscovite

K, Al Silicate

High

Hydrolysis

Quartz

SiO₂

Very high

Dissolves only very slowly

Table 2 lists common rock types and their mineralogic composition and relative resistance to chemical weathering. Note how the resistance of a rock to chemical weathering is related to its mineral composition and the rate at which those minerals weather (refer to Table 1).

TABLE 2: CHEMICAL WEATHERING OF COMMON ROCKS

TYPE OF ROCK	DOMINANT MINERALS PRESENT IN ROCK	RESISTANCE TO CHEMICAL WEATHERING
Granite	Quartz and K-feldspar	High
Basalt	Olivine, Pyroxene	Moderate
Quartz-cemented Sandstone	Quartz and K-feldspar	High
Calcite-cemented Sandstone	Quartz, K-feldspar and Calcite	Low
Siltstone	Quartz, K-feldspar, and Clays	High
Shale	Quartz and Clays	High
Limestone	Calcite	Low
Rock Salt	Halite	Low
Rock Gypsum	Gypsum	Low
Slate	Quartz, Biotite and/or Muscovite	High
Phyllite, Schist	Quartz, K-feldspar, Biotite and/or Muscovite, and Amphibole	High
Gneiss	Quartz, K-feldspar, Plagioclase, Biotite, and Amphibole	High
Marble	Calcite	Moderate
Quartzite	Quartz	Very High