Case hardening is a weathering

Weathering

Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...

 phenomenon of rock surface induration. It is observed commonly in: felsic alkaline rocks, such as nepheline syenite

Nepheline syenite

Nephelene syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are also known...

, phonolite

Phonolite

Phonolite is a rare igneous, volcanic rock of intermediate composition, with aphanitic to porphyritic texture....

 and trachyte

Trachyte

Trachyte is an igneous volcanic rock with an aphanitic to porphyritic texture. The mineral assemblage consists of essential alkali feldspar; relatively minor plagioclase and quartz or a feldspathoid such as nepheline may also be present....

; pyroclastic rock

Pyroclastic rock

Pyroclastic rocks or pyroclastics are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic...

s, as pyroclastic flow

Pyroclastic flow

A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...

 deposit, fine air-fall deposits and vent-filling pyroclastic deposits; sedimentary rock

Sedimentary rock

Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

s, as sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

 and mudstone

Mudstone

Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...

.

Weathering process

Chemical weathering alters the minerals constituent of rock surface. Decomposition of mafic and opaque minerals releases ions and colloids of iron, magnesium, calcium and sulphur. Alteration of feldspar

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

s and feldspathoid

Feldspathoid

The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks....

s releases silica colloid. These materials are reached and transported by surface water. The remnant materials are highly aluminous and siliceous. They could have certain mechanical firmness of own minerals, however no cohesion. Therefore, physical disintegration of the rock takes place to form the surface.

In certain cases, the weathered surface obtains mechanical firmness higher than subsurface. The reached materials dissolved in the surface infiltrates in the weathered surface and cement the silica-aluminous remnant materials. The surface induration by means of this process is named case hardening . The physical weakness of the subsurface in comparison with the surface is called eventually core softening.

Natural occurrence

This phenomenon takes place in various types of rocks, being notable in porous ones, for example sandstone, quartzite and volcanic ash. The cement effect by silica observed in sandstone and quartzite is prominent. . A similar phenomenon is observed in felsic alkaline rocks, such as nepheline syenite, alkaline syenite, phonolite, and trachyte, because of weathering vulnerability of nepheline and alkaline feldspar . The case hardening on trachytic clasts of volcanic breccia shows peculiar fabrics.

The Mars scientific exploring machine Spirit has observed case hardening present of the rock surface present at Gusev meteorite Crater. This phenomenon is attributed to the weathering by means of surface water, being considered to be an evidence of liquid water in a far past on that planet .

Mineral dissociation

On the rock surface in the case hardening of alkaline felsic rocks, the weathering selectively affects certain minerals. Nepheline is very sensitive to weathering and is altered to natrolita and cancrinite. On the outcroping surface of a nepheline syenite, phonolite or nepheline syenite gneiss, the minerals formed by nepheline alteration appear white in colour on the dark-coloured weatherd background. After the nepheline alteration products are leached, small holes are formed on the rock.

A similar phenomenon takes place also in selective alteration of alkaline feldspar. This mineral is more resistant than nepheline. However, water infiltrates along cleavage planes and the weathering advances easily up to the mineral core. Because of this phenomenon, physical disintegration of alkaline feldspar takes place. The selective elimination of nepheline and alkaline feldspar is named mineral dissociation. When this phenomenon is highly advanced, the altered rock surface show the fabric similar to vesicular texture, called pseudovesicular structure .