Geology

Minnesota Rock and Mineral ID


click on a rock or mineral name to see a photo
Igneous rocks, metamorphic rocks and sedimentary rocks

Minerals


Minerals are homogeneous, naturally occurring, inorganic, crystalline solids with a fixed chemical composition. The crystal structure and chemical formula are necessary for identification.

Chemical formula may be determined by physical and chemical properties.
Structure may be determined by the shape of the crystal.
Secondarily, use cleavage, color, hardness, density, luster (metallic or non-metallic), streak color.
Cleavage - breakage along planes of atomic weakness.
Calcite contains carbonate ion, will react with 5% HCl to produce CO2.
Calcite will react very quickly [CaCO3],Dolomite [CaMgCO3)2] will react much more slowly - heat acid or powder the mineral to get the CO2 production.
Some minerals have magnetic properties, especially magnetite.

Common rock forming minerals

Elements % by mass
oxygen 46.6
silicon 27.7
aluminum 8.1
iron 5.0
calcium 3.6
sodium 2.8
potassium 2.6
magnesium 2.1
Others 1.5

Silicates are based on the silicon tetrahedron SiO4-4

quartz, feldspar, mica, hornblende, olivine, pyroxene, clays all contain silicates
The silicon tetrahedron may share the oxygens - this changes the chemical structure and forms chain silicates (single and double - double have more sharing of oxygen).

Sheet silicates (called phyllosilicates) are in micas and clays

A framework (tectosilicate) forms quartz [SiO2], a 3D sharing of oxygen with silicon. Also occurs in feldspar (ie KAlSi3O8)

Olivine is Mg or Fe silicates. Ions of Mg and Fe are about the same size.

Plagioclase minerals are NaAlSi3O8 albite - low temperature, and CaAl2Si2O8 anorthite - high temperature

 kaolinate - one of several clay minerals that form from the decomposition of aluminum silicates, especially feldspar. It appears clay-like and is usually chalk white, although it may be stained red, brown or black. It has a dull earthy luster and can be cut or shaped with a knife. No cleavage, H=1.2, becomes plastic when moistened. [Al4Si4O10(OH)8]

 olivine - olive-green color with equidimensional glassy grains that lack cleavage. Olivine usually occurs in aggregates of small crystals. Non-metallic luster, olive green, cleavage absent, H=6.5-7, sometimes yellowish [Fe,Mg)2SiO4]

 hematite - metallic luster, brown to reddish brown streak, usually steel gray in color, granular, fibrous, or micaceous, h=5-6, uneven fracture.

 chert, quartz - nonmetallic luster, harder than glass, no cleavage, concoidal fracture, H=7, variable color, translucent to opaque, dull or clouded luster, colors from white, gray and red to black. [SiO2]

 magnetite - attracts magnets, metallic luster, black streak, black to dark gray color, H=6, commonly occurs in granular masses [Fe3O4]

 pyrite - metallic luster, black to greenish black streak, brass yellow, cubic crystals, striated facets common, H=6-6.5, uneven fracture [FeS2]

 calcite - nonmetallic luster, light color, softer than glass, prominent cleavage in 3 directions at about 75°, effervesces in HCl, H=3, colorless, white or pale yellow [CaCO3]

 amphibole (also known as hornblende) - nonmetallic luster, cleavage in 2 directions at approximately 60° and 120°, dark green to black or brown, long prismatic 6 sided crystals, H=6 [complex Na, Ca, Mg, Fe and Al silicates]

 chalcopyrite - metallic luster, greenish black streak, golden yellow, may tarnish purple, H=4, [CuFeS2]

 plagioclase feldspar - nonmetallic luster, harder than glass, good cleavage in 2 directions, white to gray, striations on some cleavage planes [CaAl2Si2O8 dark color]

 plagioclase feldspar - nonmetallic luster, harder than glass, good cleavage in 2 directions, white to gray, striations on some cleavage planes, lighter in color than the calcium plagioclase[NaAlSi3O8]

 pyroxene - nonmetaqllic luster, dark green to black, harder than glass, prominent cleavage, short prismatic 8 sided crystals. [complex Ca, Mg, Fe, Al silicates]

chlorite - green to dark green, nonmetallic luster, cleavage in 1 direction, commonly occurs in foliated or scaly masses, nonelastic plates, H=2-2.5, [hydrous Mg, Fe, al silicates]

 quartz - many varieties, see chert

 potassium feldspar - commonly light to dark pink, nonmetallic luster, good cleavage in 2 directions, pearly to vitreous luster, H=6-6.5 [KAlSi3O8]

 staurolite - rhombic cross section, no streak,
 

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Rocks

Igneous rocks are primary rocks - formed in volcanoes, they crystalize from silicate magma
Intrusive, plutonic, phaneritic - formed beneath the surface causing large crystals visible to the naked eye.
Extrusive, volcanic, aphanitic - formed at the surface causing small crystals.
Combinations are called porphyritic - common in volcanic rocks, the visible crystals are called phenocrysts.
 

Igneous rocks come in 4 families with each having an intrusive (listed first) and an extrusive member

1. Granite - Rhyolite. 10-40% quartz, 15-30% potassium feldspar (responsible for the pink color), 0-33% plagioclase (high Na), and 8-15% biotite and amphibole. These rocks are high in potassium, silicon and sodium, low in iron, magnesium and calcium. They are characteristically light colored.

2. Diorite - Andesite. intermediate in composition between 1 & 3. Potassium feldspar and quartz are present in minor amounts only. They are characteristically gray in color.

3. Gabbro - Basalt. no quartz, 25-70% plagioclase (high Ca), 25-75% olivine, pyroxene and amphibole - ferromagnesian minerals. These rocks crystallize from magmas that are relatively high in iron, magnesium, and calcium, but deficient in silica. Rock coloration is characteristically black or dark green.

4. Peridotite family. characterized by 65-100% olivine, 0-25% pyroxene, 0-5% Ca plagioclase and 0-10% ore minerals.

5. Syenite - high potassium feldspar with no or very little quartz. Otherwise like granite.
 

In IDís, quartz is irregular, glassy grains, commonly clear to smoky appearance; muscovite is brass-colored flakes associated with quartz or K feldspar; K-feldspar has porcelain luster, commonly colored pink, white or gray; plagioclase us usually gray or white in granite, dark-bluish color in gabbro, striations are common; biotite is visible as small black flakes with perfect cleavage in one direction, reflects light; amphibole is long black crystals in a light-colored matrix; pyroxene has short, dull, greenish-black minerals in darker rocks with cleavage in 2 directions at 90°; olivine is glassy with light-green grains.


The composition and texture of the rocks are dependent on the Bowens Reaction Series which describes the different kinds of chemical composition of minerals that are possible at different temperatures. In general the lighter color rocks high in sodium are from minerals formed at lower temperatures and darker colored rocks high in calcium and olivine are formed at higher temperatures.
 

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The Rock Cycle

 

 
 
 
 
 
 
 
 
 
 
 
 
 


 
 
 
 
 
 
 

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Sedimentary Rocks

 
Clastic Sedimentary Rocks often layered but the layers are not deformed.
1. >2 mm (gravel) is Conglomerate if it has rounded grains and Breccia if it has angular grains.
2. 2 mm to 1/16 mm (sand) is sandstone (quartz sandstone if nearly 100% quartz, arkose if abundant in feldspar, graywacke if poorly sorted mixture of quartz, feldspar and rock fragments in a fine-grained matrix).
arkose - feldspar (small ìwhiteî crystals) rich sandstone
quartz -  uniform quartz crystal sandstone
graywacke - mud rich sandstone
3. 1/16 mm to 1/256 mm (silt) is siltstone
4. < 1/256 mm (clay) shale. Shale is not glossy but otherwise like slate.

Chemical Sedimentary Rocks deposited by chemical precipitation
1. limestone contains calcite, may contain fossils
2. dolomite or dolestone (contains MgCO3 - reacts with HCl when powdered)
3. chert, flint etc
4. halite (rock salt)
5. gypsum
6. ironstone

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Metamorphic Rocks formed by pressure and or heat. Often layered (but not always) and deformed.

 
Foliated (low to high grade order)
1. gneiss - coarse to fined grained, coarsely to finely banded or streaked, contains quartz, feldspar, micas and hornblende
2. schist - finely foliated or schistose, breaks along foliation, micas predominate, talc, chlorite, hornblende, garnet, quartz etc.
3. phyllite - submicroscopic grain size (bigger than slate) has cleavage, has a ìsheenî
4. slate - finely foliated, extremely fine grained, minerals not visible to the naked eye, glossy
Massive
1. quartzite - massive, occasionally gneissic or schistose, coarse to fine grained, equigranular, very glassy luster, breaks through grains
2. marble - massive, coarse to fined grained, usually equigranular, contains calcite, dolomite, sometimes with serpentine or other minerals, effervesces in dilute HCl, cleavages visible.
 

Low grade to high grade order & examples: (less to more heat and pressure)
chlorite (green) - flaky - chlorite slates
biotite - black mica - biotite garnet schists
garnet - a 12 sided crystal of hardness 7-7.5, commonly red
staurolite - staurolite-kyanite schists
kyanite
sillimanite - sillimanite schists
granite rocks
 

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