APPENDIX FOR ROCK ID LAB

ADDITIONAL NOTES AND DETAILED FIELD CLASSIFICATION SCHEMES

Chemical/Biochemical Precipitates:

The texture of precipitates tends to be aphanitic, that is, the grains are too small to see, or crystalline, in which grains are interlocked because of crystal growth (though the grains will NOT be silicates!).

Carbonate rocks: Limestone, which is pure calcium carbonate (CaCO3), can be told apart from Dolostone, which has some magnesium (CaMgO3) by the degree of fizziness due to HCL. Limestone will fizz strongly if HCL is dropped on a fresh surface, dolostone will fizz weakly, or will need to be powdered to fizz.

Siliciclastic Rocks: Note that clastic rocks are not always made up of silicate clasts. Limestones and Dolostones can be made up of carbonate fragments, such as fossils, pellet, oolites, or intraclasts (fragments of carbonate rocks). They are classified according to the percentage of grains, the type of grains, and the amount of lime mud (micrite) vs. crystalline carbonate present.

Igneous Rocks: The Most Common Minerals

Here, the most common minerals are listed in terms of their stability at the surface, with the least stable first, the most stable last.

1. Olivene (Mg, Fe)2 SiO4: Color ranges from very dark green, green, yellow green, or less commonly colorless. Vitreous (glassy) luster. Conchoidal fracture (this is how glass breaks) common. Usually, crystals are not well developed (termed subhedral or anhedral). Well-formed crystals are called euhedral.

2. Pyroxene (Mg, Fe)2 (Si2O6): Dark green to black, forms rectangular, stubby crystals which are square in cross section. 3. Amphibole (Mg, Fe)7 (Si8O22) (OH)2: Hornblende is the most common amphibole. Like pyroxene, it is dark green to black and forms elongated crystals, but it has two distinguishing features. Hornblende crystals tend to be narrower and more needle-like than pyroxene, and in cross-section, hornblende has a diamond shape.

4. Feldspar (Na, Ca, K) Al1-2Si3-2O8: The information given above is geared towards plutonic rocks. In lavas, plagioclase often occurs as distinctive, rectangular white laths, and alkali feldspar is often represented by sanidine, which forms very clear, very glassy grains with flat faces.

5. Mica K(Mg, Fe)3 (Si3AlO10) (OH)2: Micas are easily distinguishable; they form hexagonally shaped books consisting of very thin sheets, which are easily flaked off (along cleavage planes). They are shiny, with a vitreous (glassy) to pearly luster. Also, micas are quite soft, and can be scratched with one's fingernail.

If the mica is light, it is probably muscovite, if dark, biotite.

6. Quartz (SiO2)

Plutonic igneous rocks are classified primarily according to the ratios of the above minerals with the addition of feldspathoids (don't worry about the feldspathoids for now, they are not particularly common); this classification is simplified for field use (Figure 1a). You are most likely to run into rocks from the APQ triangle.

A mineral name is assigned on the triangular diagrams by estimating the ratios of the corner minerals on a given triangle. First determine which of the corner minerals you have in your rock, so you can use the appropriate triangle. Then estimate the percentage of these 3 minerals in the rock (Figure 1b). Normalize these percentages to 100%, and then plot these percentages as lines on the diagram. The intersection of the lines is the classification of your rock. Note the "oid" endings denote a field identification.

Lavas: The field classification of lavas is based on mineralogy of phenocrysts, that is, the visible grains or crystals in the rock (Figure 2). The important minerals are those from our list above. Some of the more common terms are:

basalt = dark-colored, no quartz phenocrysts
andesite = medium-colored, often purplish, no quartz phenocrysts
rhyolite and dacite = light-colored, with quartz phenocrysts.

Volcanoclastic Rocks

Volcanoclastic rocks are subdivided into pyroclastic rocks, which are composed of fragments produced by volcanic explosions, and epiclastic rocks, in which the pyroclastic fragments have been reworked and deposited by water, wind, or ice (you can consider these as sedimentary rocks). There is a third category of autobrecciated rocks, but we won't worry about them right now.

Volcanoclastic rocks can be classified genetically, that is, we need to determine what processes were responsible for their formation in order to assign a name, or they can be classified descriptively. Following is the descriptive classication.

Pyroclastic rocks: pyroclastic fragments and pyroclastic rocks are both classified according to fragment size:

 *This is the corresponding clast size in the clastic rock classification scheme.

Note that the fragments in pyroclastic rocks can also be derived from crystals or lithics.

Epiclastic rocks:

for epiclastic rocks, use the classification for clastic sedimentary rocks, but add volcanoclastic as a prefix. For example, a rock with 1mm sized fragments, many of which are pyroclastic fragments, would be a volcanoclastic sandstone.