Friday, January 9, 2015

What a Geologist Sees - Part 8



Despite all of the fancy lab equipment these days, it is still necessary for a geologist to often make field identifications of minerals and rocks.

As mineral color may be affected by impurities (trace elements), one of the diagnostics that we use for mineral identification is "mineral cleavage".

Cleavage is the characteristic by which when a mineral breaks, it leaves behind a flat surface. The number of cleavage directions and the quality of the cleavage (on any and all of these cleavage directions) is affected by the internal structure of the mineral. Perfect cleavage in a mineral is because of inherent planes of weakness within the crystal structure (lattice). Micas are examples of minerals with one direction of perfect cleavage and cleavage surfaces are parallel to each other, on opposite sides of a mineral specimen.

Various minerals can have 1, 2, 3, 4, even 6 directions of cleavage (as in Sphalerite - the zinc, iron sulfide mined on the Gore family property in Tennessee). In minerals with more than one direction of cleavage, the angles between the cleavage planes are important.

In the examples above, we have three common minerals, all of which may be transparent to translucent and colorless in the absence of impurities. One way of distinguishing between them is to recognize the differences in cleavage (as explained in the photo). Halite (table salt) breaks into almost perfect cubes with 90 degree angles between all three directions of cleavage - which we call "cubic cleavage". Galena, a heavy, silvery lead sulfide also has cubic cleavage.

Calcite has three directions of cleavage, but none of them are at 90 degree angles to the others. This we call "rhombic cleavage".

Selenite (gypsum) occurs in several forms, in this clear crystalline form, it's one strong direction of cleavage causes it to break into thin, flat sheets, while a secondary, weaker direction of cleavage affects the shape of the specimen margins. [Hint: Selenite is one of only two common minerals that are softer than your fingernail, i.e., you can scratch it.] Differing degrees of Hardness (resistance to scratching) can be used also as a diagnostic tool. [That may be covered in a future post.]

If a broken surface is not smooth enough to classify as a cleavage surface, then it is termed as "fracture". Quartz is an important mineral that does not have cleavage (some suggest it has a very weak cleavage, but for the sake of simplicity, we will stick with the concept of no cleavage for quartz). Pyrite, olivine (peridot) and garnets are examples of other minerals with no cleavage. Some minerals, especially in the "quartz family" exhibit a distinctive type of fracture - "conchoidinal fracture", the curving type of fracture that you see in glass. The fracture link explains some of the other types of mineral fracture.

Geology students commonly study individual minerals first, in hand-size samples (if possible) to become familiar with the cleavage and other characteristics of the minerals, before we start studying minerals as smaller components of rocks. Usually we study igneous rocks after minerals, as igneous rocks are the original source of most minerals.

Cleavage can have a larger impact than some imagine. When flat, platy minerals such as micas are present in certain rocks, especially metamorphic rocks, if they are aligned with each other, they can form zones of weakness, which can affect the way rocks break. Knowing the cleavage characteristics in a diamond is important as it affects how a larger diamond can be split into to numerous smaller diamonds for faceting purposes.

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