Provide Website Feedback. Department of Earth and Environmental Science. The University of Waterloo acknowledges that much of our work takes place on the traditional territory of the Neutral, Anishinaabeg and Haudenosaunee peoples.
Our main campus is situated on the Haldimand Tract, the land granted to the Six Nations that includes six miles on each side of the Grand River. Our active work toward reconciliation takes place across our campuses through research, learning, teaching, and community building, and is centralized within our Indigenous Initiatives Office. Skip to main Skip to footer. Earth Sciences Museum. Walking tours Book our space Group Programs Volunteers. Optical properties such as relief, color, and birefringence tend to increase in intensity with the iron content.
Most amphiboles show oblique extinction with respect to the long axis, as they crystallize in the monoclinic system. Only two rock-forming amphiboles, anthophyllite and gedrite, crystallize in the orthorhombic system and have, hence, straight extinction. Amphiboles have high positive relief. CPL PPL Above : heart-shaped amphibole left and lamellar phlogopite right in a lamprophyre with primary carbonate groundmass. Amphibole is commonly colored, pleochroic, and shows moderate to high interference colors.
Aillikite from Aley, British Columbia, Canada. Field of view: 9 mm. Photo by Alessandro Da Mommio. Amphiboles and asbestos Many amphibole minerals have a tendency to form acicular to fibrous crystals, which can produce tiny silicate fibers. There is evidence that these fibers are linked to asbestosis, a serious lung conditions caused by the exposure to asbestos. Asbestos is a commercial and material term that groups together different minerals that from a mineralogical point of view belong to different groups or supergroups.
The current definition of asbestos includes chrysotile serpentine group and cummingtonite — grunerite amosite , crocidolite the fibrous variety of riebeckite , anthophyllite, tremolite, and actinolite, all belonging to the amphibole supergroup.
These minerals used to be quarried because they are excellent thermal and electric insulators and their fibrous habit made their extraction and processing easy. However, the tiny silicate fibers they produce when processed can be breathed. We can build the structures of the various sheet silicates by starting with the octahedral layers similar to the structures of brucite or gibbsite, as discussed last semester in Earth Materials. Review your notes from the clay minerals discussion.
Three varieties of serpentine are known. Antigorite and Lizardite are usually massive and fine grained, while Chrisotile is fibrous. As discussed above, the imperfect fit of the octahedral layers and the tetrahedral layers causes the crystal structure to have to bend.
In Antigorite the bending of the sheets is not continuous, but occurs in sets, similar to corrugations, as shown here. In Chrisotile, the bending of the sheets is more continuous, resulting in continuous tubes that give the mineral it's fibrous habit. The Chrisotile variety is commonly referred to as asbestos. Occurrence - Serpentine is found as an alteration product of Mg-rich silicates like pyroxene and olivine.
It results due to hydration. Thus, serpentine is commonly found pseudomorphed after olivines and pyroxenes in altered basic and ultrabasic igneous rocks, like altered peridotites, dunites, and sometimes basalts and gabbros. It is commonly associated with minerals like magnesite MgCO 3 , chromite, and magnetite.
If the rock is made up almost entirely of serpentine, it is called a serpentinite. Properties - Because the serpentines usually occur either as fine-grained aggregates or fibrous crystals, optical properties are difficult to determine. Most of the time, serpentine can be distinguished by its characteristic pseudomorphing of other crystals like olivines and pyroxenes.
In hand specimen it generally tends to have a dark green color with a greasy luster. In thin section it is clear to pale green to pale yellow, but does not show pleochroism, shows a generally low relief compared to minerals like olivine and pyroxene with which it is associated, and show very low order interference colors due to its low birefringence.
It is probably best know for its low hardness. Although it has a micaceous structure, it is so easily deformed, that crystals are rarely seen. Occurrence - Like serpentine, talc requires an environment rich in Mg. It is therefore found in low grade metamorphic rocks that originated as ultrabasic to basic igneous rocks.
Rocks composed almost entirely of talc have a greasy feel and are referred to as soapstone. Properties - Talc is most easily distinguished in hand specimen by its low hardness, greasy feel, and association with other Mg-bearing minerals. In thin section, talc is colorless, biaxial negative with a 2V of 0 to 30 o. The large number of small offset cleavage planes may make it tough to spot the angles at which the two cleavages intersect. The solution is to slowly rotate the mineral in your hand and spot the flashes of reflected light.
Even if offset, multiple parallel cleavage surfaces will reflect the light at one time. The white is calcite This close-up of a rough hornblende cleavage surface shows how the many offset but parallel cleavage surfaces reflect the light all at once.
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