Week 12 Lecture

sedimentary minerals in metamorphic rocks

Two causes of metamorphism

Changes in mineralogy and texture (foliation, grain size)

Protolith types (parent rock)

Ultramafic - peridotites, pyroxenites

Mafic (basalt, gabbro)

Pelites (aluminum shales. Most common sediment)

Carbonates (limestone, dolostone)

Quartz rocks (chert, sandstone)

Quartzo-feldspathic rock

What do metamorphic minerals reflect?

protolith composition, and the pressure-temperature conditions of the metamorphism they went through

Paragenesis

Order of formation of minerals in a rock, and of those that were removed

Garnet

Pyralspite group (different cation), pyrope, almandine, spessartine

Ugrandite group (substituting cation Ca), uvarovite, grossular, andradite

Aluminosilicate polymorphs

Al2SiO5 formula

Andalusite

Kyanite

Sillimanite

Staurolite

Fe3-4Al18Si8O48H2-4

Monoclinic (pseudo orthorhombic)

Penetration twins (cross)

Where might high pressure, low temp conditions occur?

Subduction zones. The subducting oceanic plate creates enormous stress, but has had more time to cool and refrigerates the continental plate

Diopside

CaMgSi2O6

Single chain silicate, clinopyroxene

Produced through metamorphism of carbonates; must be impure limestone

Amphiboles

Anthophyllite (MG7Si8O22(OH)); orthorhombic, found in mg-rich ultramafic protoliths

Cummingtonite-grunerite; orthorhombic, common in Precambrian iron formations

Tremolite-actinolite; monoclinic, found in ultramafic protoliths. Aluminum poor most often, if not is hornblende

Glaucophane (blueschist); clino-amphibole, formed from basaltic protolith found in subduction zones

Wollastonite

CaSiO3

Pyroxenoid

Formed through high grade metamorphism; associated with limestones

commercial substitute for asbestos

Talc

Found in ultramafic protoliths

Low grade mineral formed from retrograde metamorphism

Sheet silicate, soapstone

Paint extender, filler, ceramic tiler

Chlorite

Sheet silicate with brucite-like sheets

Triclinic or monoclinic. One direction of cleavage

Low grade greenschist

Present in many metamorphic rocks. Found around hydrothermal veins and ore deposits

Serpentine minerals

Formed through hydrothermal alteration of ultramafic rock

Polymorphs include antigorite, lizardite, chrysotile (asbestos)

Has different structural arrangement of T-O layers

Epidote-clinozoisite

monoclinic

paired and single tetrahedra

Clinozoisite is the iron-deficient end member, while epidote is the iron-rich end member

commonly found in igneous and metamorphic rocks. They can also form as hydrothermal alteration products

Cordierite

Uncommon orthorhombic ring silicate, seen in garnet

common in pelites, favored by low pressure, high temp

May be difficult to distinguish from plagioclase feldspar in thin section. Has pleochroic haloes

Titanite

AKA sphene. independent tetrahedra, monoclinic

common accessory mineral. Can be dated due to uranium presence

Found in metamorphic and igneous rocks. Common in calc-silicates, mafic rock, and granites

Corundum

High grade metamorphic rock with barrel-shaped crystals

Hardness of 9