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Thin Section: Laf105
This thin section contains some feldspar and quartz microphenocryst surrounded by a matrix of volcanic glass and some white needle-shaped microlites. Oikocryst is present, with feldspar inclusion. Many feldspars show oscillatory zoning and polysynthetic twinning, which suggests that the environment is not stable. Fiamme texture indicate that the original are formed when ignimbrites are emplaced at high temperature, and the lower portions typically become welded by the internal heat in the settled pile to a vary hard rock. They can usually be found in a pyroclastic flow deposit. Most of the crystal is euhedral or subhedral, and some crystal has a spherulitic texture. Therefore, this thin section might originate from a silicic pyroclastic flow deposit that cools quickly.
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Thin Section: IETHb-1
This thin section contains feldspar phenocrysts surrounded by a matrix of feldspar and olivine microlite crystals. Many of the feldspars exhibit quality examples of twinning. Simple, tartain plaid, and Carlsbad twinning can be observed. The igneous textures indicate that the cooling process was initially slow, but cooled rapidly in an eruption to create this porphyritic texture. Throughout the sample there are areas full of volcanic glass with a holohyaline texture. The magma itself was not at equilibrium with the phenocrysts frozen inside, which can be seen through crystals with multiple embayments. An embayment occurs when the magma dissolves a phenocryst by corroding it from the outside faces into the inner parts of the crystal. Compositionally, the magma appears to be mafic, but has varying bands of minerals depending on the location within the thin section.
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Scoria:TLC018
Opaque, dull luster, dark brownish/metallic gray colored, extremely porous, igneous rock approximately 6cm in length. Scoria is mafic (generally basaltic) in nature with very small vesicles (
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Flow Banding: TLC014
This is a rock with many parallel continuous distinct fine layers. There are mainly two colors of layers—black and white, so there may be two major compositions. The pattern is called flow banding.
Flow banding is created in two ways. Mingling of two magmatic liquids in the magma chamber will create such pattern on the wall rock. Or when eruption happens, viscous lava can leave such pattern on the contact surface. Because lava usually cools quickly, the grain size of the rock is small.
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Pumice: TLC002
Volcanic ejecta approximately 18 cm long. Felsic in nature, and extremely porous. Impressions of gas bubbles are clearly visible.
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Pumice:IHQ
Pumice is a light colored, porous, igneous rock that forms during very explosive volcanic eruptions such as an eruption from a stratovolcano. Pumice differs from scoria by its smaller vesicles, thinner vesicle walls and being light colored and less dense. Vesicles are air bubbles that are trapped in the rock during the cooling process.
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Diopside:S1507
Chemical Formula: CaMgSi2O6
Compositional Family: calcium magnesium silicate
Structural Family: Ca-clinopyroxene/single chain silicate
Crystal System: monoclinic
Location: Dekalb, St. Lawrence County, NY
Texture: alone
Crystal Form: single, short prismatic crystals with good terminations
Color: greenish-gray, white, tannish
Luster: vitreous/dull earthy
Hardness: 5 to 6
Streak Color: white to light green
Relative Density: medium (3.3-3.6)
Transparency: translucent
Cleavage/Fracture: 2 fair to good prismatic directions, meet at 87 and 93 degrees
Common Associations: contact and regional metamorphic rocks
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Biotite:S1440
Chemical Formula: K(Mg,Fe2+)(Al,Fe3+)[(Al,Si)3O10](OH,F)2 Compositional Family: iron aluminum silicate
Structural Family: sheet silicate/phyllosilicate
Crystal System: monoclinic
Location: Franklin, NJ
Texture: in mass
Crystal Form: micaceous mass
Crystal Habit: elastic mica flakes/sheets
Color: brown, black, brownish black
Luster: vitreous/pearly
Hardness: 2.5-3
Streak Color: white
Relative Density: medium (2.8-3.4)
Transparency: opaque
Cleavage/Fracture: 1 perfect direction
Common Associations: metamorphic rocks (schist, gneiss); igneous rocks (granite, rhyolite)
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Quartz:52089
Characteristic quartz crystals emerging from solidified sediment. Layers of sediment are above and below crystals of quartz. 6-sided crystals are very clear. Vitreous luster with a deep brown/red color. This sample was found at Crystal Mountain, Colorado. The chemical formula is SiO2. Quartz is a framework silicate.
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Orthoclase: S1248
KAlSi3O8 (a feldspar). A pink, monoclinic, pearly crystal with 90o cleavage angle and uneven fracture. Commonly found in low temperature metamorphic rocks, as well as pegmatites and conglomerates.
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Augite: S1567
(Ca,Na)(Mg,Fe,Al)(Al,Si)2O6 (a pyroxene). Black, vitreous, monoclinic prism with cleavage at 87o and 93o. Commonly found in basalt, diabase, and gabbro, as well as with metamorphic serpentines. Dana 325. Specimen number S1567
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Dunite with Chromite
Collection: Lafayette College Student Collection
Color: green
Luster: vitreous
Formula: (Mg, Fe)2SiO4
Hardness: 7
Fracture
Habit: Massive
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Biotite: 51431A
Collection: Lafayette College Student Collection
Color: black
Luster: vitreous
Formula: K(Fe, Mg)3AlSi3O10(OH)2
Hardness: 2.5-3.0
Cleavage: one perfect
Habit: Lamellar
Typical Occurrence: Igneous rock, Metamorphic rock, Sedimentary rock
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Albite: 51267
Collection: Lafayette College
Location of origin: Virginia
Name on collection tag: Amelia C.H.
Formula: NaAlSi3O8
Hardness: 6- 6.5
Cleavage: perfect on {001}, very good on {010}
Color: white to gray
Luster: non-metallic, vitreous
Habit: commonly tabular
Crystal system: Triclinic
Compositional family: plagioclase feldspar group
Relative density: 2.6 - 2.65
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Quartz: 51178
Collection: Lafayette College
Location of origin: Hot Springs, Arkansas
Formula: SiO2
Structural group: Framework Silicate
Hardness: 7
Cleavage: None, conchoidal fracture
Color: colorless or gray
Luster: non-metallic, vitreous
Habit: has a vast range of habits
Crystal system: Hexagonal
Compositional family: Silicates (Silica group)
Relative Density: 2.6-2.7
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Orthoclase: 51258
Lafayette College Mineral Collection
Student Collection
Mineral: orthoclase
Sample number: 51258
Chemical formula: KAl〖Si〗_3 O_8
Color: brown / orange
Luster: vitreous
Hardness: 6
Cleavage: uneven fracture
Compositional family: framework silicate
Structural family: potassium feldspar group
Crystal system: monoclinic
Crystal habits: prismatic and tabular