1. Name: Max Barnett StudentNumber:00266746
Figure 2. Zoned
plagioclasein the
center. Field of
view is 2mm.
Mt. Hood
Petrographic
Description
Sample: #2
Location: Mt. Hood
Name: Porphyritic andesite
Textures: Porphyroaphanitic
Locality: Mt. Hood in the volcanic arc
region
Description: A majority of the rock is groundmass where there are few large phenocrysts. Much
of the plagioclase is compositionally zoned. There is no orientation in any of the grains to
determine direction of a flow. In hand sample the rock has a dark green-grey color. Plagioclase
constitutes 80%, hornblende 20% of the bulk composition. Plagioclase makes up most of the
groundmass. Phenocrysts of hornblende are apparent in viewing the thin section with the
naked eye.
Description of Minerals Present
plagioclase: Most, if not all, of the plagioclase is
zoned in this sample. It is apparent in the zoning
rings (shown at the right). Plagioclase comprises
most of the phenocrysts shown in thin section. The
groundmass is heavily concentrated with plagioclase.
The crystals range in size between a few
micrometers and 1mm.
hornblende: This sample shows great specimens of
hornblende oriented on different axes. There is
some deformation to some of the hornblende, such
as oxidized rims, with parts of opaque minerals
present inside of the hornblende crystals. The sizes
of the crystals range from a few micrometers to
0.5mm. In PPL, hornblende shows pleochroism from
light brown to a slightly darker brown.
Figure 3. The
sampleto the left
shows hornblende
in XPL looking
down the Z-axis.
Hornblende is
characteristic in
thin section with
its 60/120 degree
cleavage.Field of
view is 2mm.
Figure 1. Hand sampleof Mt. Hood #2. Phenocrysts are 0.3-
0.6mm in length. Length of sampleis 10cmand 6cm in width.

2. Name: Max Barnett StudentNumber:00266746
Figure 4. Thin section slideof Mt. Hood
sample#2. Phenocrysts of hornblende
and plagioclasearevisiblewithout the aid
of a hand lens.

3. Name: Max Barnett StudentNumber:00266746
CRB-17
Petrographic
Description
Sample: CRB-17
Location: Imnaha Formation of the CRFB
province, Idaho
Name: Porphyritic basalt
Textures: Porphyroaphanitic with large
euhedral phenocrysts of plagioclase
Locality: Flow unit
Description: The massive, fractured rock has an aphanitic groundmass with phenocrysts of
plagioclase visible in hand specimen with the aid of a hand lens and in a slide as a colorless
mineral when viewing with the naked eye. Under thin section, the olivine is noticeably altered
to iddingsite, a yellow-green mineral showing growth within the olivine crystals. The
groundmass is composed of green to brown to black colored opaque minerals. Rapid cooling is
evident due to size, shape, and lack of alignment of crystals. The ground mass with opaque
minerals constitutes 60%, plagioclase 25%, olivine and iddingsite 15% of the bulk composition.
Description of Minerals Present
plagioclase: Subhedral to euhedral cumulate
crystals make up approximately half of the modal
mineralogy of plagioclase. Polysynthetic twinning
and Carlsbad twinning are visible in XPL. Crystals
vary between 0.5-2.5cm in length. Larger crystals
are noticeably larger in hand sample. Crystals
show zoning. Possibly one of the first crystals to
form due to size and shape.
olivine: Colorless, subhedral crystals displaying
high positive relief and fracture in PPL. High
birefringence in the third order observed under
XPL. Crystals are mostly under 1.0cm in width in
this sample. One of the first minerals to
Figure 1. Hand sampleof CRB-17. Plagioclasephenocrysts
are dark brown to dark green and visiblewith the naked
eye. Sample is 18cmlongand 10cmthick in some parts.
Figure 2. Angular crystals of plagioclaseat10x power under
XPL on left. Subhedral plagioclasephenocrystat4x power
under XPL on right.
Figure 3. Olivine(left)
displayinghigh
birefringencein the third
order at 10x power under
XPL.

4. Name: Max Barnett StudentNumber:00266746
crystallize due to size and shape based on Bowen's
Reaction Series.
iddingsite: Cryptocrystalline, has no definite shape,
and does not change its yellow-green color from
PPL to XPL. Crystals are non-pleochroic and show
no birefringence. The growth of iddingsite is
found within olivine crystals and replacing the
shape of pre-existing olivine crystals.
opaque minerals: Seen in PPL and XPL as the
intercumulus groundmass. Green, brown, and
black in color and have weak or no pleochroism.
They are the last crystals to crystallize, being
intercumulus minerals and are fine-grained to
cryptocrystalline.
Figure 4. Iddingsitein PPL(left) and XPL (right) showing
littleif any variation in color under different polarized
light.The iddingsiteis located within olivinegrains,taking
on its shapebut has no fracture, unlikethe original
crystals.
Figure 5. Plagioclasephenocrysts are
visiblein thin section without the aid of a
microscope.The aphanitic groundmass
with opaque minerals is also apparent.

5. Name: Max Barnett StudentNumber:00266746
Summary
Both samples had visible phenocrysts of plagioclase even though their locality and bulk
composition was different. CRB-17 contained larger phenocrysts of plagioclase in a finer
groundmass than Mt. Hood. CRB-17 seems to be more mafic than Mt. Hood sample #2 based
on the modal mineralogy and observing the amount of darker colored minerals in the
groundmass of CRB-17. CRB-17 was the only one that had altered minerals, some olivine
altered to iddingsite.