The rock found using IUGS classification with normalized mineral modes in steps 2 and 3. Composed of plagioclase and olivine mainly, with other inclusions of magnetite, pyrite, and biotite.
MEGASCOPIC: A gray, medium-grained rock consisting of feldspar, quartz, biotite, and muscovite
MICROSCOPIC: Subhedral-granular with euhedra sodic plagioclase whereas quartz and feldspar are anhedral. Magnetite and minor sphene are the other accessories along with trace mincropegmatite. In addition to some sericite, a small amount of secondary epidote is present.
Macro: Hand specimen is medium-grained dark gray rock, over 90% of the rock is made up of about equal amounts of plagioclase and pyroxenes, with the rest made up small olivine grains and possibly magnetite.
Micro: Thin section show large amounts of well twinned plagioclase. Augite/diopside also common showing good cleavage, but they are largely anhedral. Small olivine grains within pyroxenes.
Location: Silverton Calorado
Geologic Age: Miocene
Macrodescription: A purplish-gray rock consisting chiefly of plagioclase, orthoclase, and greenish hornblende with small amounts of quartz
Heterogranular texture with a few large cloudy orthoclase crystals with poikilitic plagioclase and mafic inclusions. Some potash feldspar has rims of granophyre which also forms interstitial patches. Dominant plagioclase ranges in size considerably and shows gradation zoning. Augite is partly or wholly uralitized to very weakly pleochroic green hornblende. Biotite is in part chloritized. Coarse granular magnetite is common; apatite and zircon are rare.
The trachytic texture is reflected in the oriented nature of microlites around the embayed plagioclase phenocryst. This distinct orientation suggests a flow direction, the movement of microlites around the larger grain.
Sieve structure is composed of deep and irregular embayments. The cause of this structure is debated to be due to advanced resorption or from rapid crystal growth enveloping a melt.
Anhedral is a textural term used to describe crystals that lack characterisitic faces. Anhedral crystals are not necessarily irregular in shape, but could be closely packed together which would dictate crystal face formation.
A bay-like formation caused by a reaction with the melt (resorption). Resorption is the re-fusion or dissolution of a mineral back into a melt or solution from which it formed.
This thin section displays the igneous equigranular texture. This entails that most of the crystals in the slide have a similar size within a reasonable range.
This sample is an orbicular granite. Varying in size, their shapes are ovoid masses of radiating crystals, commonly concentrically banded, found in some granites.
Fiamme – a piece of pumice that has been severely compressed by the weight of the overriding volcanic material in a deposit. They appear as long, black and “lense-shaped” fragments in the deposit
Pyroclastic – texture composed of many different types/shapes of volcanic debris, created during an explosive eruption.
Pipe vesicles are holes in extrusive igneous rocks. They form as gas filled lavas reach the surface and the gases in the lava expand coming out of solution and escape upwards.
