Chart 1: Type, Texture, Composition

 

General Rock Type Texture – ave.
size of minerals
General color and/or composition – Miscellaneous observations Rock Name

IGNEOUS
ROCKS

Interlocking
homogenous
crystalline
texture – no
preferred
orientation to the
mineral grains

Fine grainedExtrusive, volcanic Felsic – Light colored Rhyolite
Intermediate Andesite
Mafic – Dark colored Basalt
Medium grainedDikes, sills, etc. Felsic – Light colored – – – – – –
Intermediate Dacite
Mafic – Dark colored Diabase
Coarse grainedGenerally intrusive Felsic – Light colored Granite
Intermediate Diorite
Mafic – Dark colored Gabbro
Ultramafic Peridotite
Glassy Dark to black – felsic (DOES NOT
follow normal color index)
Obsidian
Frothy Felsic – Light colored Pumice
Mafic – Dark colored Scoria

SEDIMENTARY
ROCKS

Consolidated detrital clasts, chemical precipitates, and/or biological residue

Coarse Fragments Rounded clasts Conglomerate
Angular clasts Breccia
Sand sized fragments Clean quartz (w/feldspar?) Sandstone
Dirty w/rock fragments & clay Graywacke
Fine grained – cannot see individual clasts Nonfoliated, “clay the size” Siltstone
Foliated, “clay the mineral” Shale
Chemical – fine grain Soft – passes fizz test Limestone
Hard – fails fizz test Chert
Fossiliferous Mostly shell fragments Coquina

METAMORPHIC
ROCKS

Interlocking non- homogenous crystalline texture – commonly with a preferred orientation to the mineral grains

FO

L

I

A

T

E

D

Very fine grained-no visible minerals Dull – passes “tink test” Slate
Foliated, shiny due to increased size of micaceous minerals (almost see them) Phyllite
Medium to coarse grain Individual mineral grains visible. Major mineral(s) included as name modifiers Schist
(ex. Mica Schist)
Color banded Alternating layers of light (felsic)
and dark (mafic) minerals
Gneiss
Distinct layering – often highly folded and contorted Alternating layers of felsic igneous rock (light) and mafic gneiss (dark) Migmatite

Non-foliated,

with non-oriented grains

Soft – passes fizz test Marble
Hard – fails fizz test Quartzite

Interlocking hornblende crystals

Amphibolite

 

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