Earth Science: Mineral and Rock Identification

Comprehensive visual vocabulary flashcards identifying 35 diagnostic rock and mineral specimens from introductory geology, covering physical properties, crystal habits, cleavages, and classifications.

Biotite Mica

A dark-colored phyllosilicate mineral characterized by one perfect basal cleavage direction that allows it to peel easily into thin, flexible, elastic sheets; rich in iron and magnesium.

Satin Spar (Gypsum)

A fibrous variety of gypsum ($CaSO_4 \times 2H_2O$) with a distinctive silky or chatoyant luster, defining a Mohs hardness of 2 that can be easily scratched by a fingernail.

Magnetite

An iron oxide mineral ($Fe_3O_4$) with an iron-black metallic to submetallic luster, Mohs hardness of $5.5\text{--}6.5$, dark gray-to-black streak, and strong natural ferromagnetism.

Graphite

A native element mineral composed of elemental carbon ($C$) with a metallic to submetallic luster, steel-gray color, greasy feel, and extreme softness (Mohs hardness $1\text{--}2$) that readily marks paper.

Hematite

The principal ore of iron ($Fe_2O_3$), commonly occurring in specular (glittering metallic) or earthy red forms, uniquely identified by its characteristic reddish-brown streak.

Calcite

A major carbonate mineral ($CaCO_3$) with a Mohs hardness of 3, featuring three directions of perfect rhombohedral cleavage at oblique angles and vigorous effervescence when exposed to dilute hydrochloric acid ($HCl$).

Plagioclase Feldspar

A common tectosilicate solid-solution series ($NaAlSi_3O_8$ to $CaAl_2Si_2O_8$) featuring two directions of cleavage at approximately $90^\circ$ and diagnostic fine, parallel twinning striations on cleavage surfaces.

Potassium Feldspar (Orthoclase / Microcline)

A framework silicate mineral ($KAlSi_3O_8$) typically exhibiting salmon-pink to white color, a Mohs hardness of 6, two cleavage directions intersecting at $90^\circ$, and perthitic exsolution lamellae without striations.

Muscovite Mica

A hydrous potassium-aluminum phyllosilicate mineral that splits into transparent, colorless, flexible, and elastic sheets along a single perfect basal cleavage direction.

Hornblende (Amphibole)

A dark green to black double-chain inosilicate mineral forming prismatic, bladed crystals with two directions of cleavage intersecting at characteristic angles of approximately $56^\circ$ and $124^\circ$.

Augite (Pyroxene)

A dark green to black single-chain inosilicate mineral forming stubby prismatic crystals with two distinct cleavage planes intersecting at nearly right angles (approximately $87^\circ$ and $93^\circ$).

Talc

An extremely soft hydrous magnesium silicate mineral ($Mg_3Si_4O_{10}(OH)_2$) defining hardness 1 on the Mohs scale, characterized by a pearly to greasy luster and a distinctly soapy or slippery tactile feel.

Pyrite

An iron sulfide mineral ($FeS_2$), commonly known as 'Fool\'s Gold', with a pale brass-yellow metallic luster, Mohs hardness of $6\text{--}6.5$, greenish-black streak, and cubic or pyritohedral crystals with striations.

Olivine

A high-temperature nesosilicate mineral series ($(Mg,Fe)_2SiO_4$) with an olive-green color, vitreous luster, conchoidal fracture, granular sugary texture, and a Mohs hardness of $6.5\text{--}7$.

Chalcopyrite

A major copper iron sulfide ore mineral ($CuFeS_2$) with a golden brass-yellow metallic luster often exhibiting iridescent tarnish, distinguished from pyrite by its lower Mohs hardness of $3.5\text{--}4$.

Galena

The primary ore of lead ($PbS$), characterized by a silver-gray color, brilliant metallic luster, perfect cubic cleavage in three directions at $90^\circ$, lead-gray streak, and high specific gravity ($\approx 7.5$).

Quartz (Milky Quartz)

A common framework silicate mineral ($SiO_2$) defined by a Mohs hardness of 7, vitreous luster, lack of cleavage, and prominent conchoidal fracture.

Halite

Naturally occurring rock salt ($NaCl$), displaying perfect cubic cleavage in three directions at $90^\circ$, a Mohs hardness of 2.5, vitreous luster, high water solubility, and a distinct salty taste.

Basalt

A dark-colored, fine-grained (aphanitic) extrusive igneous rock of mafic composition, predominantly composed of calcium plagioclase feldspar and pyroxene, forming the oceanic crust.

Andesite

An intermediate extrusive igneous rock with an aphanitic to porphyritic texture, intermediate in composition between basalt and rhyolite, primarily consisting of plagioclase feldspar and amphibole or pyroxene.

Diorite

A coarse-grained (phaneritic) intrusive igneous rock of intermediate composition, distinguished by a classic 'salt-and-pepper' appearance composed of light plagioclase feldspar and dark amphibole or biotite.

Rhyolite

A light-colored, fine-grained (aphanitic) extrusive igneous rock of felsic composition, rich in silica and chemically equivalent to granite, consisting primarily of quartz and potassium feldspar.

Gabbro

A coarse-grained (phaneritic), dark-colored intrusive igneous rock of mafic composition, chemically equivalent to basalt, composed mostly of calcium-rich plagioclase and clinopyroxene.

Siltstone

A fine-grained clastic sedimentary rock composed of cemented silt-sized mineral particles ($\frac{1}{256}\text{ to }\frac{1}{16}\,\text{mm}$), lacking the fine-scale laminations or fissility characteristic of shale.

Shale

A fine-grained clastic sedimentary rock composed of clay and silt particles, characterized by distinct fissility that causes it to readily split into thin, fragile sheets along closely spaced bedding planes.

Sandstone

A medium-grained clastic sedimentary rock composed of cemented sand-sized mineral particles or rock grains ($\frac{1}{16}\text{ to }2\,\text{mm}$ in diameter), predominantly formed of durable quartz grains.

Conglomerate

A coarse-grained clastic sedimentary rock containing rounded to sub-rounded gravel or pebble-sized clasts ($> 2\,\text{mm}$) held together within a matrix of finer sand, silt, and mineral cement.

Coquina

A poorly consolidated, highly porous biochemical sedimentary rock (a variety of limestone) composed almost entirely of sorted, mechanically abraded, and cemented mollusk shell fragments.

Gneiss

A high-grade, foliated metamorphic rock characterized by distinct compositional banding (gneissic banding) composed of alternating layers of light-colored felsic minerals and dark-colored mafic minerals.

Marble

A non-foliated metamorphic rock formed through the recrystallization of limestone or dolostone under heat and pressure, composed of interlocking calcite crystals that readily fizz in dilute acid.

Phyllite

A foliated metamorphic rock intermediate in grade between slate and schist, featuring a wavy, crinkled foliation surface and a distinctive glossy, silky sheen (phyllitic luster) due to fine-grained micas.

Slate

A very fine-grained, low-grade foliated metamorphic rock derived from the metamorphism of shale or mudstone, renowned for slaty cleavage that enables it to split into flat, durable sheets.

Quartzite

A very hard, non-foliated metamorphic rock produced by the metamorphism of quartz sandstone, characterized by interlocking quartz crystals that fracture cleanly across individual grains rather than around them.

Amphibolite

A dark, medium-to-coarse-grained metamorphic rock composed primarily of amphibole (mainly hornblende) and plagioclase feldspar, typically formed under medium-to-high temperature and pressure conditions.

Hornfels

A fine-grained, dense, non-foliated metamorphic rock produced by contact metamorphism (baking) of shale, mudstone, or basalt adjacent to an igneous magma intrusion.