Rock & Mineral Identification Database

What's the difference between a rock and a mineral?

Rocks are made up of one or more minerals.

Minerals are homogenous, naturally occurring, inorganic solids. Each mineral has a definite chemical composition and a characteristic crystalline structure. On Earth, about 3,000 different minerals have been described. Watch this video for more information. Rocks and Minerals.

Unlike Earth, Mars has no tectonic plates, nor is there evidence of recent volcanic activity or water on the surface of Mars. Scientists expect to see igneous and sedimentary rocks on Mars. But, so far, there has been little evidence of metamorphic rocks.

Igneous Rocks

Igneous rocks are formed by volcanic processes. Underground molten rock (called magma) cools slowly and produces large grained rocks. Above ground molten rock (lava) cools quickly and produces small grained rocks. Igneous rocks are often very hard. Watch this video for more information. Igneous Rocks: How to know about igneous rocks

Sedimentary Rocks

Sedimentary rocks are made from small pieces of other rocks, and on Earth, from animal and plant remains, which are swept away by water or wind, and then settle in horizontal layers. As the layers build up, the pressure from above cements the sediments together and turns them into rocks. Fossils are found in sedimentary rocks. Watch this video for more information. Sedimentary Rocks: How to know about sedimentary rocks

Metamorphic Rocks

Metamorphic rocks are formed when heat or pressure (or both) change any rock into a new form with new minerals. The heat may come from lava, magma or just by being deep in the Earth. The pressure may come from collisions of tectonic plates or the weight of overlying rock. If there is enough pressure for long enough the new rock will often be platy or flaky.

So far, there has been little evidence of metamorphic rocks on Mars


Importance: Evidence of volcanic activity.

Mineral content: Iron, Magnesium, Manganese, Olivine, Pyroxene, Plagioclase, Silica oxide, Biotite, Hornblende.

Formation: Basalt is formed in the mantle of the earth. It comes to the surface through volcanoes.

Environments: Volcanoes, lava plains (like Western Victoria) and ridges in the centre of deep oceans.


Importance: Strong evidence of past water

Chemical Formula: Fe2O3

Appearance: Typical silver-grey coloured .

Formation: hematite most commonly precipitates out of water as it evaporates. Some hematite forms in igneous processes.

Environment: Warm shallow oceans, hot mineral springs; hydrothermal vents and volcanoes.


Chemical formula: Na / Ca Al2Si3O8

Appearance: colourless, white, grey, brown rectangle (tabular) minerals

Formation: Plagioclase is found in the earth’s crust. It is a common mineral found in granitic rocks (igneous) and in sedimentary rocks like sandstone.


Chemical elements: Potassium, Magnesium, Iron, Aluminium, Silicon, Oxygen, Fluoride

Chemical formula: (K2(Mg,Fe)3AlSi3O10(OH,O,F2)2)

Appearance: dark brown or black hexagonal – shaped crystals

Formation: Found in igneous rocks like granite and in sedimentary rocks such as mudstone.


Chemical formula: (Ca2 (Mg, Fe)4 Al(Si7Al)O22(OH,F)2 )

Appearance: dark green rectangular crystals

Formation: Hornblende is an igneous mineral which forms in the earth under various temperatures and pressures.


Chemical Formula: SiO2

Appearance: can consist of clear, white, yellow, pink, purple and black – coloured crystals.

Formation: Quartz is the most common mineral found on the earth’s surface. Quartz can be present in both igneous and sedimentary rocks.


Importance: Possible evidence of rockslides or volcanic explosions

Mineral content / structure: Angular fragments or rock and pebbles held together by mudstone or occasionally by sandstone.


- Environments with high energy, but not a lot of movement where the rock is broken up but not carried away

- Broken rock fragments are buried in mud that binds them together

- Volcanic breccias are bound together by mud, or ash, or a mixture of both.

Environments: Volcanoes, landslides and cliff collapses


Chemical Formula: KFe (III)3(OH)6(SO4)2

Appearance: yellow / brown

Formation: Jarosite is a mineral which forms in oxygenated zones (e.g. groundwater) where sulphide deposits interact to form sulphuric acid. Jarosite is a by-product mineral from this process under low temperate conditions (<100 degrees Celsius).

Jarosite forms in highly acidic water rich in iron and sulphur, such as in the Rio Tinto river in Spain.


Importance: Evidence of possible water in the past.

Structure/ Mineral content: Grains of sand held together by clay or a chemical cement - this clay or cement is called a matrix.

Formation: Sand is buried and compressed under other sediment; water brings in clay or cement that binds the sand.

Environments: Past wind or water erodes rock and separates the sand grains from smaller silt particles and larger pebbles, e.g. ocean and large lake shorelines, beaches, rivers and deserts.


Importance: Strong evidence of past water

Formation: Carbon dioxide from the atmosphere dissolves in water, then combines with metal ions in solution. Evaporation of the water leaves deposits of carbonates. Calcium carbonate can also be formed by living things such as plankton and corals to build shells and skeletons.

Environments: Carbonates are found where there is water, e.g. caves or oceans.


Importance: possible evidence of past life!

Mineral content: composed almost entirely of calcium carbonate (CaCO3).

Formation: On Earth, most limestone is made up of the remains of small shelled organisms, corals or algae. It can also be precipitated as shallow water evaporates, or formed where water trickles through rocks.

Environments: Always forms in the presence of water, which has little or no movement. e.g. warm shallow seas or lakes, deep oceans or caves.


Mineral content: Iron, Magnesium, Manganese, Olivine, Pyroxene, Plagioclase, Silica oxide, Biotite, Hornblende.

Formation: Scoria is a hard, light-weight volcanic rock. Gas escapes from magma as it rises from beneath a volcano, which produces a less dense magma before it reaches the surface as lava. Any excess gas is released as the lava begins to cool at the surface. Scoria appears to have air pockets, known as vesicles, which makes it extremely light .

Environments: Scoria is found on the surface and is often associated with explosive or effusive volcanoes. Scoria also forms where volcano eruption chambers intersect with the water table or water bodies.


Importance: Strong evidence of past water

Mineral content: composed of rounded fragments of any type of rock, held together by a muddy or sandy glue.

Formation: small and large pebbles are carried by water current or ice movement and then deposited; deposits are buried in mud or sand, which binds them together.

Environments: created in river systems, glaciers and occasionally in large undersea landslides.


Importance: Strong evidence of water in the past.

Mineral content: minute grains of silt or clay bound together.


  • Fine, muddy sediment is buried and compressed

  • The water is forced out leaving the fine sediments bound together

Environments: Forms in environment where there is water that is not flowing e.g. bottoms of lakes, deep oceans, swamps or river estuaries.


Chemical Formula: Fe (CO3)2

Appearance: yellow, dark brown and black colours. Siderite has a metallic and lustre surface.

Formation: Found In hydrothermal areas where naturally heated water interacts with iron and carbon dioxide.


Mineral Content: Composed primarily of silica quartz, hornblende, plagioclase (Sodium rich), orthoclase (Potassium rich) and biotite in varying quantities.

Formation: Granite is generated in the mantle, as it travels up through the earth’s surface, minerals are scavenged from the surrounding rock, altering the magma and mixing different crystals into it.

Environment: Granite is found along the boundaries of continental plates, or is sometimes pushed up through continental crust to the surface.


Importance: Strong evidence of deep mantle volcanic eruptions and no exposure to water.

Chemical Formula: Fe2SiO4 or Mg2SiO4. Formation: On Earth olivine forms in the mantle. At about 360 kilometres below the surface and temperatures up to 1900o Celsius. Unstable at the Earth's surface. Weathers quickly.

Environment: Brought to the surface by volcanoes.


Mineral content: composed of almost any type of sedimentary rock, sand, clay, gravel or boulders that happen to be below the glacier.

Formation: Till or glacial till is an accumulation of unsorted sediment. As a glacier moves ‘downstream’ it picks up anything underneath, including soils, gravel, clay, sand, pebbles and boulders, which is deposited where the glacier meets and ocean or lake. The weight of the glacier hardens the debris into rock.

Environments: Tillites are always formed by the presence of glaciers.


Chemical Formula: CaSO4 · 2H2O

Appearance: cream, pink

Formation: Gypsum is a calcium- based mineral which is commonly found in sedimentary environments as a residual salt which has been left behind from evaporated lakes (see image above), sea beds or in hydrothermal areas.


Elements: Potassium, Aluminium, Iron, Silicon, Oxygen

Chemical formula: KAlSi3O8

Appearance: white, cream, pink block–shaped mineral.

Formation: Orthoclase is found in the earth's crust. It is a common mineral found in granitic rocks (igneous) and in sedimentary rocks like sandstone.


Importance: Evidence of violent, explosive, volcanic eruption.

Structure: small sharp, angular ash particles, some larger pieces of solidified lava, and pieces of rock from the volcano vent walls.

Formation: Ash and lava particles from a violent eruption fall from the ash cloud and stick together to form deep deposits.


Tuff can be found in almost any volcanic environment where ash has been produced.