Identifying minerals: physical properties and the Mohs scale

Detail

Why physical properties matter

• A mineral is a natural, inorganic substance. It has a fixed chemical make-up and a fixed internal arrangement of atoms.
• In the field, a geologist cannot run a chemical test on every sample. So minerals are told apart by physical properties: features you can see, feel, scratch or weigh.
• No single property is enough. Several are checked together, such as streak plus hardness plus cleavage.
• A list of standard properties is used to identify minerals and gem stones: lustre, colour, crystal form, cleavage, parting, hardness, specific gravity, index of refraction, transparency and dispersion [3].

1. Crystal form

• Crystal form: the outer shape of a mineral. It comes from the way its molecules are arranged inside.
• Ordered inner arrangement → flat faces at fixed angles → a typical outer shape.

• Common shapes are cubes, octahedrons (eight faces) and hexagonal prisms (six-sided columns).

• The outside shape shows the inside order. This is why the same mineral grows in the same shape wherever it is found.

2. Cleavage and fracture

Property	What happens	Cause	Example
Cleavage	Breaks along smooth, flat planes	Weak bonds lie in fixed directions	Mica peels into thin sheets
Fracture	Breaks unevenly, with no flat plane	Bonds are equally strong in all directions	Quartz breaks with curved, glass-like surfaces

• Cleavage: some minerals, when struck in the right place, split more easily in one direction than another. The flat surfaces they leave lie parallel to some of the crystal faces [3].
• Indian link: mica's perfect cleavage makes it useful as thin sheets in electrical and electronic goods. India's mica belts are in the Koderma–Gaya–Hazaribagh belt (Jharkhand), Ajmer (Rajasthan) and the Nellore mica belt (Andhra Pradesh) (NCERT Class 10).

3. Lustre

• Lustre: how the surface of a mineral reflects light. It is about the quality of the shine, not the colour.
• Types:
• Metallic: shines like metal (galena, pyrite).
• Silky: soft sheen from fine fibres (asbestos).
• Glossy / vitreous: shines like glass (quartz).

4. Colour and streak

• Colour can mislead. Small amounts of impurities change it. Quartz, for example, can be clear, pink, purple or smoky.
• Streak: the colour of a mineral's powder.
• Method: rub the mineral on a rough, unglazed porcelain plate, called a streak plate [4].
• A streak plate works only for minerals up to about 6.5 on the Mohs scale. Harder minerals scratch the plate instead of leaving powder [4].

• Why it works better: two samples of the same mineral usually give the same streak even when their colours differ [4].

• Hematite (an iron ore, Fe₂O₃) is the classic example.

• Its colour can be black, steel-grey, silver-grey, brown, reddish-brown or red [5].

• Its streak is always red-brown.

• Indian link: hematite is the most important industrial iron ore. It is mined at Bailadila (Chhattisgarh) and in the Odisha–Jharkhand belt (NCERT Class 10).

• Other minerals have their own streaks. High-iron chromite, for example, gives a dark brown to black streak [6].

5. Transparency and structure

• Transparency describes how much light passes through:
• Transparent: you can see clearly through it (clear quartz, diamond).
• Translucent: light passes through, but no clear image (some gypsum).

• Opaque: no light passes through (hematite, graphite).

• Structure: how the crystals are arranged and joined in a sample, such as fine-grained, coarse-grained, fibrous or granular.

6. Specific gravity

• Specific gravity: how heavy a mineral is compared with an equal volume of water.
• Exact definition: the ratio of a substance's weight to the weight of an equal volume of water at 4 °C [3].
• Use: two minerals of the same size can feel very different in the hand. Metallic ores such as galena and hematite feel much heavier than quartz or calcite.

7. Hardness and the Mohs scale

• Hardness: how well a mineral resists being scratched.
• The Mohs scale of hardness was made in 1812 by the German mineralogist Friedrich Mohs. It is a rough measure of how well a smooth surface resists scratching or wearing [2].
• Rule: a harder mineral scratches any softer one. A mineral that scratches fluorite (4) but not apatite (5) has a hardness between 4 and 5.

Hardness	Mineral	Note
1	Talc	Softest; feels soapy
2	Gypsum	Scratched by a fingernail
3	Calcite	Main mineral of limestone and marble
4	Fluorite
5	Apatite
6	Orthoclase feldspar
7	Quartz	Scratches glass
8	Topaz
9	Corundum	Ruby and sapphire are varieties
10	Diamond	Hardest natural substance

• Mnemonic: "Tall Girls Can Fight And Other Queer Things Can Do" (Talc, Gypsum, Calcite, Fluorite, Apatite, Orthoclase, Quartz, Topaz, Corundum, Diamond).

8. The Mohs scale is ordinal, not proportional

• The scale gives only the order of hardness, not how much harder one mineral is than another.
• The 10 standard minerals were given arbitrary numbers [2].
• The scale is not linear. A step of one does not mean the same increase in hardness each time [2].

• In absolute tests, corundum (9) is about twice as hard as topaz (8), but diamond (10) is almost four times as hard as corundum (9) [2].

• Absolute hardness is measured with indentation tests such as Knoop and Vickers: a tip is pressed into the surface, instead of scratching it [2].

• So diamond is far more than "ten times" as hard as talc.

9. Everyday hardness checks

Tool	Approx. hardness	What it tells you
Fingernail	~2.5	Scratches talc and gypsum
Copper coin	~3.5	Scratches calcite
Glass / steel knife	~5.5	Scratches apatite but not orthoclase
Quartz	7	Scratches glass

10. Trap: graphite and diamond

• Graphite and diamond are allotropes (different crystal forms of the same element), and the element is carbon [7].
• Diamond: each carbon atom is bonded strongly to four others in a tetrahedral (pyramid-like) 3-D network.

• Result: hardness 10, the hardest natural substance, often colourless and transparent [7].

• Graphite: carbon atoms form rings of six in flat, widely spaced sheets. The sheets are loosely held together.

• Result: hardness only 1–2. It is soft, greasy, dark grey to black and opaque [7].

• Graphite conducts heat and electricity well. Diamond is an insulator (it does not conduct electricity) [7].

• Lesson: a mineral's properties depend on its internal structure, not only on its chemical make-up.

Prelims Hooks

• The Mohs scale (Friedrich Mohs, 1812) runs from 1 to 10: Talc, Gypsum, Calcite, Fluorite, Apatite, Orthoclase, Quartz, Topaz, Corundum, Diamond.
• Trap: "Diamond is ten times as hard as talc" is wrong. The scale is ordinal and non-linear, and diamond is almost 4 times as hard as corundum alone [2].
• Trap: graphite (1–2) and diamond (10) are both pure carbon. The difference comes from their atomic structure (layers vs a tetrahedral network) [7].
• Streak (the colour of the powder on unglazed porcelain) is more reliable than colour. Hematite always gives a red-brown streak.
• Cleavage example: mica (sheets). Fracture example: quartz (uneven, curved break).
• Specific gravity is weight compared with an equal volume of water at 4 °C [3].
• Field checks: fingernail ~2.5, copper coin ~3.5, glass or steel knife ~5.5. Quartz (7) scratches glass.
• Corundum (9) is the mineral of ruby and sapphire. Calcite (3) is the main mineral of limestone and marble.
• Lustre types: metallic (galena), silky (asbestos), glossy/vitreous (quartz).

Mains Points

• Identification guides mineral exploration (GS-III):
• Field geologists use streak, hardness, lustre and specific gravity to spot ore minerals before costly lab tests.

• Example: hematite's red-brown streak helps survey iron-ore belts such as Bailadila (Chhattisgarh) and Odisha–Jharkhand.

• Physical properties decide industrial use:

• Mica's cleavage (thin, insulating sheets) suits electronics.
• High hardness makes corundum and diamond useful as abrasives and cutting tools.

• Graphite's softness and conductivity suit pencils, lubricants and battery electrodes.

• Structure matters as much as chemistry (GS-I):

• The graphite–diamond contrast shows why rocks made of the same elements can weather and erode at very different rates.
• This links mineral make-up to how landforms develop in the rock cycle.