1. Minerals II: Physical Properties and Crystal Forms From: http://webmineral.com/data/Rhodochrosite.shtml

2. Physical properties of minerals In a previous lecture, we discussed that minerals have both a definite chemical formula and a definite crystal lattice – both of which determine the properties of the mineral In this lecture, we will discuss what those properties are!

3. The Physical Properties of Minerals Color Streak Luster Hardness Cleavage/Fracture Crystal Form Specific Gravity Special Properties

4. Important Physical Properties II Color - Although an obvious feature, it is often unreliable to use to determine the type of mineral. –Color of a mineral may be useful for determining any impurities that are present in the mineral –For example – Rose quartz has iron impurities; green quartz has chlorite impurities

5. Important Physical Properties III Streak - The color of a mineral in its powdered form; obtained by rubbing the mineral against an unglazed porcelain plate. –Streak is usually less variable than color. –Streak can be VERY different from color!

6. Important Physical Properties I Luster - This property describes the appearance of reflected light from the mineral's surface. Luster is either “metallic” or “nonmetallic” Nonmetallic minerals are described using the following terms: vitreous, pearly, silky, resinous, and earthy.

7. Important Physical Properties V Hardness - This is the resistance of the mineral to abrasion or scratching. This property doesn't vary greatly from sample to sample of the same mineral, and thus is highly diagnostic. A value is obtained by comparing the mineral to a standard scale devised by Moh, which is comprised of 10 minerals ranging in hardness from talc (softest) to diamond (hardest).

8. Fingernail Hardness (2.5) Scratches Gypsum (2)

9. Mohs’ Hardness Scale

10. Important Physical Properties VI Cleavage - Orientation and number of planes of weakness within a mineral. Directly reflects the orientation of weak bonds within the crystal structure. This feature is also very useful Fracture - This describes how a mineral breaks if it is not along well defined planes. In minerals with low symmetry and highly interconnected atomic networks, irregular fracture is common.

11. Planer Cleavage in Mica

12. Weak Bonding Yields Basal Cleavage

13. Rhombohedral Cleavage in Calcite

14. Conchoidal Fracture in Glass

15. Important Physical Properties IV Crystal form or habit - The shape of crystals generally reflects the internal arrangement of their atoms in their crystal lattice. This can be altered, however, if the mineral crystallized in an environment that did not allow it to grow properly. Imagine wearing shoes that are too tight – your foot will change its shape to match the shoes!

16. Crystal Forms: Quartz

17. Feldspar

18. Intergrown cubic crystals of fluorite

19. Density and Specific Gravity Density - Defined as the mass divided by the volume. Specific Gravity - Ratio of the mass of a substance to the mass of an equal volume of water. Note that D water = 1 g/cm 3. S.G. is unitless. Examples - quartz (SiO 2 ) has a S.G. of 2.65 while galena (PbS) has a S.G. of 7.5 and gold (Au) has a S.G. of 19.3.

20. Special Properties Magnetism - Magnetite (Fe 3 O 4 ) has this property and it can be used to distinguish it from other non-magnetite iron oxides, such as hematite (Fe 2 O 3 ). Double Refraction - Seen in calcite crystals. Light is split or refracted into two components giving rise to two distinct images. Taste/Smell – Halite has a salty taste; Sulfur smells of rotten eggs Reactions with acid – Calcite fizzes on reaction with acid

21. Plagioclase striations

22. Calcite Double Refraction