Thursday January 19, 2012 (Physical Properties of Minerals)

The Launch Pad Thursday, 1/19/12 Identify the four objects.

Assignment Currently Open Summative or Formative Date Issued Date Due Date Into Grade Speed Last Day Launch Pads 1/4 - 1/6 F1 1/6 Project: Fossils S1 1/91/13 Lab – Fossils F4 1/12 Quiz 15 S2 1/13

Announcements I will not be available after school today. The Project on Fossils is way overdue.

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Minerals - The Building Blocks of Rocks Physical properties of minerals Streak When you rub a mineral on a flat surface, the color of the powder left behind on the streak plate is the mineral's streak. The streak and color of some minerals are the same. For others, the streak may be quite different from the color.

Although the color of a mineral may not be very helpful in identification, the streak, which is the color of the powered mineral, can be very useful.

Minerals - The Building Blocks of Rocks Physical properties of minerals Hardness Mineral hardness is based on the ability of one natural sample of matter to scratch another. Diamond is the hardest known naturally occurring substance, whereas talc is the least hard.

Minerals - The Building Blocks of Rocks Physical properties of minerals Hardness The hardness of a material is measured against the Mohs scale by finding the hardest material that the given material can scratch, and/or the softest material that can scratch the given material. For example, if some material is scratched by apatite but not by fluorite, its hardness on the Mohs scale would fall between 4 and 5.

Figure 2.13 Mohs Scale of Hardness

Minerals - The Building Blocks of Rocks Physical properties of minerals Cleavage In mineral terms, cleavage describes how a crystal breaks when subject to stress on a particular plane. If part of a crystal breaks due to stress and the broken piece retains a smooth plane or crystal shape, the mineral has cleavage. A mineral that never produces any crystallized fragments when broken off due to stress has no cleavage.

Cleavage

Minerals - The Building Blocks of Rocks Physical properties of minerals Habit Mineral habit is the shape of how a mineral tends to form. Types of mineral habits include columnar, blocky, acicular (needle-like), granular, lamellar or foliated (sheets easily split apart), botryoidal (like a bunch of grapes), fibrous, radiating, or dendritic (like a fern).

The mineral quartz often exhibits good crystal shape

Pyrite (fool’s gold) has two common crystal forms Figure 2.11

Figure 2.12A bladed habit

Figure 2.12B prismatic habit

Figure 2.12C banded habit

Figure 2.12D botryoidal habit

Minerals - The Building Blocks of Rocks Physical properties of minerals Luster The luster of a mineral is the way its surface reflects light. In simplest terms, minerals have a metallic or non-metallic luster. A metallic luster is a shiny, opaque appearance similar to a bright chrome bumper on an automobile. Other shiny, but somewhat translucent or transparent lusters (glassy, adamantine), along with dull, earthy, waxy, and resinous lusters, are grouped as non-metallic.

The freshly broken sample of galena (right) displays a metallic luster, while the sample on the left is tarnished and has a submetallic luster.

Minerals - The Building Blocks of Rocks Physical properties of minerals Color The color of a mineral is one of its most obvious attributes, and is one of the properties that is always given in any description. Color results from a mineral’s chemical composition, impurities that may be present, and flaws or damage in the internal structure. Color in minerals is caused by the absorption, or lack of absorption, of various wavelengths of light.

Minerals - The Building Blocks of Rocks Physical properties of minerals Fracture Fracture is a description of the way a mineral tends to break. It is different from cleavage and parting, which are generally clean flat breaks along specific directions. Different minerals will break in different ways and leave a surface that can be described in a recognizable way. Is the broken area smooth? Irregular? Jagged? Splintery? These are some of the ways of describing fracture. Although many minerals break in similar ways, some have a unique fracture and this can be diagnostic.

Minerals - The Building Blocks of Rocks Physical properties of minerals Fracture The most common fracture type is conchoidal. This is a smoothly curved fracture that is familiar to people who have examined broken glass. Sometimes described as a clam-shell fracture. Quartz has this fracture type and almost all specimens that have been broken, demonstrate this fracture type very well.

Conchoidal Fracture Figure 2.16

Minerals - The Building Blocks of Rocks Physical properties of minerals Fracture Another common type is subconchoidal. Similar to conchoidal, just not as curved, but still smooth. Andalusite can show this type.

Minerals - The Building Blocks of Rocks Physical properties of minerals Fracture Uneven is a type that is basically self explanatory. It is a common type that is found in anhydrite.

Minerals - The Building Blocks of Rocks Physical properties of minerals Fracture Jagged fractures have sharp points or edges that catch on a finger that's rubbed across the surface. Usually this indicates a metal such as copper, a metal alloy or some sulfides or oxides.

Minerals - The Building Blocks of Rocks Physical properties of minerals Fracture Splintery is a fracture type that occurs in fibrous or finely acicular minerals and in minerals that have a relatively stronger structure in one direction than the other two. Chrysotile serpentine is a typical mineral with splintery fracture, and kyanite is an example of a non-fibrous mineral that has this fracture.

Minerals - The Building Blocks of Rocks Physical properties of minerals Fracture Earthy is a fracture that produces a texture similar to broken children's clay. It is found in minerals that are generally massive and loosely consolidated such as limonite.