Lab I: Hand Sample Description & IUGS Classification Schemes

Purpose

In this lab, you will have the opportunity to refine your observational skills and build your petrographic vocabulary through construction of detailed rock descriptions. You will increase your familiarity and proficiency with igneous rock classification schemes. You will develop a more complete appreciation for the accuracy and precision achieved during point counting.

Preparation

Please read all of Chapter 2 and pages and pages 67-76 in Chapter 3 (IUGS classification scheme) prior to coming to lab.

In addition, refresh your memory of the definitions of the following terms. Most of these terms are discussed in Chapter 2.

Degree of Crystallinity

• Holocrystalline
• Hypocrystalline
• Holohyaline

Grain Size

• aphanitic
• phaneritic
• microlites
• crystallites
• fine-grained
• medium-grained
• coarse-grained
• very coarse-grained
• megacrysts
• pegmatite

Grain Size Distribution

• equigranular
• inequigranular
• seriate
• porphyritic (bimodal, trimodal; be sure to describe the distinct populations separately)
• glomeroporphyritic
• microporphyritic

Grain Shape

• euhedral
• subhedral
• anhedral

Additional textural terms

• graphic, micrographic
• granophyric
• vermicular
• mantle, corona, rapakivi & antirapakivi
• poikiolitic
• trachyitic
• vesicular