00799 - Petrography

Course Unit Page


This teaching activity contributes to the achievement of the Sustainable Development Goals of the UN 2030 Agenda.

Quality education

Academic Year 2020/2021

Learning outcomes

Students will learn the basic tools to describe and classify igneous, metamorphic and silicoclastic rocks, and understand the petrogenetic processes and their geologic significance. Students will acquire the know-how to classify in the field (= outcrop scale) and in the lab (hand sample + thin sections) igneous, metamorphic and silicoclastic rocks

Course contents

Igneous rocks - The magma: physical and chemical features. Upwelling and solidification of magmas: intrusive vs. volcanic settings. Origin of mantle magmas. Magmatic differantiation: fractional crystallzation, gravitative setting and cumulus rocks, assimilation. Bowen's reaction series. The role of Pressure and fluids on the origin of magmas. Origin of anatectic magmas. Mixing and mingling of magmas. Mineralogical and chemical of igneous rocks. Classification of magmatic rocks. Trace element and isotopes to fingerprint magmatic sources. Magmatic series and their relation to different geodynamic settings.

Metamorphic rocks - The drivers: Pression, Temperature, Deformation and Fluids. Microstructures and their classification. How to classify a metamorphic rock. ;etamorphic reactions and the record they left into the rocks. Metamorphic grade and facies. Facies series. Geotherm and Pressure-Temperature-time paths. Regional metamorphism (orogenic and sea floor) and contact metamorphism. Analysis and interpretation of mineral assemblages in metabasites and metapelites.

Petrography of Sedimentary rocks - Wheathering, transport and deposition of sediments. Diagenetic processes. Structures, textures, composition and classification of clastic, carbonatic, evaporitic and derived from biological activity.

Lab activities - (Natural Sciences) Classification of hand-sized samples of igneous, metamorphic and sedimentary (silicoclastic) rocks. (Geological Sciences) Classification of hand-sized samples of igneous, metamorphic and sedimentary (silicoclastic) rocks and texture analysis of thin sections of igneous, metamorphic and sedimentary (silicoclastic) rocks by polarizing optical microscopy 

Field Trip - A single-day excursion with practical exercises of classification of igneous and metamorphic rocks


Klein C., Philpotts A. (2018) Mineralogia e Petrografia (1a ed. italiana), Bologna, Zanichelli.

D'Argenio F., Innocenti F., Sassi P.F. (1994) Introduzione allo studio delle rocce, Torino, UTET.

Teaching methods

Class lectures augmented by

  • Petrography Lab
  • Optical microscopy lab
  • Field Excursion

Assessment methods

The transformation of teaching into virtual teaching inevitably leads to changes in the assessment of the learning objectives.

The new assessment procedure is an oral exam in which we will test

  1. skills in the recognition of images of rocks (also at the microscale for Geological Sciences) and
  2. knowledge of the main topics covered in class.

As for point (1), the student must be able to:

  • know and understand the basic terms (= basic knowledge) to describe a rock at the different observation scales,
  • apply basic knowledge to the proposed images,
  • logically organize the information the student will be able to obtain from the analysis of the images,
  • critically evaluate what has been observed, highlighting the limits of observation and how one could overcome these limits.

A practical example:

I observe an image of a phaneritic rock with a low color index (<40) and in which I certainly recognize quartz. The structure is hypidiomorphic granular. It could be a tonalite, a granodiorite, a monzogranite, a sienogranite or an alkali-feldspar granite.

The correct classification is based on the evaluation of the alkalifeldspar / plagioclase modal ratio and, secondly, on the type of femic minerals.

The possible answers are (listed in order of increasing critical approach, which is proportional to the vote ...)

1. it is a granitoid,

2. it can be a tonalite or a granodiorite (or, for example, a granite or a granodiorite, or a sienogranite or an alkalifeldspar granite)

3. it can be a tonalite or a granodiorite (or one of the other two pairs of lithotypes of the previous point); to improve the classification, I would try to estimate the orthoclase vs plagioclase modal abundances. To do this, I would observe on the hand sample how many minerals reflect light on their cleavage planes (orthoclase) compared to those that do not have this property because they have a less developed cleavage (plagioclase). Also, I would try to better understand the type of femic mineral because tonalites often have amphibole while granites are more likely to have biotite as the main femic.

All the answers are correct but the answer (3) shows a greater petrographic competence. It is this type of competence that we seek (and reward with good marks).

From a practical point of view, the examination procedure is as follows:

  • Registration via almaesami,
  • The exam platform will probably be MSTeams,
  • During the oral session, the camera must be switched on and the student must have an identification document,
  • The teacher will share the screen with the images (macro and micro for Geological Sciences; only hand samples for Natural Sciences),
  • The student will have control of the screen to indicate what she/he describes,
  • If the image recognition part is positive, the topics discussed in class are checked,
  • If the image recognition part is negative, for example, a granite is exchanged for rhyolite, the candidate does not continue to the oral exam.

Teaching tools

  • Video Projectors supporting Power Point presentations
  • Hand-sized samples
  • Thin sections
  • Polarizing Optical Microscopes
  • Mineral and rock collections of the Museo di Mineralogia Luigi Bombicci

Office hours

See the website of Roberto Braga