A widely used classification of sandstone types which divides sandstones into arenites (less than 15% of rock is mud matrix) and wackes (more than 15% but less than 75% of the rock is mud matrix). The arenites are subdivided into quartz arenite (more than 95% quartz grains), arkosic arenite (more than 25% feldspar and more feldspar than rock fragments), subarkose arenite (5–25% feldspar grains and more feldspar than rock fragments), lithic arenite (more than 25% rock fragments and more rock fragments than feldspar), and sublitharenite (5–25% rock fragments and more rock fragments than feldspar). The wackes are divided into quartz wacke (more than 95% quartz grains), lithic wacke (more than 5% rock fragments and more rock fragments than feldspar), and feldspathic (arkosic) wackes (more than 5% feldspar and more feldspar than rock fragments).
Dott classification
- Orgel diagram
- Orgueil meteorite
- orientable
- Orientale Basin
- Oriental faunal realm
- orientalism
- Oriental region
- orientation
- orientation survey
- orifice
- orifice plate meter
- Origen (c.185–c.254)
- origin
- original horizontality
- original position
- original sin
- origination
- origin of elements
- origin of life
- origin principle of taxation
- Origins Program
- Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer
- O-ring
- Orion
- Orion Arm