How to identify Porphyry

To correctly identify Porphyry (Porphyritic igneous rock), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Highly variable, depending on the mineralogy of the phenocrysts and groundmass. Can range from light (felsic, e.g., pink, white, grey) to dark (mafic, e.g., black, dark grey, green).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Variable, typically dull to vitreous in the groundmass, with phenocrysts exhibiting their characteristic lusters (e.g., vitreous for quartz and feldspar, pearly for mica).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Porphyritic, defined by two distinct crystal sizes: larger, visible phenocrysts embedded in a finer-grained or glassy groundmass. The phenocrysts are typically 1 mm to several centimeters in size, while the groundmass crystals are often microscopic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Phenocrysts often display well-developed crystal forms (euhedral to subhedral), reflecting their slower growth. Groundmass crystals are typically anhedral or too small to discern form.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Cleavage is observed in individual phenocrysts according to their mineralogical properties (e.g., feldspars exhibit good cleavage, quartz has none). The groundmass typically shows no macroscopic cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Forms in both intrusive (plutonic) and extrusive (volcanic) settings. Intrusive porphyries (e.g., porphyritic granite) form in shallow crustal intrusions (dikes, sills, laccoliths). Extrusive porphyries (e.g., porphyritic andesite, rhyolite) form from volcanic eruptions where magma cools rapidly at the surface.

Key Facts

Hardness

Variable, depending on constituent minerals. Phenocrysts like quartz (7) and feldspar (6-6.5) are hard, while the groundmass can be softer or harder depending on its composition and crystallinity.

Specific Gravity

Variable, typically 2.5 to 3.0 g/cm³, depending on mineral composition. Felsic porphyries are generally lighter (2.5-2.7), mafic porphyries are denser (2.8-3.0).

Crystal System

Not applicable to the rock as a whole; individual phenocrysts exhibit their respective crystal systems (e.g., quartz is trigonal, feldspars are triclinic or monoclinic).

Color

Highly variable, from light grey, pink, red, or white to dark grey, green, or black.

Luster

Variable, from dull to vitreous, depending on the minerals present and the texture of the groundmass.

Transparency

Opaque to translucent in hand specimen.

Fracture

Conchoidal to uneven, depending on the groundmass and presence of phenocrysts.

Cleavage

Absent in the rock as a whole; present in individual phenocrysts according to their mineral properties.

Composition

Silicate minerals, primarily feldspars (plagioclase, orthoclase), quartz, micas (biotite, muscovite), amphiboles (hornblende), pyroxenes (augite), and sometimes olivine. The specific proportions define the rock type (e.g., rhyolite, andesite, basalt, granite, diorite, gabbro).

Physical characteristics

• Crystal Habit: Phenocrysts are typically euhedral to subhedral, often prismatic or tabular. Groundmass crystals are anhedral or cryptocrystalline.
• Cleavage Type: Not a characteristic of the rock as a whole. Individual phenocrysts exhibit their specific cleavage (e.g., feldspars have 2 directions at ~90 degrees).
• Fracture Type: Conchoidal (if glassy groundmass or quartz phenocrysts), uneven, or hackly.
• Tenacity: Brittle.
• Luster Type: Variable, typically dull to vitreous.

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