Vogesite Identification Guide

What Vogesite Looks Like

Vogesite is a lamprophyre, a dark, fine-grained igneous dike rock. It is defined by hornblende (amphibole) as the dominant dark phenocryst, with orthoclase (alkali feldspar) dominant in the groundmass.

• Color: dark grey to greenish-black or brownish-black.
• Luster: dull groundmass with glossy black amphibole crystals.
• Texture: porphyritic; shiny hornblende (and sometimes biotite/augite) crystals set in a fine feldspathic matrix, often with well-formed dark crystals.
• Form: occurs as dikes and sills, not large bodies.

Step-by-Step Field Checklist

1. Confirm a dark, fine-grained dike rock rather than a coarse plutonic rock.
2. Spot the phenocrysts. Look for prominent black, prismatic, glossy hornblende crystals in the matrix.
3. Check for two generations of dark minerals (as both phenocrysts and groundmass), typical of lamprophyres.
4. Test hardness/heft. Hard, dense, heavy in the hand.
5. Note the setting. Thin dikes cutting other rocks support a lamprophyre identification.

Key Diagnostic Tests

• Hardness: mineral-dependent; amphibole and feldspar both ~5.5-6.5, so a knife barely marks it.
• Density: moderately high, ~2.8-3.0 g/cm3 (mafic).
• Magnetism: weak (minor magnetite possible).
• Acid: usually no reaction unless carbonate alteration is present (lamprophyres can be carbonate-altered and may fizz locally).
• Hand-lens mineralogy: hornblende + orthoclase is the key, distinguishing Vogesite from its relatives.

Common Look-Alikes (and how Vogesite is classified)

Lamprophyres are told apart by dominant dark mineral + dominant feldspar:

• Minette: biotite + orthoclase (mica-dominant, not hornblende).
• Kersantite: biotite + plagioclase.
• Spessartite: hornblende + plagioclase (Vogesite has orthoclase instead).
• Basalt: lacks abundant glossy hornblende phenocrysts and is plagioclase + pyroxene; basalt is a lava flow rock, while Vogesite is a dike rock with conspicuous amphibole.

The mnemonic: Vogesite = hornblende + orthoclase.

Where It Is Found

Named for the Vosges Mountains (France), vogesite occurs in dike swarms associated with alkaline and calc-alkaline magmatism worldwide, including parts of Europe and North America.