Websterite Identification Guide
A field guide to Websterite, a two-pyroxene ultramafic rock (pyroxenite), and how to separate it from peridotite and hornblendite.
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What Websterite Looks Like
Websterite is a type of pyroxenite, an ultramafic plutonic rock made essentially of both orthopyroxene and clinopyroxene (roughly equal proportions), with little or no olivine.
- Color: dark green, brownish-green to greenish-black.
- Luster: sub-metallic to vitreous on cleavage surfaces of pyroxene.
- Texture: medium- to coarse-grained, granular, interlocking pyroxene crystals; often quite uniform and heavy.
- Form: layers, lenses and cumulate bands in layered intrusions and mantle/ophiolite sections.
Step-by-Step Field Checklist
- Confirm an ultramafic, dark, dense rock dominated by dark minerals (few or no feldspars).
- Identify pyroxene: stubby prismatic grains with two cleavages meeting near 90 degrees, sub-metallic to glassy luster.
- Look for two pyroxenes. Subtle color/luster differences between bronzy orthopyroxene and greener clinopyroxene indicate websterite.
- Check for absence of olivine (no sugary olive-green grains), distinguishing it from peridotite.
- Heft it: noticeably dense and heavy.
Key Diagnostic Tests
- Hardness: pyroxene ~5.5-6.5; a knife only faintly marks it.
- Cleavage: pyroxene has two cleavages intersecting at ~87/93 degrees (near right angles), a key diagnostic (amphibole cleaves at ~56/124 degrees).
- Density: high, ~3.2-3.4 g/cm3.
- Magnetism: weak unless accessory magnetite/spinel present.
- Acid: no reaction (unless altered to carbonate).
Common Look-Alikes
- Peridotite (dunite/harzburgite/lherzolite): contains abundant olivine (granular olive-green grains that weather brown); websterite is olivine-poor and pyroxene-dominated.
- Hornblendite/amphibole rocks: amphibole cleavage angles are ~60/120 degrees, not ~90; amphibole grains are more elongate/needle-like.
- Orthopyroxenite (bronzitite) / clinopyroxenite: single-pyroxene rocks; websterite specifically contains BOTH ortho- and clinopyroxene in significant amounts.
- Gabbro: contains substantial plagioclase feldspar (pale grains); websterite has little to no feldspar.
Where It Is Found
Websterite (named for Webster, North Carolina) occurs in layered mafic-ultramafic intrusions, ophiolite complexes, and as mantle xenoliths and cumulate layers worldwide.
Frequently asked questions
What is Websterite made of?
It is a pyroxenite composed of both orthopyroxene and clinopyroxene in roughly comparable amounts, with little or no olivine.
How is Websterite different from peridotite?
Peridotite is olivine-rich, while websterite is olivine-poor and dominated by two pyroxenes; the presence or absence of granular olive-green olivine is the key clue.
How do you tell pyroxene from amphibole in Websterite?
Pyroxene cleaves at about 90 degrees and forms stubby prisms, whereas amphibole cleaves at about 60/120 degrees and forms more elongate, needle-like crystals.
Is Websterite a heavy rock?
Yes. Being ultramafic and pyroxene-rich, it has a high density of roughly 3.2-3.4 g/cm3 and feels noticeably heavy for its size.