Anthophyllite

Anthophyllite

Anthophyllite • amphibole group — idealized Mg₇Si₈O₂₂(OH)₂ (Fe → ferroanthophyllite) Crystal system: Orthorhombic • Series: anthophyllite–gedrite Mohs: ~5.5–6 • SG: ~2.9–3.2 • Luster: vitreous–pearly Cleavage: two directions ~56° & 124° (amphibole‑style) • Habit: bladed, prismatic, fibrous, radiating Associates: talc, chlorite, cordierite, olivine (forsterite), orthopyroxene, hornblende, garnet

Anthophyllite — Bladed Greens from Metamorphic Heat

Anthophyllite is an amphibole that prefers the quiet company of metamorphic rocks. It grows as bladed prisms, silky fibrous mats, and sometimes radiant sprays, wearing earthy greens and browns that feel straight from the hillside. It’s a textbook mineral for teaching cleavage angles (the classic amphibole “V”) and a subtle beauty when paired with snow‑white talc or soft grey chlorite.

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What it is
An orthorhombic amphibole rich in magnesium (iron‑rich = ferroanthophyllite), part of the anthophyllite–gedrite series
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Why it captivates
Elegant bladed crystals, soft woodland colors, and that satisfying amphibole 56°/124° cleavage that makes structural geology click
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Care snapshot
Mid‑hardness but can be splintery; avoid impact & abrasion; dust gently. For fibrous pieces, handle softly and keep surfaces undisturbed

Identity & Family 🔎

An amphibole with order

Anthophyllite is one of the few orthorhombic amphiboles (many cousins are monoclinic). It belongs to the anthophyllite–gedrite series, shifting chemistry by swapping Mg with Al and Fe, which nudges color from pale straw–green toward richer brown‑greens.

Name & meaning

Its name nods to “leaf‑like” aggregates—think stacks of pages or slender blades. Under the right conditions, it also grows in fine, silky fibers that form felted masses in metamorphic rocks.

Cleavage memory trick: Amphiboles (like anthophyllite) break at ~56° & 124°; pyroxenes break closer to ~90°. Once you see the “V,” you never unsee it.

Where It Forms 🧭

Metamorphosed magnesium rocks

Anthophyllite thrives where Mg‑rich rocks (like ultramafics or magnesian sediments) are cooked during regional metamorphism. It often grows alongside talc, chlorite, and orthopyroxene, marking amphibolite‑facies conditions.

Contact zones & skarns

At intrusive contacts with dolostones and Mg‑rich sediments, it can join forsterite, spinel, and cordierite in handsome skarn‑style assemblages—textbook metamorphic mosaics.

Retrograde stories

Later fluids may soften anthophyllite into talc along cleavage planes, producing a two‑tone look: hard blades framed by soft, silky alteration.

Pressure organizes the orchestra; anthophyllite plays the clarinet line—clear, woody, and beautifully directional.

Palette & Habit Vocabulary 🎨

Palette

  • Pistachio to olive — Mg‑rich tones.
  • Brown‑green — Fe‑richer or weathered blades.
  • Pearly silver — on fresh cleavage and fibrous sheens.
  • Grey — common in massive aggregates.

Luster is vitreous on fresh faces, pearly–silky on fibrous surfaces; edges can show a subtle satiny glow.

Habit words

  • Bladed/prismatic — elongated crystals with crisp terminations.
  • Radiating sprays — starburst clusters from a central point.
  • Fibrous — felted mats or veinlets of fine fibers.
  • Granular/massive — interlocking amphibole mosaics in schists.

Photo tip: Low, raking light makes cleavage planes flash; a matte dark plinth keeps olive tones from washing out.

Physical & Optical Details 🧪

Property Typical Range / Note
Chemistry Mg₇Si₈O₂₂(OH)₂ with Fe substitution to ferroanthophyllite; Al‑rich trend toward gedrite
Crystal system / Group Orthorhombic amphibole
Hardness (Mohs) ~5.5–6
Specific gravity ~2.9–3.2 (rises with Fe)
Cleavage / Fracture Two good cleavages at ~56° & 124°; fracture splintery–uneven
Luster / Transparency Vitreous to pearly; translucent on thin edges, otherwise opaque
Optics Biaxial (+); RI ~1.61–1.66; birefringence up to ~0.02; pleochroism pale straw→olive→brown
Streak White
Alteration Common to talc, chlorite, serpentine along cleavage & rims
Treatments None typical; polished slabs are uncommon due to splintery fracture
Plain‑English optics: Rotate a thin flake and watch greenish tones shift—amphibole pleochroism doing a quiet color‑change.

Under the Loupe 🔬

Cleavage “V”

Two intersecting cleavage sets meet near 56° and 124°. On broken ends, they form a neat, repeated V—very satisfying to spot.

Splintery ends

Fresh breaks look splintery with tiny spear‑like shards. Edges often show pearly flashes under side light.

Alteration halos

Look for talc or chlorite along cracks—a pale rim that softens the outline and adds texture contrast.

Look‑Alikes & Mix‑ups 🕵️

Hornblende (common amphibole)

Often darker and monoclinic; chemistry is Ca‑bearing. Cleavage angles are similar, but hornblende rarely shows the orthorhombic, bladed look of anthophyllite sprays.

Tremolite–actinolite

Whiter to grass‑green amphiboles; typically more fibrous and Ca‑rich. Thin edges of tremolite can be nearly colorless.

Orthopyroxene (enstatite)

Two cleavages near 90° (pyroxene style), not the amphibole V. Often more bronzy and has different luster.

Cummingtonite–grunerite

Monoclinic amphiboles with brown tones; optical tests or detailed habit comparisons help separate them from anthophyllite.

Quick checklist

  • Olive–brown blades or sprays?
  • Distinct cleavage V at ~56°/124°?
  • Pearly edges; talc/chlorite fringes? → Anthophyllite.

Localities & Notes 📍

Where it shines

Classic metamorphic belts in Scandinavia (Norway, Finland), the Appalachians (USA), and parts of the Alps and India produce handsome bladed clusters and felted aggregates with talc and chlorite.

How people use it

As a collector mineral and a metamorphic indicator—its presence helps geologists read pressure–temperature histories. Lapidary uses are rare due to splintery fracture.

Labeling idea: “Anthophyllite — orthorhombic amphibole — bladed/ fibrous — associates (talc, chlorite, cordierite) — locality.” Clean and informative.

Care & Display 🧼⛰️

Handling

  • Support from beneath; anthophyllite can be splintery along cleavage.
  • Use a puffer or very soft brush; avoid vigorous rubbing that lifts tiny shards.
  • Keep away from gritty surfaces that could scratch pearly faces.

Mounting & storage

  • Seat on foam or acrylic cradles; secure gently at the base.
  • For very fine fibrous pieces, display under a clear cover to keep surfaces undisturbed.
  • Transport snugly so blade tips don’t rattle.

Photography

  • Low, directional light to ignite the pearly sheen.
  • Black flags tame hotspots; a pale side reflector opens shadows between blades.
  • Angle to show the cleavage V—it’s the amphibole signature.
Display idea: Pair a bladed cluster with a talc schist containing felted anthophyllite—same mineral, two textures. Visitors love the comparison.

Hands‑On Demos 🔍

Cleavage compass

Shine a narrow beam along a broken edge and trace the two gleaming planes—measure the angle (you’ll land near 56°/124°). It’s a mini structural‑geology lesson.

Alteration contrast

Under a loupe, find pale talc or green chlorite rims along anthophyllite blades. The texture change tells a cooling‑fluids story in millimeters.

Anthophyllite is mountain building in miniature—orderly, directional, and quietly strong.

Questions ❓

Why orthorhombic matters?
It sets anthophyllite apart from most amphiboles (often monoclinic). The symmetry shows up as crisp, bladed habits and specific optical behavior.

Is it always green?
No—Mg‑rich pieces look pale straw to olive; Fe‑rich ferroanthophyllite trends brown‑green. Weathering and alteration add grey and pearly tones.

Can it be cabbed?
Rarely. The splintery fracture and cleavage make cutting tricky. Collectors prefer natural clusters and matrix pieces.

What’s with anthophyllite–gedrite?
They’re a solid‑solution series. As Al increases (gedrite side), colors deepen and some properties shift subtly; many specimens plot between the endmembers.

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