Metamorphic Minerals

• Actinolite
• Alurgite
• Andalusite
• Andalusite (chiastolite)
• Antigorite
• Anthophyllite-Gedrite
• Biotite
• Brucite
• Calcite
• Cassiterite
• Charoite
• Chlorite
• Chloritoid
• Chrysocolla
• Cordierite
• Corundum
• Corundum (ruby)
• Kyanite
• Kornerupine
• Kyanite (reticite)
• Deerite
• Dumortierite
• Epidotes
Allanite
Pistacite
Piemontite
Zoisite

• Fassaite
• Fluorite
• Glaucophane
• Garnet
• Hercynite
• Hedenbergite
• Hydrogrossular
• Howieite
• Humites
• Jadeite
• Lawsonite
• Lazulite
• Lazurite
• Malachite
• Minnesotaite
• Muscovite
• Olivine
• Omphacite
• Omphacite Cr
• Pargasite
• Piemontite (Praborna)
• Pyrite
• Prehnite
• Pumpellyite
• Quartz
• Rhodonite
• Rutile
• Sapphirine
• Scapolite
• Serpentine
• Sillimanite
• Sillimanite (fibrolite)
• Staurolite
• Stilpnomelane
• Sugilite
• Talc
• Taramellite
• Thulite
• Tinaksite
• Tinzenite
• Titanite
• Tremolite
• Tourmaline
• Vesuvianite
• Violan
• Wollastonite
• Yoderite
• Zussmanite

Texture and Microstructure

• Augen
• Book shelf sliding
• Boudinage
• Corona
• Decussate
• Deformation lamellae
• Deformation twins
• Crenulation cleavage
• Fringes
• Granoblastic
• Kelyphite
• Kink Bands
• Lepidoblastic
• Mesh
• Mica fish
• Mylonitic
• Nematoblastic
• Oblique foliations
• Polygonal
• Porphyroblastic
• Porphyroclastic
• Pseudomorph
• Ribbon
• S-C fabric
• Skeletal garnet
• Symplectite
• Veins

Metamorphic Rocks

• Albite greenshist
• Amphibolite (felsic)
• Amphibolite (mafic)
• Amphibolite (mylonitic)
• Amphibolite with epidote
• Andalusite schist
• Anyolite
• Augen gneiss
• Blueschist
• Blueschist (Ile de Groix)
• Blueschist (retrograde)
• Buchite
• Calc-schists
• Carbonated peridotite
• Cataclasite
• Charoitite
• Chlorite schist
• Chloritoid Schist (Ile de Groix)
• Cordierite slate
• Corundum-fuchsite rock
• Deformed oolite
• Dunite (mylonitic)
• Eclogite
• Eclogite Porto Ottiolu
• Epidosite
• Gabbro (mylonitic)
• Garnet micaschist
• Glaucophane granofels
• Granite (mylonitic)
• Granulite (felsic)
• Granulite (mafic)
• Granulite (retrograde)
• Granulite (sapphirine-bearing)
• Greenschist
• Hornfels
• Hornfels (albite-epidote)
• Hornfels (cordierite, plagioclase)
• Hornfels (corundum, plagioclase)
• Hornfels (hydrogarnet)
• Jadeite metagranite
• Jadeitite
• Kinzigite
• Lapis lazuli
• Marble
• Marble (calc-silicate)
• Marble (diopside)
• Marble (forsterite)
• Marble (mylonitic)
• Marble (tremolite)
• Mylonite
• Ocean jasper
• Ophicalcite
• Peridotite (mylonitic)
• Phyllite
• Prasinite
• Pseudotachylyte
• Pseudotachylyte (Ivrea-Verbano)
• Quartzite
• Quartzite (tourmaline)
• Rhyolite (mylonitic)
• Rodingite
• Schist
• Seafloor basalt
• Serpentinite
• Serpentinized lherzolite
• Skarn
• Skiddaw Metamorphic Aureole
Skiddaw
1) Spotted slate
2) Andalusite slate
3) Cordierite slate

• Slate
• Slate (ottrelite)
• Talc schist
• Tourmalinite
• Ultramylonite (granitic)
• Ultramylonite (mafic)

Howieite - NaMn₃Fe₉Si₁₂O₂₉(OH)₁₅

Howieite was named by Agrell et al. (1965) in honour of Prof. Robert Andrew Howie, one of the authors of the Rock-Forming Minerals book series (Deer, Howie and Zussman). The mineral howieite has a stability range that extends down to pressures at, or even below, those normally considered to represent blueschist facies metamorphism. Its structure, lying somewhere between a chain and a sheet silicate, can accommodate almost complete iron for manganese substitution as well as the more extraordinary ferrous iron for ferric iron substitution.

Optical Properties:

• Form: Bladed crystal ore plumose/radiating crystals.
• Relief: Low.
• Pleochroism: From yellow to green to a lilac-grey.
• Interference colors: II order, but are masked to some extent by the absorption.

Acicular, black howieite crystals. Laytonville Quarry, California, USA. From RRUFF.

Acicular, black howieite crystals. Laytonville Quarry, California, USA. From RRUFF.

Geology of Laytonville quarry

Laytonville quarry (red Box).

Laytonville quarry

The Laytonville Quarry is the type locality for the minerals deerite, howieite, and zussmanite (Agrell et al., 1965). Situated 8 km south of Laytonville (Mendocino County, California), is a locality where an unusual variety of minerals has been found in Franciscan glaucophane schists. The rocks exposed in the quarry include both metamorphosed sedimentary and volcanic rocks of the Franciscan Formation (Jurassic and Cretaceous age). The metasedimentary rocks include metachert, metashale, metaironstone, and metalimestone. The Laytonville quarry contains a series of deep-ocean, subduction related, sediments of varying Fe:Mn, Fe:Al and Fe²⁺:Fe³⁺ contents, but all enriched in Fe Mn carbonates. These sediments have a complex metamorphic history that has involved the early development of aegirine and garnet, followed by various phases of secondary mineral growth, all of which required the addition of water to the metamorphic assemblages under high-P, low-T conditions. During the course of an early hydration episode deerite formed within most of the ironstones, and zussmanite in certain Fe-rich, Mn-poor pelites. At a later, and apparently lower pressure period in the metamorphic history, howieite, riebeckite, stilpnomelane, and a more manganous variety of zussmanite have overtaken much of the earlier mineralogy.

Sample kindly given to me by Keith A. Peregrine.

Bibliography

• R. M. Wood: The iron-rich blueschist facies minerals: Howieite. MINERALOGICAL MAGAZINE, JUNE 197 9, VOL. 43, PP. 363-7 0
• R. M. Wood: The Laytonville Quarry (Mendocino County California) exotic block: iron-rich blueschist-facies subduction-zone metamorphism. MINERALOGICAL MAGAZINE, VOL. 45, 1982, PP. 87 99

Photo

Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 2x (Field of view = 7mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 2x (Field of view = 7mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 2x (Field of view = 7mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. XPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Green fibrous howieite crystals and black deerite crystals. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and black deerite crystals. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Green fibrous howieite crystals. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 2x (Field of view = 7mm)
Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 2x (Field of view = 7mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 2x (Field of view = 7mm)
Green fibrous howieite crystals. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. XPL image, 10x (Field of view = 2mm)
Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. XPL image, 10x (Field of view = 2mm)
Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. XPL image, 10x (Field of view = 2mm)
Green fibrous howieite crystals. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 2x (Field of view = 7mm)
Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Green fibrous howieite crystals. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Green fibrous howieite crystals. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals and stilpnomelane. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Green fibrous howieite crystals. Meta-Ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)