The hastingsite Mineral Group

Hastingsite is a member of the extended Hornblende group. There are several variations of Hastingsite with different elemental substitutions. Some of these are recognized as individual minerals, thereby making Hastingsite a mineral group, with Hastingsite the dominating member. Hastingsite is named after Hastings Co. in Ontario, which is the type locality for this mineral.

Chemical Formula

NaCa₂Fe_²⁺3Fe³⁺₄Al₂Si₆O₂₂(OH)₂

Color

Black, gray, dark green, beige, tan, brown

Crystal System

Monoclinic

Properties

Streak Light gray with a greenish tinge

Hardness 5 - 6

Transparency Translucent to opaque

Specific Gravity 3.2 - 3.6

Luster Vitreous, submetallic, dull

Cleavage 1,2 - prismatic

Fracture Uneven

Tenacity Brittle

Crystal Habits

Prismatic or tabular crystals are uncommon. They are usually flattened-with a narrow, diamond-shaped cross-section and pointed top. Most often in dense groups of platy or grainy crystals, acicular, radiating, fibrous, columnar, and massive. Crystals are sometimes striated.

Additional Information

Composition Basic sodium, calcium, iron and aluminum silicate.

In Group Silicates; Inosilicates; Amphibole Group

Striking Features Cleavage angles, environment, and crystal habit

Environment In marble deposits, metamorphic gneiss, and nepheline syenites.

Rock Type Metamorphic

Varieties

Hastingsite is the name of an individual mineral, as well as group name for several similar Hastingsite-related minerals with slight variations of elements in their chemical formula. The main variety is magenesio-hastingsite, but there are several other rare combinations defined by the IMA as individual minerals with potassium, fluorine, and chlorine. These rare varieties are not listed here.

Magnesio-hastingsite

 -   Variety of Hastingsite where magnesium partially replaces the iron where the magnesium is more dominant. Magnesio-hastingsite is recognized by the IMA as a distinct mineral species with the following chemical formula: NaCa₂Mg₄Fe³⁺Al₂Si₆O₂₂(OH)₂

Noteworthy Localities

Hastingsite is fairly common, though often perceived as Hornblende and not further identified. It generally does not make aesthetic mineral specimens, and is not commonly represented in collections. Thus, only few localities are mentioned here. The type locality of Hastingsite, where it forms in stubby crystals, is Dungannon Township, Hastings Co., Ontario, Canada. Hastingsite is very prevalent as black crystallized plates throughout the old iron mines of the Hudson Highlands and Ramapo Mountains, Rockland and Orange Counties, New York; and across the border in Passaic County, New Jersey. Large crystal plates were found in the iron mines of Monroe, Orange Co., New York; and dark gray crystals at the Sterling Hill Mine, Ogdensburg, New Jersey.

A new discovery in 2013 on the Rhein Property in Amity, Orange Co., New York has yielded some exceptionally large and sharp crystals of Magnesio-hastingsite, associated with red Spinel, embedded Graphite, and an occasional Geikielite crystal. Originally mistaken for Fluoro-pargasite, these have been reanalyzed as Magnesio-hastingsite and are by far the best examples of this mineral.

Common Mineral Associations

Calcite, Magnetite, Biotite, Phlogopite, Graphite, Diopside, Spinel, Quartz, Epidote

Distingushing Similar Minerals

Hastingsite can be very difficult to distinguish from other Hornblendes, though a locality will often be able to isolate it.

hastingsite Photos

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