Ferro-edenite
Chemical formula: NaCa<sub>2</sub>Fe<sup>2+</sup><sub>5</sub>(Si<sub>7</sub>Al)O<sub>22</sub>(OH)<sub>2</sub>
A silicate from the amphibole group, being the iron-rich analogue of edenite, forming dark green to black, prismatic crystals.
Description
## Characteristics Ferro-edenite is a mineral from the silicate group, belonging to the calcic amphiboles. It is the ferrous (Fe²⁺) analogue of edenite. It typically forms well-developed, prismatic, and often elongated crystals. Its aggregates can also be granular or fibrous. Due to its high iron content, specimens have a characteristic dark color, ranging from dark green, through greenish-black, to almost black. ## Physical properties The hardness of ferro-edenite on the Mohs scale is 5-6, which is a typical value for amphiboles. The luster is vitreous, and on fracture surfaces or in fibrous aggregates, it can be silky. The mineral is translucent, and in thicker fragments or very dark specimens, it becomes almost opaque. The density ranges from 3.2-3.4 g/cm³. It exhibits excellent, bidirectional cleavage, typical of amphiboles, with planes intersecting at angles of approximately 56° and 124°. ## Colors and varieties This mineral does not form color varieties or commercial varieties. Its color is directly related to its high iron content and remains in the range of dark greens and blacks. ## History and name The name of the mineral, given in 1946, refers to its chemical composition and structural kinship. The prefix "ferro-" indicates the dominance of iron over magnesium, and "edenite" refers to its magnesium analogue, named after the locality in Edenville, New York, USA. ## Applications Ferro-edenite has no industrial applications. It is primarily an object of interest for collectors specializing in mineral systematics, and also a subject of scientific research in the field of petrology and mineralogy.
Diagnostic features
## Identification The key diagnostic feature of ferro-edenite, like other amphiboles, is the angle between cleavage planes (approx. 56°/124°), visible in cross-sections of crystals as a characteristic rhombic outline. The dark green to black color and prismatic crystal habit are also important clues. ## Distinguishing from similar minerals Ferro-edenite is difficult to distinguish from other dark amphiboles, such as ferro-hornblende or arfvedsonite, without advanced chemical analysis. It can be distinguished from pyroxenes (e.g., augite) based on the cleavage angle – in pyroxenes, it is close to 90°. Tourmaline (schorl) can be similar, but it has a different, triangular crystal cross-section and lacks distinct cleavage. ## Crystal forms It most often occurs as columnar, prismatic crystals with a hexagonal or rhombic cross-section. It can also form granular, radial, or fibrous aggregates.
Geological environment
## Genesis Ferro-edenite is a metamorphic mineral, typical of rocks metamorphosed under amphibolite facies conditions, such as amphibolites and gneisses. It also forms in some igneous rocks, especially in syenites, monzonites, and granites, where it crystallizes from iron- and calcium-rich magma. ## Mineral associations It co-occurs with other rock-forming minerals such as plagioclase, potassium feldspar, quartz, biotite, garnets (especially almandine), as well as with other amphiboles and pyroxenes. ## Localities Significant occurrences of ferro-edenite are found in Japan (Hokkaido – type locality), Canada (the famous Mont-Saint-Hilaire locality in Quebec, where well-formed crystals have been found), USA (Adirondack Mountains in New York), as well as in Finland and Norway.
Rarity
Not very common
Collector aspects
## Quality criteria The most valued specimens by collectors are those with sharply terminated, lustrous crystals of distinct prismatic form. Crystals embedded in a contrasting rock matrix, for example, on light feldspars, are also highly regarded. Crystal size and the absence of mechanical damage significantly increase the value of a specimen. ## Popular localities Specimens from Mont-Saint-Hilaire in Canada are considered the best for collectors, distinguished by their excellent crystal development and luster.
Care and storage
## Cleaning Ferro-edenite specimens can be safely cleaned with a soft brush and distilled water. Avoid strong streams of water that could damage more delicate aggregates. ## What to avoid The mineral is sensitive to strong acids. Ultrasonic cleaners are not recommended, as vibrations can cause crystals to cleave along natural cleavage planes. ## Storage Ferro-edenite is a relatively durable mineral and does not require special conditions. It should be stored in a place not exposed to impacts and falls, which could cause it to cleave.