CUMMINGTONITE
Cummingtonite – (Mg,Fe2+)2(Mg,Fe2+)5Si8O22(OH)2 – is an inosilicate of the Amphibole Supergroup. It is a very rare mineral, characteristic of some types of metamorphic rocks. It does not currently constitute ore. A very rare fibrous variety, “amosite” (AMOSA = Asbestos Mine of South Africa) was mined as “asbestos” in South Africa.
It is defined as a monoclinic amphibole of the Mg-Fe-Mn Amphibole Subgroup. Cummingtonite forms a solid solution series with grunerite (from Fe) and is dimorphic with anthophyllite. Grunerite has less than 30% Mg, while cummingtonite has been defined as containing between 30 and 70% Mg. May contain impurities such as Mn, Al, Ti, Ca, Na and K
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Dark green, brown, grey, colorless.
Habit: Tabular, columnar, fibrous (in aggregates), granular, up to 20 cm. Rare crystals.
Cleavage: {110} perfect, typical for amphiboles.
Tenacity: Brittle.
Twinning: Simple and multiple, paralell to {100}.
Fracture: Splintery.
Mohs Hardness: 5 – 6
Parting: No.
Streak: White.
Lustre: Vitreous.
Diaphaneity: Transparent.
Density (g/cm³): 3.1 – 3.6
2. Geology and Deposits
Cummingtonite is normally found in medium to high grade, Ca-poor mafic and ultramafic metamorphic rocks such as amphibolites, cornubianites, gneisses and granulites.
It also occurs as a late mineral in some intermediate volcanic rocks such as gabbros, diorites and norites. It rarely occurs in silica volcanic rocks such as dacites. It can occur in scorpions. In Mn-rich metamorphic rocks, Mn-rich cummingtonite occurs.
Grunerite, on the other hand, occurs in metamorphic rocks derived from iron-rich sediments, banded iron formations.
3. Mineral Associations
Occurrs with:
– common silicates such as quartz, plagioclase and micas (biotite, chlorite).
– Mn silicates such as pyroxmangite, manganogrunerite and rhodonite.
– other amphiboles such as green hornblende, actinolite, actinolite-(Fe), tremolite, anthophyllite, arfvedsonite, arfvedsonite-(Mg), glaucophane and gedrite.
– other silicates such as serpentine/antigorite, cordierite, diopside, chlorite, talc and garnet.
– oxides such as hematite and magnetite.
– sulphides such as arsenopyrite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.639 – 1.671 nβ: 1.647 – 1.689 nγ: 1.664 – 1.708
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless to pale green or pale brown. With weak pleochroism and with more intense colors (darker) if it contains higher Fe contents:
X = colorless, pale yellow.
Y = pale yellow, pale brown.
Z = pale green, pale brown.
Mg-rich cummingtonite is not pleochroic.
Relief: Moderate to high.
Cleavage: Shows the perfect {110} cleavage typical of amphiboles.
In the longitudinal sections there is only one cleavage.
In the basal sections there are two cleavages that intersect at angles of 56º and 124º, as in all amphiboles. In fibrous crystals it is difficult to see cleavage.
Habits: Lamellar, tabular, columnar, acicular, fibrous, may form parallel or radial crystal aggregates.
Diamond-shaped basal section, like all amphiboles.
It normally forms aggregates parallel to subradiates of prismatic crystals.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Birefringence from 0.025 to 0.037: colors of up to 2nd and 3rd order, intense, in yellow, blue, orange, purple and red.
Birefringence increases with increasing Fe content.
Extinction: Oblique in longitudinal sections parallel to (010), ranges from 10 to 21°. parallel in sections parallel to (100). symmetrical in the basal sections.
Elongation sign: ES(+)
Twins: Very common, single or multiple, by {100}, usually as very thin polysynthetic twins parallel to the length of the grains in longitudinal section. It is an important diagnostic feature!
Zoning: No.
CONVERGENT LIGHT
Character: B(+)
2V angle: 65 – 90o
Alterations: occasionally alters to phyllosilicates such as chlorite, talc and serpentine. Hornblende overgrowths are relatively frequent.
Fine lamellae of cummingonite demixing can occur in actinolite, hornblende and arfevdsonite, as well as lamellae of these last three minerals can occur in cummingtonite.
May be confused with: other amphiboles, it may be necessary to use X-Ray Diffractometry.
Gedrite and anthophyllite have parallel extinction and commonly do not show polysynthetic twins.
Grunerite has darker colors, slightly weaker pleochroism, slightly higher interference colors, smaller extinction angles, is B(-) and occurs in other paragenesis.
Tremolite is B(-) and presents twins much more rarely; only single, non-polysynthetic twins.
Actinolite is B(-) and has lower indices.
5. Reflected Light Microscopy
Reflected light microscopy is not the recommended analytical method for the identification of cummingtonite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with cummingtonite.
Sample preparation:
PLANE POLARIZED LIGHT – PPL
Reflection color:
Pleochroism:
Reflectivity:
Bireflectance:
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy:
Internal reflections:
May be confused with:
General Characteristics: