JADEITE
Jadeite – Na(Al,Fe)Si2O6 – is an inosilicate of the Pyroxene Group, of the Clinopyroxene Subgroup. It is a rather rare mineral, typical of high-grade metamorphic terrains. In South America, for example, it practically does not occur, with the exception of two occasional, non-commercial occurrences in Brazil. It has a very high value as an ornamental stone. There are many imitations and fakes being sold as “jade”.
Jade is an ornamental rock that can be formed by jadeite (the most valued) or by nephrite, which are green amphiboles of the tremolite-actinolite series.
Jadeite is part of the jadeite-kosmochlor series. May contain H2O, Mn, Mg, Ca, K and Ti. It has six varieties, based on specific colors or higher levels of a certain impurity (e.g. Cr).
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Apple green, greenish white, violet blue, blue-green, violet, white (when pure), black.
Habit: Granular, tabular, acicular, fibrous. Crystals very rare.
Cleavage: {110} perfect.
Tenacity: Brittle.
Twinning: Simple on {100} or {001} and lamellar
Fracture: Irregular, fibrous.
Mohs Hardness: 6
Parting: On {100} possible, but not common.
Streak: White.
Lustre: Vitreous.
Diaphaneity: Transparent.
Density (g/cm³): 3.25 – 3.35
2. Geology and Deposits
Jadeite is found in low to medium temperature and high pressure metamorphic rocks such as serpentinites, quartzites, metagraywackes, pyroxenites (clinopyroxenites = “astridites”) and glaucophane-schists (from basaltic protoliths) of metamorphic belts of the Alpine Type. It forms massive, almost monomineralic rocks called jadeitites and jadeite-jade.
It can also occur in veins or as scattered grains. It is often found with quartz due to its origin by the decomposition of albite into jadeite + quartz.
3. Mineral Associations
In glaucophane-schists it occurs with glaucophane, lawsonite, low Fe pumpellyite, chromiferous epidote and albite, with or without quartz.
In quartzites, jadeite is associated with quartz, albite, sericite, graphite, titanite, calcite and chlorite.
Sometimes it occurs with chlorite, actinolite, relicts of augite and calcite.
It can also occur associated with zoisite, garnet, tremolite, calcite, aragonite, micas, itoigawaita, rengite, kosmochlor, stronalsite, niboite and fengite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.654 – 1.673 nβ: 1.659 – 1.679 nγ: 1.667 – 1.693 (literature reports very different indices from each other, as the indices increase with the increase of Fe3+, Ca and Mg contents, an increase that is not regular).
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless. It does not show pleochroism or is pleochroic in pale green.
Relief: Moderate.
Cleavage: {110} perfect. In the longitudinal sections there is only one cleavage. In the basal sections there are two cleavages that intersect at angles of 87º and 93º, typical of pyroxenes.
Habits: Usually in acicular to fibrous aggregates, more rarely in anhedral or subhedral grains in the matrix of the rock.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Birefringence from 0.005 to 0.021, resulting in colors from half 1st order to half 2nd order: white, yellow, orange, blue. With higher Fe3+ contents, anomalous interference colors appear.
Extinction: Oblique, between 33 and 40º (or 32º to 55º), difficult to determine when the habit is granular. The extinction angle increases with increasing Fe3+ content.
Parallel extinction in sections (100).
It often presents undulating forms and undulating extinction due to the deformations it underwent.
Elongation sign: It is not possible to determine.
Twins: Simple on {100} and lamellar.
Zoning: No.
CONVERGENT LIGHT
Character: B(+)
2V angle: 67 – 72º or 70 – 80º or 60º – 96º (Literature presents very different angles).
Alterations: is a product of alteration: albite = jadeite + quartz. With reduced pressure or presence of water, it transforms into amphibole (actinolite) or albite+nepheline. Can alter to analcime.
May be confused with: the moderate relief, the oblique extinction and the two cleavages intersecting at almost 90º in the basal sections characterize it as pyroxene. Its paragenesis, extinction angle and birefringence distinguish it from most other pyroxenes.
Aegirine and aegirine-augite show much stronger colors and pleochroism, are more birefringent, show different extinction angles, and occur in other paragenesis.
Jadeite with anomalous interference colors resembles zoisite.
Nephrite has a larger extinction angle.
Kyanite, which is also found in ultra-pressure rocks, is similar, but kyanite has a notably higher relief.
5. Reflected Light Microscopy
Reflected light microscopy is not the recommended analytical method for the identification of jadeite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with jadeite.
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: