DIOPSIDE

Diopside is a calcium magnesium silicate mineral, often with some iron in the CaMgSi2O6 formula. This mineral creates fine collector specimens, and transparent crystals from the classic localities are highly valued. It is faceted as a minor gemstone, and the emerald-green, chromium-rich Chrome-Diopside variety has been increasing in popularity. Some polished gemstones display asterism in the form of four-rayed stars. The stone can be confused with Dioptase because of the similarity in its name, and both are frequently green in color. However, there are different minerals, such as Dioptase, a copper silicate mineral, and Diopside, a calcium-magnesium silicate.

The color of this mineral is usually light to dark green but may also be gray, yellow, light blue, purple, and white. It may also be green with thin white streaks running through a crystal. It is rarely colorless or multicolored. White forms of Diopside will occasionally fluoresce bright powder blue in short wave ultra-violet light.

Gemstone quality diopside is found in two forms: black star diopside and chrome diopside (which includes chromium, giving it a rich green color). Due to the deep green color of the gem, they are sometimes referred to as Siberian emeralds, although they are completely unrelated minerals.

This mineral forms a series with Hedenbergite, the iron equivalent of Diopside, and may be partially replaced by it. Diopside and Hedenbergite can even occur together in a single crystal, with a core of Hedenbergite and the outer zone of Diopside. It almost always contains a slight amount of iron replacement, and pure, iron-free Diopside is rather uncommon. Additional iron in the chemical structure of Diopside will cause a darker color and decreased transparency.

This mineral usually forms as single, short prismatic crystals. Crystals may also be somewhat elongated and usually have good terminations. Also massive, grainy, columnar, bladed, radiating, fibrous, as cleavage fragments, and in disordered aggregates of elongated crystals. It may also be in v-shaped penetration twins, and crystals from certain localities have partially hollow or dissolved etchings.

The mineral is found in contact and regional metamorphic rocks in hornfels and in skarn deposits of hydrothermal metamorphic rock. It is also found in ultramafic (kimberlite and peridotite) igneous rocks. It is an essential mineral in the Earth’s mantle and is common in peridotite xenoliths erupted in kimberlite and alkali basalt.

Diopside belongs to the Monoclinic Crystal System, Prismatic Crystal Class. Its hardness is 5.5 – 6.5 on the Mohs Scale. The luster is vitreous to dull, and crystals can be transparent to opaque.

Varieties:

Chrome-Diopside – Emerald green, chromium-rich variety of Diopside.

Dekalbite – Pure Diopside free of iron impurities.

Malacolite – White or lightly tinted variety of Diopside, usually fluorescent.

Salaite – Diopside with a large amount of iron impurities, leaning towards Hedenbergite.

Schefferite – Manganese-rich variety of Diopside. A zinc-rich form also exists, and it may be called “Zinc Schefferite”.

Violane – Light blue to purple, manganese-rich variety of Diopside.

Noteworthy Localities: Several classic Diopside occurrences are in Italy. Localities there include Val d’Fassa, Trento Province; Val d’Ala and Val d’Ossola, Piedmont; Bellecombe, Val D’Aosta; and Monte Somma, Vesuvius. The purple Violane variety comes from the Prabornaz Mine in Saint-Marcel, Val D’Aosta, Italy.

Important worldwide occurrences of Diopside include Slyudyanka, Lake Baikal area, Russia; the Ziller Tal, Tyrol, Austria; Tulear Province, Madagascar; and a new find of gemmy crystals from the Kunlun Mts, Xinjiang, China. Vivid green Diopside crystals with exceptional transparency come from the Karo Mine, Merelani Hills, Arusha, Tanzania, where sharp graphite plates are associated.

The emerald-green, chromium-rich variety Chrome Diopside comes from several places, specifically Outokumpu, Finland; Konar and Nangharnar Provinces, Afghanistan; Alchuri (Shigar Valley), the Tormiq Valley, and Chamachhu, Skardu District, Pakistan; and Inagli Massif, Aldan, Russia.

In the United States, the best Diopside localities are in New York. The premier U.S. locality is De Kalb, St Lawrence County, New York, where Diopside forms in transparent and gemmy light to medium green crystals. Dark, forest green Diopside with good form and a contrasting white matrix comes from the Rose Road locality near Pitcairn, also in St Lawrence County, New York. Very large opaque crystals were found in Orange County, New York, at Amity and Edenville, and nearby at several of the old iron mines in the Hudson Highlands and Ramapo Mountains region. Other good U.S. localities are Sanford, York County, Maine; the Belvidere Mountain quarries, Lowell/Eden, Orleans & Lamoille Counties, Vermont; and the Crestmore Quarry, Riverside County, California.

Canada also has many good deposits of Diopside. Fibrous bundles of Diopside associated with bright green Grossular come from Brompton Lake, Quebec. Other famous localities in Canada are the Orford Nickel Mine, St-Denis-de-Brompton; Wilberforce and Cardiff, Haliburton County, Ontario; Bancroft, Hastings County; and the Jefferey Mine, Asbestos, Quebec, where it occurs in a unique tabular, transparent flat crystal form. White Diopside crystals are known from Dog Lake, Frontenac County, Ontario, Canada.

By Bill Jones, Sidewinder Minerals

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