THE MINERAL PHLOGOPITE

Chemistry: K Mg3 AlSi3 O10 (OH)2, Potassium magnesium aluminum silicate hydroxide.
Class: Silicates
Subclass: Phyllosilicates
Group: Micas
Uses: heat and electrical insulator for industrial purposes.
Specimens

Phlogopite is a rarer member of the mica group and is not well known even by mineral collectors. It has been mined however for its heat and electrical insulating properties which are considered superior to other micas. The typical light brown color of phlogopite is characteristic although it is difficult to distinguish brown biotite from dark brown phlogopite. The two are actually end members in a series that is dependent on the percentage of iron. Phlogopite is iron poor and biotite is iron rich. The darker color and density increase with an increase in the iron content. Biotite tends to form in a wider range of conditions than phlogopite which is limited mostly to ultramafic rocks and magnesium rich marbles and pegmatites.

Phlogopite, like other micas, has a layered structure of magnesium aluminum silicate sheets weakly bonded together by layers of potassium ions. These potassium ion layers produce the perfect cleavage. Phlogopite is rarely considered a valuable mineral specimen, but well formed crystals are rare and some are now on the market showing nice crystals. These come from the Kola Pennisula area of Russia. Single large plates or "books" of phlogopite can grow to considerable size.

PHYSICAL CHARACTERISTICS:

Color is pale brown to brown.
Luster is vitreous to pearly.
Transparency crystals are transparent to translucent.
Crystal System is monoclinic; 2/m
Crystal Habits include tabular to prismatic crystals with a prominant pinacoid termination. Phlogopite's four prism faces and two pinacoid faces form pseudo-hexagonal crystal "books". The sides of the crystal often tend to tapper and can have a "hard candy that has been sucked on, look". Also as lamellar or granular rock forming masses.
Cleavage is perfect in one direction producing thin sheets or flakes.
Fracture is not readily observed due to cleavage but is uneven.
Hardness is 2.5 - 3.
Specific Gravity is approximately 2.9+ (average)
Streak is white.
Associated Minerals are dolomitic marbles, hornblende, garnets and schorl.
Other Characteristics: cleavage sheets are flexible and elastic, meaning they can be bent and will flex back to original shape. Thin flakes show an asterism or six rayed star when a light source is viewed through the crystal due to inclusions.
Notable Occurrences include Ontario and Quebec, Canada; Russia and at many locallities in Europe.
Best Field Indicators are crystal habit, color, cleavage, elastic sheets and associations.

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Books

PHLOGOPITE specimens:
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PHLOGOPITE specimen phl-1
$ 20.00

Dims: 2-1/2" x 2" x 1/16"

Wt: 9.7 g

Kovdor, Kola Peninsula, Russia

While at the Tucson Gem and Mineral Shows, I found out that what I had marked as Biotite for a long time was actually Phlogopite from the famous Kola Peninsula area of Russia. The book shows 5 complete and undamaged hexagonal faces. The sixth, likely where the crystal was separated from the host rock, shows a small amount of boundary, but most of it is broken. The specimen's color appears to be black, but close examination of very thin sheets show a definite green coloration. I've only seen a few mica specimens with crystal form this good, and most of those were Phlogopite specimens from Russia.

phl-1 ($ 20.00)

Kovdor, Kola Peninsula, Russia

PHLOGOPITE specimen phl-2
$ 20.00

Dims: 2-3/4" x 2-1/4" x 1/16"

Wt: 12 g

Kovdor, Kola Peninsula, Russia

Once again we have a Phlogopite book with the dark, greenish-black color and pearly luster. Like others of its kind, this book has clean sides and a definite, visible pattern of growth from its center. Two of the six sides are incomplete, and one is completely missing, showing where the book was separated from its host rock. All in all, the book is very well-formed, though, and shows a definite growth pattern in its basal faces.

phl-2 ($ 20.00)

Kovdor, Kola Peninsula, Russia

PHLOGOPITE specimen phl-3
$ 16.00

Dims: 2" x 1-3/4" x 1/16"

Wt: 6 g

Kovdor, Kola Peninsula, Russia

Like other specimens from this Russian locality, this book of Biotite shows excellent hexagonal form, with 5 clean, definite sides, and an unusual, green-black color. One of the six sides is incomplete, although part of it is still evident. There is a small, three-sided hole in the crystal where it looks like a tourmaline crystal intersected the book. Actually, according to the book's growth pattern, that seems to be the spot from which the book grew outwards. Neato! I wonder if that hole is due to a tourmaline crystal or a Phlogopite "core" that later came out?

phl-3 ($ 16.00)

Kovdor, Kola Peninsula, Russia

PHLOGOPITE specimen phl-4
$ 20.00

Dims: 2-1/2" x 2-1/2" x 1/16"

Wt: 13 g

Kovdor, Kola Peninsula, Russia

Another lovely Russian Phlogopite, this book exhibits the clean sides, green-black color and visible "growth rings" that make it so weird! This particular specimen has a hole in it that signifies that a crystal of some other mineral has intersected it. One of the six sides of the book is missing(this is where it was attached to the host rock), and the two adjoining sides are incomplete, but the others are quite clean, and help to preserve the hexagonal shape.

phl-4 ($ 20.00)

Kovdor, Kola Peninsula, Russia

PHLOGOPITE specimen phl-5
$ 160.00

Dims: 5.0 x 4.0 x 3.0" (12.7 x 10.2 x 7.6 cm)

Wt: 1 lb., 13.0 oz. (821 g)

Liscombe Deposit, Monmouth Township, Wilberforce, Ontario, Canada

At least 3 large Phlogopite "books" are partly embedded in the calcite matrix of this specimen. These books appear to be in good condition, though one of the larger ones is obviously cleaved, and have good monoclinic and pseudohexagonal tabular form, with relatively well-defined edges and moderately clean faces. They are essentially black in color but have visible brown highlights. They are completely opaque, but the platelets that form the books are at least translucent as individuals, if not transparent. The books are accompanied by several green apatite crystals that are embedded in the calcite matrix- most of these are broken and intergrown with each other, but there are several which appear to be intact and in good condition. They have reasonably good hexagonal form, though their edges are all rounded, and are transparent and relatively clear, though usually internally fractured. These are some of the largest Phlogopite crystals that we have seen available.

phl-5 ($160.00)

Liscombe Deposit, Monmouth Township, Wilberforce, Ontario, Canada

PHLOGOPITE specimen phl-6
$ 65.00

Dims: 3.0 x 2.7 x 2.7" (7.6 x 6.9 x 6.9 cm)

Wt: 9.53 oz. (270.3 g)

Liscombe Deposit, Monmouth Township, Wilberforce, Ontario, Canada

A large "book" of brown-black Phlogopite blades makes up the bulk of this specimen. This book is in good condition, showing almost no human-induced damage, and has good hexagonal tabular form, though one of the edgeward "prism" faces is warped and another is essentially nonexistent. It has the classic brown-black color, pearly luster and micaceous cleavage of its species. A small amount of calcite with a few bits of apatite is attached.

phl-6 ($ 65.00)

Liscombe Deposit, Monmouth Township, Wilberforce, Ontario, Canada

PHLOGOPITE specimen phl-7
$ 70.00

Dims: 4.3 x 3.2 x 2.1" (10.9 x 8.1 x 5.3 cm)

Wt: 1 lb., 3.5 oz. (552 g)

Liscombe Deposit, Monmouth Township, Wilberforce, Ontario, Canada

A single, large Phlogopite "book" is partly embedded in the calcite host of this specimen. This book has dimensions of 4.3 x 2.1 x 0.7" (10.8 x 5.4 x 1.9 cm) and is in excellent condition, showing no appreciable damage. Its monoclinic, pseudo-hexagonal form is rather warped but still relatively good, and its black color with brown highlights, pearly luster and flaky habit are standard for the specie. The calcite base also contains 3 partial and 2 complete apatite crystals that are pale green in color, transparent, and quite clear.

phl-7 ($ 70.00)

Liscombe Deposit, Monmouth Township, Wilberforce, Ontario, Canada

PHLOGOPITE specimen phl-8
$ 375.00

Dims: 8.5 x 4.6 x 3.3" (21.6 x 11.7 x 8.4 cm)

Wt: lbs. ( kg)

Liscombe Deposit, Monmouth Township, Wilberforce, Ontario, Canada

This large display piece consists of a formation of scores of intersecting Phlogopite "books" that are accompanied by a small amount of calcite and apatite. Some of these books are quite large, reaching thicknesses of over 2" (5 cm) and diameters of 1.5" (3.8 cm). All have a reasonably good pseudohexagonal tabular form and the standard brown-black color and pearly luster on their flaky surfaces. There is light damage in a few areas, but the piece is still generally in excellent condition. Some of the apatite that is present has excellent trigonal crystal form, but all of the calcite is shapeless, and was likely mostly dissolved away with acid to expose the Phlogopite blades. It is one of the nicest pieces that I have seen.

phl-8 ($375.00)

Liscombe Deposit, Monmouth Township, Wilberforce, Ontario, Canada

PHLOGOPITE specimen phl-9
$ 37.00

Dims: 1.7 x 1.4 x 0.9" (4.3 x 3.6 x 2.2 cm)

Wt: 1.4 oz. (39 g) w/ base

Liscombe Deposit, Monmouth Township, Wilberforce, Ontario, Canada

This small cabinet specimen consists of two rather warped Phlogopite "books" that appear to have intersected during their growth. Both are in good condition, showing little human-induced damage. The larger of the two measures 1.4 x 0.9 x 0.5" (3.6 x 2.2 x 1.3 cm), and the other is only slightly smaller. The larger shows reasonably good pseudohexagonal tabular form, but the smaller is misshapen. Both have the standard brown-black color and pearly luster. They are both attached to a small amount of pale orange calcite, to which is also attached a piece of a small apatite crystal. The piece is hot-glued to an acrylic base.

phl-9 ($ 37.00)

Liscombe Deposit, Monmouth Township, Wilberforce, Ontario, Canada

PHLOGOPITE specimen phl-10
$ 60.00

Dims:1.7x1.6x1.0" (4.3x4.1x2.5 cm)

Wt: 2.0oz. (57g)

Baffin Island, Nunavut, Canada

This specimen consists of a single crystal of phlogopite measuring just under 0.5" (1.3cm) in length. This crystal is on a matrix of flesh-colred calcite, and shows excellent form. Tiny indentations in the exposed side of the crystal show where there were once crystals of blue spinel, I believe. This crystal is extremely translucent-a fact that I found very impressive. The calcite matrix on this specimen fluoresces a dark purple under short wave ultraviolet light.

phl-10 ($ 60.00)

Baffin Island, Nunavut, Canada