Basic Ferroan Dolomite Forms       Several distinct ferroan dolomite crystal forms occur at the Lamereaux road bridge locality.  The most common of these forms are illustrated in figure I to the right.   This is an unusual occurrence, as most Ohio localities will produce no more than one or two crystal forms of the species.  The most regularly observed form in Ohio is the simple rhombohedron, labeled A in figure I.  Minute examples of these can be found in abundance lining pockets at many dolostone quarries in Western Ohio.   The second most common form known as the curved or saddle-shaped rhombohedron, labeled B in figure I, is commonly found at various localities in the state, most notably the C.E. Duff and Son quarry at Huntsville, in Logan County.   While simple rhombohedrons and curved rhombohedrons are fairly common throughout Ohio, dolomite or ferroan dolomite crystals with additional modifications are extremely rare.  Two of these modified forms commonly seen at the Lamereaux road bridge locality are labeled C and D in figure I.  The first of these, illustrated as C, is a positive rhombohedron(a) modified by a negative rhombohedron(b).  The second, illustrated as D, has an additional positive rhombohedral face(c).  These additional faces vary in size as described later in this article.

Fig I.   basic forms of dolomite and ferroan dolomite crystals from  septaria at the Lamereaux road bridge locality; A) simple  rhombohedron, B) curved rhombohedron, C) rhombohedron(a)  with negative rhombohedron(b), D) rhombohedron(a) with negative  rhombohedron(b) and positive rhombohedron (c).

Simple Rhombohedral Form     Sharp, straight rhombohedrons of near perfect form occur regularly in the 1st and 2nd generations of ferroan dolomite growth in septaria at the Lamereaux road bridge locality.  These crystals are generally vitreous, slightly translucent and less than 1 cm in size.  Crystals from these two generations are often tan or light yellowish brown in color.  A common example is displayed in the photo to the  rhombohedral ferroan dolomite crystal measures 5 mm West branch of the Huron River at Lamereaux road bridge Monroeville, Huron Co., Ohio lower left.   Isolated pockets with crystals of this simple form occur in later generations, but are not as common.   The photo to the right displays an example from the 3rd generation.  Although the form is often modified, this generation of ferroan dolomite is typically dark brown to black in color.  Ferroan dolomite crystals like this have often been mistaken for Fluorite by amateur collectors.   However, while the color and clarity are very similar if not identical to Fluorite specimens from the White Rock quarry at Clay Center, the form is clearly incorrect.

dark amber brown ferroan dolomite crystals (specimen Joseph W. Vasichko) largest crystal measures 7 mm West branch of the Huron River at Lamereaux road bridge Monroeville, Huron Co., Ohio

Curved Rhombohedral Form     The classic form for dolomite and ferroan dolomite is the curved rhombohedron or saddle shaped crystal group.  Crystals of this type are fairly common throughout the Midwest, including certain Ohio localities.  At the Lamereaux road bridge locality, the best curved crystals occur regularly in the 5th generation of ferroan dolomite growth.  Since the  curved rhombohedral ferroan dolomite crystal measures 8 mm West branch of the Huron River  at Lamereaux road bridge Monroeville, Huron Co., Ohio     curved rhombohedral ferroan dolomite crystal measure 6 mm West branch of the Huron River  at Lamereaux road bridge Monroeville, Huron Co., Ohio majority of pockets in septaria do not show this later generation, curved crystals are not as common. The 4th and the rarely present 6th generations of ferroan dolomite growth also produce micro curved crystals which often create a pseudo-like shell around earlier generations of ferroan dolomite or  other minerals.   Earlier generations do not tend to produce curved crystals as commonly.  A clear example of a slightly curved crystal is displayed in the photo to the upper left.  The cause for this curvature is not fully understood, but the effect is always amplified in larger crystals.   The reason for this is that smaller crystals will group themselves at a slightly offset angle to each other to form the larger saddle shaped crystals.   This property is best displayed in the photo to the upper right.  Notice the size of the individual rhombohedrons in comparison to the overall size of the saddle shape groups.   The pearl-like luster of the species is best seen on these curved crystals when a sheen of light sweeps across the curved faces.  The photo to the lower left exhibits a typical example of this effect.  Curved crystals are frequently twinned, and occasionally form individual flower-like crystal groups.  These are possibly the most attractive of the ferroan dolomite crystals to be found at the locality.  This is especially true when they occur as isolated individuals on earlier generations of ferroan dolomite of a different color. A classic example of this is displayed in the photo to the lower right.

curved saddle shaped ferroan dolomite crystals (specimen Joseph W. Vasichko) field of view is 4.1 cm x 2.7 cm Huron River at Lamereaux road bridge Monroeville, Huron Co., Ohio   twinned saddle shaped ferroan dolomite crystal group (specimen Joseph W. Vasichko) twinned group measures 1.7 cm Huron River at Lamereaux road bridge Monroeville, Huron Co., Ohio

rhombohedral crystals with negative rhombohedral modification  largest crystals measures 6 mm West branch of the Huron River at Lamereaux road bridge Monroeville, Huron Co., Ohio

Negative Rhombohedral Modification As mentioned previously, dolomite or ferroan dolomite crystals with any modification to the rhombohedron are generally rare.  One of the modifications which does occur at the Lamereaux road bridge site with some frequency is the negative rhombohedral modification.   This form is illustrated as crystal C in figure I.  The ‘b” faces  represent the negative rhombohedral modification.   In this description, it is assumed that the basic crystal form is a positive rhombohedron.  This type of modification adds 6 triangular shaped faces to the main rhombohedron.The photo to the left displays a typical cluster of crystals with the negative rhombohedral modification.   These negative rhombohedral faces are generally crude, rounded and difficult to discern as true faces.  It is generally necessary to view these under magnification to identify the modification.   A close up of this modification is pictured in the photo to the right.  This modification varies in size from very slight to near equal proportion to the main crystal form, but is often distorted by increase in size. negative rhombohedral modification  modification measures 2 mm West branch of the Huron River at Lamereaux road bridge Monroeville, Huron Co., Ohio

Positive Rhombohedral Modification       The more common modified ferroan dolomite crystal observed at the Lamereaux road bridge locality has both a negative rhombohedral modification and a positive rhombohedral modification.  This form is illustrated in figure I as crystal D.   While the “b” face represents the negative rhombohedral modification, the “c” face represents the positive rhombohedral modification.  The positive rhombohedral modification adds 6 elongate rhombus shaped faces along the edges of the basic rhombohedron.  With the addition of this face and dependant on proportion, the negative rhombohedral modification becomes a five sided face.   Like the negative rhombohedral modification, this additional modification varies in size and is generally crude.  The photo to the right displays a typical group of ferroan dolomite crystals showing both modifications.   Notice the five sided negative rhombohedral modification.  These crystals are from the 3rd generation of ferroan dolomite, as is evident by the rich dark brown color.  The 2nd, 3rd and 4th generations tend to produce the most crystals with either modification.   No crystals have been observed with the positive rhombohedral modification alone.   Ferroan dolomite crystals with additional modifications have not been observed at the locality.

rhombohedral crystals with both positive and negative rhombohedral modifications  largest crystal measures 9 mm West branch of the Huron River at Lamereaux road bridge Monroeville, Huron Co., Ohio

Ferroan Dolomite Habits     In addition to the 4 basic forms, several distinct habits can be observed in ferroan dolomite crystals from the Lamereaux road bridge locality.   Many of these habits are broadly observed in particular generations throughout septaria at the locality, while others appear to be more unique to individual septaria.  It should be noted, that all of the crystals in one pocket of the same generation will typically be of similar form and habit.   Figure II to the right displays some of the more common growth habits observed at the locality including convex growth or multiple growth hillocks, concave growth or hopper shape, stacked growth, epitaxial growth, multiple step growth, and twin growth.  It is possible for any of the 4 crystal forms to occur in any of these habits.  Couple this with the fact that generations of ferroan dolomite crystals are often affected by the generation which came before and it becomes evident why there are so many varieties of ferroan dolomite to be found at the locality.  Other ferroan dolomite habits do exist, but are not as regularly encountered and therefore have not been included in this description.

Fig II.  common habits observed in dolomite and ferroan dolomite  crystals from septaria at the Lamereaux road bridge locality; 1)  multiple growth hillocks, 2) hopper growth, 3) stacked,  4) epitaxial, 5) step growth,  6) twinned

Multiple Growth Hillocks     Although one of the less common habits of ferroan dolomite found at the Lamereaux road bridge locality, multiple-growth hillocks do occur with some frequency throughout the septaria.  This habit has been illustrated by crystal 1 in figure II.  This type of growth is not linked to any particular generation, but has been observed more frequently in the 2nd, 3rd and 4th generations.   The reason for the hillocks is not completely clear, but they do appear to grow simultaneously with the central crystals.   In general, the hillocks are crude in comparison to the central crystal, but are of identical form, color and luster.  The number of hillocks on ferroan dolomite crystals varies.  Often crystals will show only one hillock.  Crystals with more than 3 hillocks on any one face are very rare.  Generally, each face of the crystal will have the same number of hillocks.  The thickness of these hillocks also varies.  An example of growth hillocks is photographed to the right.   This crystal shows 2 hillocks on all of its faces.  It should be noted that these are some of the thicker hillocks observed on ferroan dolomite from the locality.   In general, the hillocks are thinner, less distinct and not as photogenic.  Most of the crystals in this pocket show similar hillocks.

multiple growth hillocks on ferroan dolomite crystal measures 7 mm West branch of the Huron River at Lamereaux road bridge Monroeville, Huron Co., Ohio

Hopper Shaped Crystals     Concave or hopper shaped ferroan dolomite crystals are occasionally observed at this locality.  While the term hopper shaped may not be entirely accurate for this form, the mechanism that caused the imperfection is likely the same.  In a true hopper shaped crystal the crystal edges grew faster than the faces, forming a skeletal crystal.  Often, hopper shaped crystals will show a concave series of steps on each face.   This type of crystal generally grows very fast.   While the ferroan dolomite crystals at this locality did not grow quickly, the crystal edges occasionally did grow slightly faster than the faces.  Because of this, ferroan dolomite crystals from this locality will often only show one step.    In addition, these concave faces are usually irregularly shaped and are often filled with smaller faces that attempted to complete the overall form.   In general, crystals will exhibit the hopper shape on every face, but the size and form are often different on each side.  The photo to the right exhibits a clear example of this habit.  Note the small raised crystals faces that attempted to fill the center of the large irregularly shaped concave face.  While this imperfection has been observed in multiple generations, it is most common in second generation crystals.

hopper growth on ferroan dolomite crystal measures 8 mm West branch of the Huron River at Lamereaux road bridge Monroeville, Huron Co., Ohio

Stacked Rhombohedrons     One of the more complex ferroan dolomite crystal habits observed at the Monroeville locality is the stacked rhombohedron, as illustrated by crystal 3 in figure II.   These crystals appear to be the composite of a series of stacked rhombohedrons of decreasing size that generally grow upward away from the matrix.   Crystals of this form are often elongate with slightly curved faces.  This habit of crystal is most commonly found in the later generations of ferroan dolomite growth.  Crystals may occur as individuals or as  stacked rhombohedron of ferroan dolomite crystal measures 5 mm West branch of the Huron River  at Lamereaux road bridge Monroeville, Huron Co., Ohio clusters of crystals lining pockets.   The number of visibly stacked rhombohedrons will often vary slightly from crystalto crystal, but crystals from the same pocket or mineralized seam will be of similar form.  Rarely, broken individuals of this form are hollow, indicating a possible pseudomorphic growth.  Because of the overall shape created by the upward step growth of these crystals, the term “pagoda shaped” has occasionally been applied to this form.  The crystal displayed to the left is a sharp individual of this form.   A careful eye can discern several stacked rhombohedrons in this single crystal.  The photo to the right displays a cluster of more rounded crystals of this same form.

cluster of stacked rhombohedrons of ferroan dolomite field of view is 3.8 cm x 4.3 cm West branch of the Huron River at Lamereaux road bridge Monroeville, Huron Co., Ohio